**Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials**

Youssef S. Al Jabbari1,2,\* and Spiros Zinelis3,4

*1Director, Dental Biomaterials Research and Development Chair, School of Dentistry, King Saud University, Riyadh, 2Prosthetic Dental Sciences Department, School of Dentistry, King Saud University, Riyadh, 3Department of Biomaterials, School of Dentistry, University of Athens, Athens 4Dental Biomaterials Research and Development Chair, School of Dentistry, King Saud University, Riyadh 1,2,4Saudi Arabia 3Greece* 

### **1. Introduction**

52 Nondestructive Testing Methods and New Applications

Sikora,R.; Chady, T.; Gratkowski, S.; Komorowski, M. & Stawicki, K. (2003). Eddy Current

Valsan, M.; Sundararaman, D.; Bhanu Sankara Rao, K. & Mannan, S.L. (1995). High

Valsan, M.; Nagesha, A.; Bhanu Sankara Rao, K. & Mannan, S.L. (2002). High temperature

Weinstock. H. & Nisenoff, M. (1985) Nondestructive evaluation of metallic structures using

Weinstock, H. & Nisenoff, M. (1986) Defect detection with a SQUID magnetometer *Review of* 

Weinstock, H. (1991). A review of SQUID magnetometry applied to nondestructive

Weinstock, H. (Ed.), (1995) *SQUID sensors: Fundamentals, Fabrication and Applications*,

pp. 427-434, ISBN 0-7354-0117-9, Bellingham, Washington, July 2002 Tavrin, Y.; Siegel, M. & Hinken, J.-H. (1999). Standard method for detection of magnetic

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(Berlin: deGruyter) pp. 843–847 ISBN 0899251447

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0019-493X

704

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Testing of Thick Aluminum Plates with Hidden Cracks. *Review of progress in Quantitative nondestructive evaluation*, Vol. 22, AIP Conference Proceedings, Vol. 657,

defects in aircraft engine discs using a HTS SQUID gradiometer. *IEEE Trans. Appl.* 

Temperature Low Cycle Fatigue of Steels and their Welds, *Metall. Mater. Trans*.,

low cycle fatigue and creep-fatigue interaction behaviour of 316 and 316(N) weld metals and their weld joints. *Trans Ind Inst Met*. Vol. 55, No. 5 pp. 341-348, ISSN

a SQUID gradiometer, *SQUID '85, Proc. 3rd Int. Conf. On Superconducting Quantum Interference Devices and their Applications* (eds) Hahlbohm, H.D. & L¨ubbig, H.

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evaluation. *IEEE Trans. Mag.,* Vol. 27, No. 2, (March 1991), pp. 3231-3236, ISSN

Dental biomaterials consist of a wide range of synthetic products that are normally used to restore patients' oral health, function and aesthetic appearance. Generally, dental biomaterials are classified on the basis of their atomic bonding into metallic, ceramic, polymer and composite materials. In addition to this classification, biomaterials can be classified according to their interactions with the surrounding oral tissues (Mano et al. 2004). Based on the tissue responses, biomaterials can be divided into three different categories commonly known as bioinert, bioactive and bioresorbable materials.


Further to these previous classifications of dental biomaterials, it is important to mention that the final products normally placed in the oral cavity are produced in two different ways. The first involves industrial line production, wherein thousands of identical dental

<sup>\*</sup> Corresponding Author

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 55

locating and sizing any external/internal structural defects. The acquired information is then assessed to evaluate whether the tested devices meet acceptable criteria or whether they should be rejected. Taking advantage of the NDT capabilities, the quality of the end products is increased while the level of reliability is enhanced. The application of these sophisticated NDT methods can easily be conducted industrially to produce acceptable dental devices. However, conducting extensive systematic quality control for custom-made dental devices is restricted owing to 1) the tremendous increase in relative costs, 2) the need for availability of specifically designed testing equipments in dental offices and laboratories and 3) the need for well-trained dentists and dental technicians to operate these equipments. Besides all these factors, if proper NDT is applied routinely to custom-made dental devices,

Although there are a number of limiting factors for routine application of NDT in dentistry, NDT has a variety of important applications in the dental field. The best example is X-ray radiography, which is used for the detection of pores that are typically located in thin regions of cast custom-made dental devices (i.e. clasp shoulders of a cast metal removable partial denture framework). The early detection of pores at these critical regions and stage will allow the dentist and/or dental technician to correct the problem before the final delivery of the prostheses to the patients. The existence of such pores will lead to catastrophic premature failure in the oral cavity. In research applications, NDT contributes significantly to the determination of adverse effects of the oral environment on dental biomaterials/devices and vice versa. In addition, NDT is very helpful in studies of the interaction mechanisms between the oral environment and dental biomaterials as a result of intraoral aging. A variety of NDT methods can be used to fully characterize dental devices preoperatively (as received) and after retrieval from the oral cavity by tracking the occurrence of any changes during intraoral aging. Stereomicroscopy, variable pressure SEM (VPSEM) without the need for a conductive coating and X-ray microanalysis have been used to monitor the morphological and elemental alterations after intraoral aging of dental devices. In addition, computed X-ray micro-tomography (micro-XCT) has been used to correlate the locations and sizes of internal defects found preoperatively in all ceramic bridges with the fracture origin under clinical conditions. Such information acquired from micro-XCT regarding the defect locations will provide tremendous insights into the underlying mechanism of intraoral degradation. Other examples for applications of NDT in dentistry are 1) noninvasive and *in situ* testing of intraoral implant stability utilizing RFA and 2) detection of hidden or sub-surface caries (tooth decay) by utilizing X-radiography

it will dramatically increase the total cost of already expensive dental services.

and laser technology measuring laser fluorescence within the tooth structure.

daily dental practice, noninvasive diagnosis and dental biomaterials research.

**treatment** 

**2. Applications of NDT for quality assurance purposes during dental** 

The aim of this chapter is to present and discuss the applications of current technologies for NDT of dental biomaterials covering a wide range of NDT techniques and their impacts on

Commonly, NDT techniques are performed by dentists and/or dental technicians to ensure the proper quality of custom-made dental devices. For example, they carry out routine checks by the naked eye or using a stereoscope for the marginal integrity of dental restorations (crowns) for precision before the restorations are delivered permanently to the patients. However, it is important to mention that precision in dentistry is a subjective term

devices are manufactured, such as dental implants, endodontic files and orthodontic wires/brackets. The second is the production of custom-made devices that are produced in dental laboratories, such as crowns, fixed partial dentures, removable partial dentures, complete dentures and orthodontic devices (Figure 1).

As with any industrial application, the structural integrity of dental devices has to be tested to preserve the reliability of function and avoid premature failures. This testing starts with quality control aiming to discard any defective items, while the end products should comply with safety regulations and materials specifications (i.e. ISO standards). Information for assuring proper quality is acquired through destructive and non-destructive testing. These techniques are easily distinguished from each other, as the latter leaves the tested devices undamaged and suitable for additional testing and/or permanent usage in patients. Nondestructive testing (NDT) involves a broad spectrum of analytical techniques, including macroscopic/microscopic visual inspection, eddy current testing, radiography, X-ray computed tomography (XCT), resonance frequency analysis (RFA) and many others. NDT aims to qualitatively and/or quantitatively characterize the tested devices by detecting,

Fig. 1. a) Representative examples of industrially-made dental devices: 1) orthodontic archwire made of Ni-Ti alloy, 2) endodontic file made from Ni-Ti alloy, 3) Mini-orthodontic implant made of cp-Ti and 4) dental implant made of Ti. b) Industrially-made orthodontic brackets fabricated from stainless steel alloy. c) Custom-made ceramo-metal crowns. d) Custom made removable partial denture made of Cr-Co alloy and acrylic resin.

devices are manufactured, such as dental implants, endodontic files and orthodontic wires/brackets. The second is the production of custom-made devices that are produced in dental laboratories, such as crowns, fixed partial dentures, removable partial dentures,

As with any industrial application, the structural integrity of dental devices has to be tested to preserve the reliability of function and avoid premature failures. This testing starts with quality control aiming to discard any defective items, while the end products should comply with safety regulations and materials specifications (i.e. ISO standards). Information for assuring proper quality is acquired through destructive and non-destructive testing. These techniques are easily distinguished from each other, as the latter leaves the tested devices undamaged and suitable for additional testing and/or permanent usage in patients. Nondestructive testing (NDT) involves a broad spectrum of analytical techniques, including macroscopic/microscopic visual inspection, eddy current testing, radiography, X-ray computed tomography (XCT), resonance frequency analysis (RFA) and many others. NDT aims to qualitatively and/or quantitatively characterize the tested devices by detecting,

Fig. 1. a) Representative examples of industrially-made dental devices: 1) orthodontic archwire made of Ni-Ti alloy, 2) endodontic file made from Ni-Ti alloy, 3) Mini-orthodontic implant made of cp-Ti and 4) dental implant made of Ti. b) Industrially-made orthodontic brackets fabricated from stainless steel alloy. c) Custom-made ceramo-metal crowns. d) Custom made removable partial denture made of Cr-Co alloy and acrylic resin.

c d

complete dentures and orthodontic devices (Figure 1).

locating and sizing any external/internal structural defects. The acquired information is then assessed to evaluate whether the tested devices meet acceptable criteria or whether they should be rejected. Taking advantage of the NDT capabilities, the quality of the end products is increased while the level of reliability is enhanced. The application of these sophisticated NDT methods can easily be conducted industrially to produce acceptable dental devices. However, conducting extensive systematic quality control for custom-made dental devices is restricted owing to 1) the tremendous increase in relative costs, 2) the need for availability of specifically designed testing equipments in dental offices and laboratories and 3) the need for well-trained dentists and dental technicians to operate these equipments. Besides all these factors, if proper NDT is applied routinely to custom-made dental devices, it will dramatically increase the total cost of already expensive dental services.

Although there are a number of limiting factors for routine application of NDT in dentistry, NDT has a variety of important applications in the dental field. The best example is X-ray radiography, which is used for the detection of pores that are typically located in thin regions of cast custom-made dental devices (i.e. clasp shoulders of a cast metal removable partial denture framework). The early detection of pores at these critical regions and stage will allow the dentist and/or dental technician to correct the problem before the final delivery of the prostheses to the patients. The existence of such pores will lead to catastrophic premature failure in the oral cavity. In research applications, NDT contributes significantly to the determination of adverse effects of the oral environment on dental biomaterials/devices and vice versa. In addition, NDT is very helpful in studies of the interaction mechanisms between the oral environment and dental biomaterials as a result of intraoral aging. A variety of NDT methods can be used to fully characterize dental devices preoperatively (as received) and after retrieval from the oral cavity by tracking the occurrence of any changes during intraoral aging. Stereomicroscopy, variable pressure SEM (VPSEM) without the need for a conductive coating and X-ray microanalysis have been used to monitor the morphological and elemental alterations after intraoral aging of dental devices. In addition, computed X-ray micro-tomography (micro-XCT) has been used to correlate the locations and sizes of internal defects found preoperatively in all ceramic bridges with the fracture origin under clinical conditions. Such information acquired from micro-XCT regarding the defect locations will provide tremendous insights into the underlying mechanism of intraoral degradation. Other examples for applications of NDT in dentistry are 1) noninvasive and *in situ* testing of intraoral implant stability utilizing RFA and 2) detection of hidden or sub-surface caries (tooth decay) by utilizing X-radiography and laser technology measuring laser fluorescence within the tooth structure.

The aim of this chapter is to present and discuss the applications of current technologies for NDT of dental biomaterials covering a wide range of NDT techniques and their impacts on daily dental practice, noninvasive diagnosis and dental biomaterials research.

### **2. Applications of NDT for quality assurance purposes during dental treatment**

Commonly, NDT techniques are performed by dentists and/or dental technicians to ensure the proper quality of custom-made dental devices. For example, they carry out routine checks by the naked eye or using a stereoscope for the marginal integrity of dental restorations (crowns) for precision before the restorations are delivered permanently to the patients. However, it is important to mention that precision in dentistry is a subjective term

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 57

As mentioned previously, dental technicians routinely evaluate the quality of their fabricated devices. For example, they use a stereomicroscope to examine devices made by casting. If they detect any external large casting porosity, they will correct and repair the problem by filling the pores with solder materials or they may need to remake the whole casted device before they pass it for clinical application. In addition, they regularly evaluate the passivity of the fit of their cast prosthesis superstructure utilizing a stereomicroscope. If they detect any misfit between the cast superstructure and the supporting teeth and/or implants, they will correct the problem using recasting, soldering, welding and electrodischarging machining techniques (Contreras et al. 2002; Ntasi et al. 2010; Romero et al.

Many dental devices (such as crowns, and fixed and removable partial dentures) are traditionally manufactured by casting dental alloys. Recently, the preparation of metal-free crowns and bridges has introduced the concept of casting ceramic materials. However, the mechanical stability and biocompatibility of dental devices depend on the properties of the materials and on the accuracy of the manufacturing process. Unfortunately, the dental casting procedure unavoidably leads to the development of pores in dental cast frameworks owing to gas entrapment or shrinkage, which may adversely affect the quality and efficacy of dental devices. The development of undesirable porosity is dependent on various factors and is a common complication for precious, semiprecious and base contemporary dental alloys (Elliopoulos et al. 2004; Li et al. 2010; Neto et al. 2003; Ucar et al. 2011; Zinelis 2000). This may negatively affect the long-term mechanical stability. For example, when there are cast external porosities, corrosion resistance decreases because of crevice formation and

In industrial applications, internal voids can be readily investigated by employing X-ray examination, and the same technique has been adopted in dental practice as a nondestructive method for the same purposes. Pores can be easily distinguished as dark regions on radiographs, thereby providing significant information for the size, location and distribution of imperfections. The same methodology can be readily applied to identified internal voids in dental cast frameworks (Dharmar et al. 1993; Eisenburger et al. 2002; Eisenburger et al. 1998; Elarbi et al. 1985; Mattila 1964; Wictorin et al. 1979). However, the visibility of voids and the picture quality depend on the combination of the material to be tested and the analytical conditions applied for X-ray testing (Eisenburger & Addy 2002).

Going back to the principle of this technique, it must be noted that the attenuation of a narrow beam of monoenergetic photons with specific E and intensity Io passing through a

 I=Io e-μ(ρ, Ζ, Ε)\*t (1) where I is the photon intensity that goes out to the sample, and μ is the linear attenuation coefficient that depends on the material density, atomic number Z and beam energy. Therefore, the X-ray absorption depends on the atomic number, the density of the material and the energy of radiation. Accordingly, different dental alloys can be penetrated by X-rays to different extents during testing. As stated above, various precious, semiprecious and base

homogeneous material of thickness t is determined by Lambert's low:

2000; Zinelis 2007).

**2.2 X-ray testing** 

plaque accumulation in the oral cavity.

and varies from one dentist to another. This means that proper precision and acceptability are dependent on a dentist's personal experience and skills. Therefore, this type of testing/inspection is optional and in many situations is performed without specific requirements to accept or reject custom-made dental devices (e.g. crowns). Internal defects in cast and welded metallic custom-made devices are not uncommon and X-ray testing is performed for early detection of such defects. This section will present the dental applications of stereoscopic and X-ray examinations and testing.

### **2.1 Stereomicroscopic examination and testing**

Stereomicroscopic examination is performed routinely by dental technicians and/or dentists to evaluate the quality of recently fabricated custom-made dental devices (e.g. crowns). Detection of any problems at this stage will allow for proper correction before the final prosthesis insertion in the patient's mouth. A recent study found that implant retaining screws deteriorate over a period of time when they are used to hold a dental prosthesis in a patient's mouth (Al Jabbari et. al, 2007a). Therefore, the authors encouraged dentists who provide extensive implant treatment in their practice to equip their offices with a stereomicroscope to enable regular evaluation of the quality of the tiny retaining implant screws at follow-up appointments. A low power stereomicroscope was found to be a powerful and useful tool for evaluating the quality of the external structures/surfaces of tiny dental devices (e.g. prosthetic retaining screws) (Al Jabbari et. al, 2007a). As illustrated in Figure 2, the stereomicroscope was able to clearly reveal the damage and deterioration of an implant screw head and threads, whereas it is almost impossible for a dentist to observe such damage and deterioration under the naked eye. The great advantage of performing this type of NDT in dental offices is that it will allow dentists to replace any severely damaged or deteriorated retaining screws with new ones. Failure to detect such deterioration and damage may lead in the future to a more complicated and expensive dental treatment for patients (Al Jabbari et. al, 2007b).

Fig. 2. Threaded segment (a) and slotted head segment (b) & (c) of new and retrieved implant retaining screws examined under stereomicroscope. Examination of these tiny dental implant devices reveals significant threads deterioration/thinning "black arrows" and screw head damage "(c)" when compared to the intact threads profile "red arrows" and normal slotted head (b).

As mentioned previously, dental technicians routinely evaluate the quality of their fabricated devices. For example, they use a stereomicroscope to examine devices made by casting. If they detect any external large casting porosity, they will correct and repair the problem by filling the pores with solder materials or they may need to remake the whole casted device before they pass it for clinical application. In addition, they regularly evaluate the passivity of the fit of their cast prosthesis superstructure utilizing a stereomicroscope. If they detect any misfit between the cast superstructure and the supporting teeth and/or implants, they will correct the problem using recasting, soldering, welding and electrodischarging machining techniques (Contreras et al. 2002; Ntasi et al. 2010; Romero et al. 2000; Zinelis 2007).

### **2.2 X-ray testing**

56 Nondestructive Testing Methods and New Applications

and varies from one dentist to another. This means that proper precision and acceptability are dependent on a dentist's personal experience and skills. Therefore, this type of testing/inspection is optional and in many situations is performed without specific requirements to accept or reject custom-made dental devices (e.g. crowns). Internal defects in cast and welded metallic custom-made devices are not uncommon and X-ray testing is performed for early detection of such defects. This section will present the dental

Stereomicroscopic examination is performed routinely by dental technicians and/or dentists to evaluate the quality of recently fabricated custom-made dental devices (e.g. crowns). Detection of any problems at this stage will allow for proper correction before the final prosthesis insertion in the patient's mouth. A recent study found that implant retaining screws deteriorate over a period of time when they are used to hold a dental prosthesis in a patient's mouth (Al Jabbari et. al, 2007a). Therefore, the authors encouraged dentists who provide extensive implant treatment in their practice to equip their offices with a stereomicroscope to enable regular evaluation of the quality of the tiny retaining implant screws at follow-up appointments. A low power stereomicroscope was found to be a powerful and useful tool for evaluating the quality of the external structures/surfaces of tiny dental devices (e.g. prosthetic retaining screws) (Al Jabbari et. al, 2007a). As illustrated in Figure 2, the stereomicroscope was able to clearly reveal the damage and deterioration of an implant screw head and threads, whereas it is almost impossible for a dentist to observe such damage and deterioration under the naked eye. The great advantage of performing this type of NDT in dental offices is that it will allow dentists to replace any severely damaged or deteriorated retaining screws with new ones. Failure to detect such deterioration and damage may lead in the future to a more

complicated and expensive dental treatment for patients (Al Jabbari et. al, 2007b).

Fig. 2. Threaded segment (a) and slotted head segment (b) & (c) of new and retrieved implant retaining screws examined under stereomicroscope. Examination of these tiny dental implant devices reveals significant threads deterioration/thinning "black arrows" and screw head damage "(c)" when compared to the intact threads profile "red arrows" and

(b)

(c)

applications of stereoscopic and X-ray examinations and testing.

**2.1 Stereomicroscopic examination and testing** 

(a)

normal slotted head (b).

Many dental devices (such as crowns, and fixed and removable partial dentures) are traditionally manufactured by casting dental alloys. Recently, the preparation of metal-free crowns and bridges has introduced the concept of casting ceramic materials. However, the mechanical stability and biocompatibility of dental devices depend on the properties of the materials and on the accuracy of the manufacturing process. Unfortunately, the dental casting procedure unavoidably leads to the development of pores in dental cast frameworks owing to gas entrapment or shrinkage, which may adversely affect the quality and efficacy of dental devices. The development of undesirable porosity is dependent on various factors and is a common complication for precious, semiprecious and base contemporary dental alloys (Elliopoulos et al. 2004; Li et al. 2010; Neto et al. 2003; Ucar et al. 2011; Zinelis 2000). This may negatively affect the long-term mechanical stability. For example, when there are cast external porosities, corrosion resistance decreases because of crevice formation and plaque accumulation in the oral cavity.

In industrial applications, internal voids can be readily investigated by employing X-ray examination, and the same technique has been adopted in dental practice as a nondestructive method for the same purposes. Pores can be easily distinguished as dark regions on radiographs, thereby providing significant information for the size, location and distribution of imperfections. The same methodology can be readily applied to identified internal voids in dental cast frameworks (Dharmar et al. 1993; Eisenburger et al. 2002; Eisenburger et al. 1998; Elarbi et al. 1985; Mattila 1964; Wictorin et al. 1979). However, the visibility of voids and the picture quality depend on the combination of the material to be tested and the analytical conditions applied for X-ray testing (Eisenburger & Addy 2002).

Going back to the principle of this technique, it must be noted that the attenuation of a narrow beam of monoenergetic photons with specific E and intensity Io passing through a homogeneous material of thickness t is determined by Lambert's low:

$$\mathbf{I} = \mathbf{I}\_o \cdot \mathbf{e} \star \mu (o, \mathbf{z}\_r \cdot \mathbf{E})^\* \mathbf{t} \tag{1}$$

where I is the photon intensity that goes out to the sample, and μ is the linear attenuation coefficient that depends on the material density, atomic number Z and beam energy. Therefore, the X-ray absorption depends on the atomic number, the density of the material and the energy of radiation. Accordingly, different dental alloys can be penetrated by X-rays to different extents during testing. As stated above, various precious, semiprecious and base

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 59

when cpTi and Ti alloys are cast (Elliopoulos et al. 2004; Zinelis 2000). Figures 4c and 4d show spherical pore formation on a cast framework for a removable partial denture located critically at the shoulder of a circlet clasp, which is considered to be a common region for

Fig. 3. Mass attenuation coefficient of three dental alloys (Eisenburger & Tschernitschek 1998). The same values for pure Au, Ni and Ti are presented for comparison purposes

The main advantage of radiographic X-ray testing is the ability to provide quick valuable information about the quality of the metal framework casting by revealing the locations, numbers and sizes of internal defects that are not possible to detect by visual inspections. The occurrence of internal defects/porosity at critical regions such as the clasp shoulders of cast removable partial denture frameworks will lead to premature clinical failures. Therefore, early detection will allow proper correction of the problem prior to final delivery

Previous studies have reported that fractures and premature failures of removable partial denture frameworks range from 16% to 19% owing to the occurrence of internal porosity in cast Co-Cr frameworks (Dharmar et al. 1993; Elarbi et al. 1985; Wictorin et al. 1979). In addition, they revealed that the frequencies of the common locations of these defects were 22% to 73% at major connectors, 5% to 43% at clasps/clasp shoulders and 6% to 8% at minor connectors (Dharmar et al. 1993; Elarbi et al. 1985). Therefore, and because of the high rates of occurrence of such defects, dentists and/or dental technicians are encouraged to perform radiographic X-ray inspections at early stages of removable partial denture fabrication.

such pore formation.

(Hubbell et al. 1996).

of a prosthetic device to a patient.

metal alloys are used in the dental field that have different elemental, mechanical and physical properties (Roberts et al. 2009; Wataha 2002). Table 1 shows some representative types of dental alloys with densities ranging from 4.51 g/cm3 for Ti to 19.3 g/cm3 for pure Au. The vast majority of dental frameworks are produced by casting of precious and base metal alloys (Roberts et al. 2009) with the exception of pure gold for use in dentistry, which employs an electroforming technique (Vence 1997). As can be expected from equation (1) and the density values of the base metal alloys in Table 1, lower levels of X-ray absorption facilitate X-ray penetration. Low absorption coefficients and a high energy beam are needed to penetrate the thick metallic parts of cast dental frameworks. Figure 3 demonstrates the attenuation coefficients of some of the dental alloys presented in Table 1. As can be seen in Figure 3, the absorption coefficients decrease with increasing beam energy. This is not the case for pure Au and Au-based dental alloys where the absorption coefficient rises at the energy of 80 kV because the K absorption edge limits the penetration of Au and Au-based dental alloys to 0.6 mm (Eisenburger & Tschernitschek 1998). Non-precious and base dental alloys such as Ni-Cr, Co-Cr and commercially pure Ti (cpTi) can be penetrated by up to several millimeters depending on the accelerating voltage and exposure time (Eisenburger & Addy 2002; Eisenburger & Tschernitschek 1998).


Table 1. Brand names, elemental compositions, alloy types and densities of some representative dental alloys used for the production of metalloceramic crowns and bridges. The alloys are sorted in descending order of the density. The attenuation coefficients of the alloys indicated by asterisks are shown in Figure 3.

As examples, Figure 4 shows dental devices casted from grade II cpTi and analyzed by Xrays at 70-kV tube voltage, 8-mA beam current and 0.32-s exposure time. In Figure 3a, it can be seen that large pores are located at the connector areas of a cast framework for a fixed partial denture. Similar sized pores can be seen in cast rectangular specimens used for the determination of metalloceramic bonding strength values (Figure 4b). The spherical shape of these pores is a strong indication of gas entrapment, which is a typically reported problem

metal alloys are used in the dental field that have different elemental, mechanical and physical properties (Roberts et al. 2009; Wataha 2002). Table 1 shows some representative types of dental alloys with densities ranging from 4.51 g/cm3 for Ti to 19.3 g/cm3 for pure Au. The vast majority of dental frameworks are produced by casting of precious and base metal alloys (Roberts et al. 2009) with the exception of pure gold for use in dentistry, which employs an electroforming technique (Vence 1997). As can be expected from equation (1) and the density values of the base metal alloys in Table 1, lower levels of X-ray absorption facilitate X-ray penetration. Low absorption coefficients and a high energy beam are needed to penetrate the thick metallic parts of cast dental frameworks. Figure 3 demonstrates the attenuation coefficients of some of the dental alloys presented in Table 1. As can be seen in Figure 3, the absorption coefficients decrease with increasing beam energy. This is not the case for pure Au and Au-based dental alloys where the absorption coefficient rises at the energy of 80 kV because the K absorption edge limits the penetration of Au and Au-based dental alloys to 0.6 mm (Eisenburger & Tschernitschek 1998). Non-precious and base dental alloys such as Ni-Cr, Co-Cr and commercially pure Ti (cpTi) can be penetrated by up to several millimeters depending on the accelerating voltage and exposure time (Eisenburger

Brand name Mass content (%) Type Density

Ir<1.0, Fe<1.0, Mn<1.0, Ta<1.0, High Au 18.9

Sn:0.8,In:1.5,Ir:0.1,Re:0.2,Ta:0.2. High Au 17.9

Pd:2.0, Zn:1.2, Ir: 0.1 High Au 15.5

Ru: 0.5; Ga: 6.0; Ge: 0.5 Pd based 11.7

Si:1.5, Mn<1.0, Al<1.0 Ni-Cr 8.4

cp Ti

(gradeII) 4.5

Si:0.9, Mo:0.6, Fe:0.5, B:0.3, Li<1.0 Co-Cr 7.8

Electroforming\* Au: 99.9 Pure Au 19.3

C<0.08, Η< 0.013, Fe < 0.25

representative dental alloys used for the production of metalloceramic crowns and bridges. The alloys are sorted in descending order of the density. The attenuation coefficients of the

As examples, Figure 4 shows dental devices casted from grade II cpTi and analyzed by Xrays at 70-kV tube voltage, 8-mA beam current and 0.32-s exposure time. In Figure 3a, it can be seen that large pores are located at the connector areas of a cast framework for a fixed partial denture. Similar sized pores can be seen in cast rectangular specimens used for the determination of metalloceramic bonding strength values (Figure 4b). The spherical shape of these pores is a strong indication of gas entrapment, which is a typically reported problem

Table 1. Brand names, elemental compositions, alloy types and densities of some

(gr/cm3)

& Addy 2002; Eisenburger & Tschernitschek 1998).

IPS d.SIGN 98 Au:85.9, Pt:12.1, Zn:1.5, In<1.0

Degudent U94\* Au:76.0,Pt:9.6,Pd:8.9,Ag:1.2,Cu:0.3

Degulor M\* Au:70.0,Ag:13.5,Au:8.8,Pt:4.,

Degupal G\* Pd:77.3, Ag:7.2, Au:4.5, Sn:4.0,

4 all Ni:61.5, Cr:25.7, Mo:11.0,

Ti\* Ti:balance,Ο< 0.2,Ν< 0.06,

IPS d.SIGN 30 Co:60.2,Cr:30.1,Ga:3.9,Nb:3.2

alloys indicated by asterisks are shown in Figure 3.

when cpTi and Ti alloys are cast (Elliopoulos et al. 2004; Zinelis 2000). Figures 4c and 4d show spherical pore formation on a cast framework for a removable partial denture located critically at the shoulder of a circlet clasp, which is considered to be a common region for such pore formation.

Fig. 3. Mass attenuation coefficient of three dental alloys (Eisenburger & Tschernitschek 1998). The same values for pure Au, Ni and Ti are presented for comparison purposes (Hubbell et al. 1996).

The main advantage of radiographic X-ray testing is the ability to provide quick valuable information about the quality of the metal framework casting by revealing the locations, numbers and sizes of internal defects that are not possible to detect by visual inspections. The occurrence of internal defects/porosity at critical regions such as the clasp shoulders of cast removable partial denture frameworks will lead to premature clinical failures. Therefore, early detection will allow proper correction of the problem prior to final delivery of a prosthetic device to a patient.

Previous studies have reported that fractures and premature failures of removable partial denture frameworks range from 16% to 19% owing to the occurrence of internal porosity in cast Co-Cr frameworks (Dharmar et al. 1993; Elarbi et al. 1985; Wictorin et al. 1979). In addition, they revealed that the frequencies of the common locations of these defects were 22% to 73% at major connectors, 5% to 43% at clasps/clasp shoulders and 6% to 8% at minor connectors (Dharmar et al. 1993; Elarbi et al. 1985). Therefore, and because of the high rates of occurrence of such defects, dentists and/or dental technicians are encouraged to perform radiographic X-ray inspections at early stages of removable partial denture fabrication.

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 61

**3. Applications of NDT for research purposes in the dental biomaterials field**  Besides the valuable applications of NDT in quality assurance of dental devices, NDT makes significant contributions and has important applications in the dental biomaterials research field. Normally, dental biomaterials in the oral cavity are exposed to various aggressive conditions. When biomaterials are placed intraorally in the form of filling materials or prosthetic appliances, they go through a variety of degradation mechanisms such as fatigue, wear, corrosion and discoloration. Therefore, studies of the degradation mechanisms of biomaterials are important toward the design of new biomaterials with increased efficacy

Laboratory or *in vitro* testing is widely applied to dental biomaterials to determine the properties of the materials. However, *in vitro* testing cannot provide any reliable information that can predict the *in vivo* behavior of biomaterials because the conditions of the oral environment cannot be simulated experimentally in research laboratories. Accordingly, NDT can be effectively used over a long period of time to monitor the occurrence of changes in a specific dental biomaterial or device resulting from intraoral aging. Generally, NDT of a retrieved specific dental biomaterial or device that has been placed in a patient's mouth for a reasonable time and comparison with the properties of a new (unused) biomaterial/device will provide more useful and significant information about the degradation mechanism resulting from long-term use *in vivo*. Under this general concept, a variety of NDT methods and techniques have been developed such as micro-XCT, VPSEM-EDS, optical profilometry

The main two limiting factors in conducting such research protocols are ethical and cost reasons. For example, a permanently placed filling dental biomaterial cannot be retrieved from the mouth for only research purposes. Retrieval of a biomaterial and replacement with a new one will subject the tooth to additional unnecessary clinical procedures. Multiple clinical procedures will cause repeated insults to a specific tooth that may lead to pulpal irritation and/or necrosis. Needless to say, that is ethically unacceptable. Similarly, a successful dental appliance or device cannot be retrieved from the mouth solely to conduct NDT before it fails, since making a new one is costly for the patient and there is no

There are two additional obstacles that may also make the performance of NDT in retrieved dental biomaterials infeasible. First, it requires frequent patient follow-ups before the biomaterials and/or devices are retrieved. Unfortunately, not all patients will accept many follow-ups for research purposes only. Second, many retrieved dental biomaterials and/or devices are unsuitable for the conduction of certain types of NDT. For example, XRD analysis requires flat surfaces of a few square millimeters in dimension, a requirement that

Currently, computed tomography is extensively used in the medical field for diagnostic/treatment purposes. During the last two decades, new bench-top models have been introduced for use in the characterization of materials that employ similar principles to medical computed tomography. The only difference is that the bench-top models have an isotopic resolution capable of reaching a few tens of nanometers. Contrary to medical

guarantee that the newly fabricated device will be as successful as the first one.

and longevity.

and X-ray diffraction (XRD).

is hard to fulfill in dental devices.

**3.1 Micro-XCT** 

Early detection of these casting defects and imperfections may allow easy repair or simple remaking of the metal framework. On the other hand, late detection may mandate remaking of the entire removable prosthesis, which is considered to be a costly choice to be performed by dentists, to avoid intraoral premature failures of a removable prosthesis.

Fig. 4. X-ray images of dental casting. a) A dental cast framework for fixed partial denture, b) cast specimens for testing metalo-ceramic bond strength c) a cast framework for removable partial denture and d) High magnification of the highlighted region in (c) and it shows internal porosity occurrence (small arrow) at the clasp shoulder.

Another valuable dental application of radiographic nondestructive X-ray testing is in retrieval analysis studies. Conventional dental X-ray units can be readily used for X-ray analysis of dental alloys, except for pure Au and Au-based alloys where the penetration depth is limited to approximately 0.6 mm and thus increasing acceleration voltage to 120 kV is not possible with conventional dental X-ray machines. Microfocus X-ray systems (CRX 1000/CRX 2000; CR Technology Inc., Aliso Veijio, CA, USA) have been used to detect the occurrence of internal defects in prosthetic retaining implant screws made of gold-based alloys (Al Jabbari et al, 2007a). For NDT, utilization of a microfocus X-ray machine is possible and very valuable for evaluating various tiny dental devices made from precious, semiprecious and non-precious alloys. However, this type of machine is known to be mainly useful for dental biomaterials research purposes.

### **3. Applications of NDT for research purposes in the dental biomaterials field**

Besides the valuable applications of NDT in quality assurance of dental devices, NDT makes significant contributions and has important applications in the dental biomaterials research field. Normally, dental biomaterials in the oral cavity are exposed to various aggressive conditions. When biomaterials are placed intraorally in the form of filling materials or prosthetic appliances, they go through a variety of degradation mechanisms such as fatigue, wear, corrosion and discoloration. Therefore, studies of the degradation mechanisms of biomaterials are important toward the design of new biomaterials with increased efficacy and longevity.

Laboratory or *in vitro* testing is widely applied to dental biomaterials to determine the properties of the materials. However, *in vitro* testing cannot provide any reliable information that can predict the *in vivo* behavior of biomaterials because the conditions of the oral environment cannot be simulated experimentally in research laboratories. Accordingly, NDT can be effectively used over a long period of time to monitor the occurrence of changes in a specific dental biomaterial or device resulting from intraoral aging. Generally, NDT of a retrieved specific dental biomaterial or device that has been placed in a patient's mouth for a reasonable time and comparison with the properties of a new (unused) biomaterial/device will provide more useful and significant information about the degradation mechanism resulting from long-term use *in vivo*. Under this general concept, a variety of NDT methods and techniques have been developed such as micro-XCT, VPSEM-EDS, optical profilometry and X-ray diffraction (XRD).

The main two limiting factors in conducting such research protocols are ethical and cost reasons. For example, a permanently placed filling dental biomaterial cannot be retrieved from the mouth for only research purposes. Retrieval of a biomaterial and replacement with a new one will subject the tooth to additional unnecessary clinical procedures. Multiple clinical procedures will cause repeated insults to a specific tooth that may lead to pulpal irritation and/or necrosis. Needless to say, that is ethically unacceptable. Similarly, a successful dental appliance or device cannot be retrieved from the mouth solely to conduct NDT before it fails, since making a new one is costly for the patient and there is no guarantee that the newly fabricated device will be as successful as the first one.

There are two additional obstacles that may also make the performance of NDT in retrieved dental biomaterials infeasible. First, it requires frequent patient follow-ups before the biomaterials and/or devices are retrieved. Unfortunately, not all patients will accept many follow-ups for research purposes only. Second, many retrieved dental biomaterials and/or devices are unsuitable for the conduction of certain types of NDT. For example, XRD analysis requires flat surfaces of a few square millimeters in dimension, a requirement that is hard to fulfill in dental devices.

### **3.1 Micro-XCT**

60 Nondestructive Testing Methods and New Applications

Early detection of these casting defects and imperfections may allow easy repair or simple remaking of the metal framework. On the other hand, late detection may mandate remaking of the entire removable prosthesis, which is considered to be a costly choice to be performed

Fig. 4. X-ray images of dental casting. a) A dental cast framework for fixed partial denture,

removable partial denture and d) High magnification of the highlighted region in (c) and it

Another valuable dental application of radiographic nondestructive X-ray testing is in retrieval analysis studies. Conventional dental X-ray units can be readily used for X-ray analysis of dental alloys, except for pure Au and Au-based alloys where the penetration depth is limited to approximately 0.6 mm and thus increasing acceleration voltage to 120 kV is not possible with conventional dental X-ray machines. Microfocus X-ray systems (CRX 1000/CRX 2000; CR Technology Inc., Aliso Veijio, CA, USA) have been used to detect the occurrence of internal defects in prosthetic retaining implant screws made of gold-based alloys (Al Jabbari et al, 2007a). For NDT, utilization of a microfocus X-ray machine is possible and very valuable for evaluating various tiny dental devices made from precious, semiprecious and non-precious alloys. However, this type of machine is known to be mainly

b) cast specimens for testing metalo-ceramic bond strength c) a cast framework for

shows internal porosity occurrence (small arrow) at the clasp shoulder.

useful for dental biomaterials research purposes.

by dentists, to avoid intraoral premature failures of a removable prosthesis.

Currently, computed tomography is extensively used in the medical field for diagnostic/treatment purposes. During the last two decades, new bench-top models have been introduced for use in the characterization of materials that employ similar principles to medical computed tomography. The only difference is that the bench-top models have an isotopic resolution capable of reaching a few tens of nanometers. Contrary to medical

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 63

Fig. 5. Micro XCT analysis of a fixed partial denture (FPD) that was fabricated from all ceramic materials. The core of the FPD was made from alumina that was veneered with dental porcelain. **a)** FPD X-ray image before the FPD was cemented in the patient's mouth showing (A) the core alumina, (B) the veneering porcelain and (C) the connector area joining the three unites of the FPD. It is easy to distinguish between the three parts because the differences in X-ray absorption. **b)** A pseudocoloring reconstruction of a perpendicular cross section for the FPD showing the alumina core, the veneering porcelain and the material used for cementation at the connector area. Pores are easily identified as white circle areas in the regions of porcelain layer and at the core porcelain interface. Red line located on the small attached image indicates the cross sectional plane while the bar indicates the absorption scale. **c)** 3D image of the reconstructed structure after digital processing helpful in total volume determination. **d)** Alternative 3D image showing the distribution of internal pores in

dental porcelain and the occurrence of big voids near and within the cement layer.

computed tomography machines, the specimens tested with bench-top models can be rotated while the detector and X-ray source are fixed within the machine. The micro-XCT scanning produces hundreds of horizontal slices for a tested specimen, which are then used to reconstruct the entire specimen. Reconstruction of a specimen is accomplished by two reconstruction algorithms commonly known as iterative and filtered back projection methods. In addition, computer software can be utilized in the development of threedimensional models, pseudocoloring and quantitative determination of geometrical features of an irregular dental biomaterial device.

Figure 5 shows a good example of NDT utilizing a micro-XCT analysis of a fixed partial denture (FPD). It reveals the importance of this tool for nondestructively analyzing the internal structure of the whole ceramic FPD. The FPD was analyzed prior to permanent cementation of the prosthesis in the patient's mouth. The analysis revealed that the joining procedure of the three different parts of the alumina core was not done properly because of the entrapment of large voids at and within the bulk of the cementing material (Figure 5d). Unfortunately, after final insertion of the FPD in the patient's mouth, it did fail at the connector area after being in service for a short period of time. Therefore, it can be said that micro-XCT is a powerful tool for evaluating the quality of industrially and/or custom-made dental devices and for failure analysis of dental biomaterials.

### **3.2 SEM-VPSEM-EPMA**

Scanning electron microscopy (SEM) combined with electron probe microanalysis (EPMA) is considered to be a powerful analytical tool for providing morphological and elemental information about tested samples at low (6×) and high (150,000×) magnifications. SEM is able to bridge the gap between optical stereomicroscopy and transmission electron microscopy. Recent advancements in SEM manufacturing technology can provide imaging of non-conductive specimens (low-vacuum SEM) and samples at 99% relative humidity (environmental SEM). These new operating modes are also known as VPSEM and confer tremendous capabilities to SEM. Additional information regarding the operation principles and applications of these new SEM models can be found in relevant previous reports (Bergmans et al. 2005; Danilatos; Danilatos 1991; Danilatos 1993; Danilatos 1994; Danilatos et al.; Kodaka et al. 1992).

In dentistry, brazing is the main joining technique for making metallic orthodontic appliances. A space maintainer is an example of the most commonly used orthodontic appliances. This appliance is made of two stainless steel tooth bands joined by a stainless steel orthodontic wire. The orthodontic wire and the two bands are joined by brazing utilizing low fusing silver brazing alloys (Figure 6). Figure 7 shows a high magnification SEM photomicrograph of an orthodontic space maintainer appliance, revealing that the soldered area joins the stainless steel bands to the orthodontic wire. NDT utilizing X-ray EDS analysis was performed at that area at two different times (before and after dental treatment). The purpose of the NDT and the analysis was to determine the effects of longterm use *in vivo* on the Ag-based brazing alloy. Small porosities (indicated by arrows in Figure 7) were used to identify the area for X-ray EDS analysis. The two spectra obtained at the two different times are shown in Figure 8, and reveal significant decreases in the Cu and Zn composition after intraoral aging.

computed tomography machines, the specimens tested with bench-top models can be rotated while the detector and X-ray source are fixed within the machine. The micro-XCT scanning produces hundreds of horizontal slices for a tested specimen, which are then used to reconstruct the entire specimen. Reconstruction of a specimen is accomplished by two reconstruction algorithms commonly known as iterative and filtered back projection methods. In addition, computer software can be utilized in the development of threedimensional models, pseudocoloring and quantitative determination of geometrical features

Figure 5 shows a good example of NDT utilizing a micro-XCT analysis of a fixed partial denture (FPD). It reveals the importance of this tool for nondestructively analyzing the internal structure of the whole ceramic FPD. The FPD was analyzed prior to permanent cementation of the prosthesis in the patient's mouth. The analysis revealed that the joining procedure of the three different parts of the alumina core was not done properly because of the entrapment of large voids at and within the bulk of the cementing material (Figure 5d). Unfortunately, after final insertion of the FPD in the patient's mouth, it did fail at the connector area after being in service for a short period of time. Therefore, it can be said that micro-XCT is a powerful tool for evaluating the quality of industrially and/or custom-made

Scanning electron microscopy (SEM) combined with electron probe microanalysis (EPMA) is considered to be a powerful analytical tool for providing morphological and elemental information about tested samples at low (6×) and high (150,000×) magnifications. SEM is able to bridge the gap between optical stereomicroscopy and transmission electron microscopy. Recent advancements in SEM manufacturing technology can provide imaging of non-conductive specimens (low-vacuum SEM) and samples at 99% relative humidity (environmental SEM). These new operating modes are also known as VPSEM and confer tremendous capabilities to SEM. Additional information regarding the operation principles and applications of these new SEM models can be found in relevant previous reports (Bergmans et al. 2005; Danilatos; Danilatos 1991; Danilatos 1993; Danilatos 1994; Danilatos et

In dentistry, brazing is the main joining technique for making metallic orthodontic appliances. A space maintainer is an example of the most commonly used orthodontic appliances. This appliance is made of two stainless steel tooth bands joined by a stainless steel orthodontic wire. The orthodontic wire and the two bands are joined by brazing utilizing low fusing silver brazing alloys (Figure 6). Figure 7 shows a high magnification SEM photomicrograph of an orthodontic space maintainer appliance, revealing that the soldered area joins the stainless steel bands to the orthodontic wire. NDT utilizing X-ray EDS analysis was performed at that area at two different times (before and after dental treatment). The purpose of the NDT and the analysis was to determine the effects of longterm use *in vivo* on the Ag-based brazing alloy. Small porosities (indicated by arrows in Figure 7) were used to identify the area for X-ray EDS analysis. The two spectra obtained at the two different times are shown in Figure 8, and reveal significant decreases in the Cu and

of an irregular dental biomaterial device.

**3.2 SEM-VPSEM-EPMA** 

al.; Kodaka et al. 1992).

Zn composition after intraoral aging.

dental devices and for failure analysis of dental biomaterials.

Fig. 5. Micro XCT analysis of a fixed partial denture (FPD) that was fabricated from all ceramic materials. The core of the FPD was made from alumina that was veneered with dental porcelain. **a)** FPD X-ray image before the FPD was cemented in the patient's mouth showing (A) the core alumina, (B) the veneering porcelain and (C) the connector area joining the three unites of the FPD. It is easy to distinguish between the three parts because the differences in X-ray absorption. **b)** A pseudocoloring reconstruction of a perpendicular cross section for the FPD showing the alumina core, the veneering porcelain and the material used for cementation at the connector area. Pores are easily identified as white circle areas in the regions of porcelain layer and at the core porcelain interface. Red line located on the small attached image indicates the cross sectional plane while the bar indicates the absorption scale. **c)** 3D image of the reconstructed structure after digital processing helpful in total volume determination. **d)** Alternative 3D image showing the distribution of internal pores in dental porcelain and the occurrence of big voids near and within the cement layer.

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 65

Fig. 8. EDS x-ray spectrum for the Ag-based alloy solder before and after intraoral aging.

Figures 8 and 9 provide additional information regarding the surface structure of the Agbased soldering alloy. Both figures confirm that the significant decreases in Cu and Zn are appended to the dissolution of the low atomic contrast second phase, which is enriched in Cu and Zn. A possible explanation is that dental biomaterials with multiple phase structures might be prone to galvanic corrosion. Of course, the presence of the stainless steel bands and wire might have an additional effect on this phenomenon. However, this is only an assumption and the verification of galvanic corrosion requires further extended research. The important contribution of this NDT and X-ray EDS analysis is significant because it confirms other previous findings that Ag-based alloys are prone to corrosion and ionic release (Grimsdottir et al. 1992; Locci et al. 2000a; Mockers et al. 2002; Staffolani et al. 1999). These findings might be a reason for mucosal irritation, which was reported in a previous study (Bishara 1995). The significant release and dissolution of Cu and Zn during intraoral aging must be taken seriously because Cu ions have toxic effects on the human body (Locci

Note the decrease in Cu, Zn after long-term use *in vivo*.

et al. 2000b; Vannet et al. 2007; Wataha et al. 2002).

**4. Dental applications of NDT as non-invasive diagnostic methods** 

measurements (Jablonski-Momeni et al. 2011; Lussi et al. 2003; Rodrigues et al. 2010).

Radiographic X-rays are used routinely in dental offices to non-invasively diagnose hard dental tissue diseases or to detect dental caries (tooth decay). However, in recent years, new technologies have been developed and introduced into the dental field for use as non-invasive diagnostic tools. The best two examples are RFA (Meredith et al. 1997) and fluorescence

Fig. 6. An orthodontic device known as space maintainer made of two bands and a wire with two soldered joints (arrows). Bands and wires were manufactured from stainless steel whereas the soldering alloy is Ag-based alloy containing Cu, Zn and Sn.

Fig. 7. Secondary Electron Images (SEI) of a joint area between the stainless steel band (A) and the orthodontic wire (C) soldered with Ag-based soldering alloy (B). (a) As-received appliance from the dental laboratory and before it was placed in the patient's mouth. (b) The retrieved appliance from the patient's mouth after it serviced for approximately 14 months of treatment period. External surface porosities (arrows) were used as a reference for locating exact area used for x-ray EDS analysis.

Fig. 6. An orthodontic device known as space maintainer made of two bands and a wire with two soldered joints (arrows). Bands and wires were manufactured from stainless steel

Fig. 7. Secondary Electron Images (SEI) of a joint area between the stainless steel band (A) and the orthodontic wire (C) soldered with Ag-based soldering alloy (B). (a) As-received appliance from the dental laboratory and before it was placed in the patient's mouth. (b) The retrieved appliance from the patient's mouth after it serviced for approximately 14 months of treatment period. External surface porosities (arrows) were used as a reference for

locating exact area used for x-ray EDS analysis.

whereas the soldering alloy is Ag-based alloy containing Cu, Zn and Sn.

Fig. 8. EDS x-ray spectrum for the Ag-based alloy solder before and after intraoral aging. Note the decrease in Cu, Zn after long-term use *in vivo*.

Figures 8 and 9 provide additional information regarding the surface structure of the Agbased soldering alloy. Both figures confirm that the significant decreases in Cu and Zn are appended to the dissolution of the low atomic contrast second phase, which is enriched in Cu and Zn. A possible explanation is that dental biomaterials with multiple phase structures might be prone to galvanic corrosion. Of course, the presence of the stainless steel bands and wire might have an additional effect on this phenomenon. However, this is only an assumption and the verification of galvanic corrosion requires further extended research. The important contribution of this NDT and X-ray EDS analysis is significant because it confirms other previous findings that Ag-based alloys are prone to corrosion and ionic release (Grimsdottir et al. 1992; Locci et al. 2000a; Mockers et al. 2002; Staffolani et al. 1999). These findings might be a reason for mucosal irritation, which was reported in a previous study (Bishara 1995). The significant release and dissolution of Cu and Zn during intraoral aging must be taken seriously because Cu ions have toxic effects on the human body (Locci et al. 2000b; Vannet et al. 2007; Wataha et al. 2002).

### **4. Dental applications of NDT as non-invasive diagnostic methods**

Radiographic X-rays are used routinely in dental offices to non-invasively diagnose hard dental tissue diseases or to detect dental caries (tooth decay). However, in recent years, new technologies have been developed and introduced into the dental field for use as non-invasive diagnostic tools. The best two examples are RFA (Meredith et al. 1997) and fluorescence measurements (Jablonski-Momeni et al. 2011; Lussi et al. 2003; Rodrigues et al. 2010).

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 67

(a) (b)

by positioning the transducer in different directions.

**4.2 Fluorescence measurements** 

(Figure 10a) (Lussi & Hellwig 2006b).

Fig. 10. (a) The Osstell device utilized normally for RFA. (b) Illustrating conduction of RFA in an animal study. It is important that the transducer be placed in same position each time a measurement is taken for a specific dental implant. Different ISQ values could be obtained

Dental caries are also known as tooth decay and result from demineralization of inorganic components of the outer layer (enamel) of the tooth structure. Released bacterial lactic acid will normally lead to tooth enamel demineralization. Detection of tooth caries at early stages is crucial because it only requires a non-costly simple treatment. However, the early stages of dental caries may not be easily detected by the naked eye during routine dental examinations. Therefore, fluorescence measurements have been recommended for early detection and diagnosis of enamel demineralization (Jablonski-Momeni et al. 2011; Lussi et al. 2003; Rodrigues et al. 2010). The structure of healthy and sound tooth enamel is characterized by a low baseline fluorescence level, while demineralized and infected enamel will have an increased fluorescence level. In addition, the fluorescence level increases as the caries process advances (Lussi et al. 2001). A recently developed device for detecting dental caries at its early stages based on fluorescence measurements is the DIAGNOdent device (KaVo, Biberach, Germany). The DIAGNOdent device emits red light at 655 nm and detects bacterial metabolites in the demineralized tooth structure (Lussi et al. 2003; Lussi et al. 2006a; Lussi et al. 2006b). The DIAGNOdent device then classifies the tested regions according to the calibrated fluorescence intensity as follows: scores 0–13 = no caries, scores 14–20 = early (incipient) enamel caries and scores above 20 = advanced dentine caries

Another recently developed fluorescence camera, VistaProof (Dürr Dental, Bietigheim-Bissingen, Germany), is used for early diagnosis of dental caries. The VistaProof emits blue light at 405 nm (Jablonski-Momeni et al. 2011) and records fluorescence from the probed tooth surfaces in the form of digital images (Rodrigues et al. 2008). Intact tooth structures show green fluorescent images, while infected and demineralized tooth structures show

Fig. 9. Backscattered Electron Images after etching of Ag-based alloy solder before (a) and after (b) intraoral aging. The Ag based alloy has two different microstructure phases with dispersion of lower atomic contrast phase in the matrix. This second phase has been completely diluted leaving behind a random distribution of surface craters. (Original magnification: 1000X).

### **4.1 RFA**

The main dental application of RFA as a diagnostic method is to quantify dental implant stability after surgical implant placement in the human jaw bone. Normally, when dental implants are placed in healthy human jaws, they will form and establish strong stable bond with the surrounding bone tissues after a period of several months and this phenomenon is known as *Osseointegration* (Albrektsson et al. 1986). It has been suggested in several studies that the stiffness of the bone–implant interface can be assessed by RFA (Valderrama et al. 2007; Sakoh J et al. 2006; Alsaadi G et al. 2007). Therefore, clinicians have been advised to utilize RFA to determine the strength and adequacy of the established bond (Osseointegration) before they restore implants with dental prostheses. A commonly used device for this purpose is the Osstell Mentor device (Integration Diagnostics, Goteborg, Sweden) (Figure 10a). As shown in Figure 10, part of the Osstell Mentor device comprises Lshaped transducers. These transducers will record all the information as an implant stability quotient (ISQ), which is a function of the bone–implant stiffness (N/µm) and the marginal bone height. The ISQ is a dimensionless quantity, and larger values indicate greater levels of interfacial bone–implant stiffness (meaning a higher established osseointegration with greater stability).

Besides the aforementioned beneficial diagnostic applications of RFA, it has been utilized extensively in research studies. Traditionally, research studies have evaluated the established bond between an implant and bone by histomorphometric tests. However, the main disadvantage of these tests is that they are destructive (Meredith N. 1998). Therefore, it has been suggested that RFA can be used periodically to evaluate the established bond (osseointegration occurrence) between an implant and bone without sacrificing the object in an *in vivo* study (Figure 10b) (Meredith N. 1998; Huang HM. et al. 2003).

Fig. 10. (a) The Osstell device utilized normally for RFA. (b) Illustrating conduction of RFA in an animal study. It is important that the transducer be placed in same position each time a measurement is taken for a specific dental implant. Different ISQ values could be obtained by positioning the transducer in different directions.

### **4.2 Fluorescence measurements**

66 Nondestructive Testing Methods and New Applications

Fig. 9. Backscattered Electron Images after etching of Ag-based alloy solder before (a) and after (b) intraoral aging. The Ag based alloy has two different microstructure phases with dispersion of lower atomic contrast phase in the matrix. This second phase has been completely diluted leaving behind a random distribution of surface craters. (Original

The main dental application of RFA as a diagnostic method is to quantify dental implant stability after surgical implant placement in the human jaw bone. Normally, when dental implants are placed in healthy human jaws, they will form and establish strong stable bond with the surrounding bone tissues after a period of several months and this phenomenon is known as *Osseointegration* (Albrektsson et al. 1986). It has been suggested in several studies that the stiffness of the bone–implant interface can be assessed by RFA (Valderrama et al. 2007; Sakoh J et al. 2006; Alsaadi G et al. 2007). Therefore, clinicians have been advised to utilize RFA to determine the strength and adequacy of the established bond (Osseointegration) before they restore implants with dental prostheses. A commonly used device for this purpose is the Osstell Mentor device (Integration Diagnostics, Goteborg, Sweden) (Figure 10a). As shown in Figure 10, part of the Osstell Mentor device comprises Lshaped transducers. These transducers will record all the information as an implant stability quotient (ISQ), which is a function of the bone–implant stiffness (N/µm) and the marginal bone height. The ISQ is a dimensionless quantity, and larger values indicate greater levels of interfacial bone–implant stiffness (meaning a higher established osseointegration with

Besides the aforementioned beneficial diagnostic applications of RFA, it has been utilized extensively in research studies. Traditionally, research studies have evaluated the established bond between an implant and bone by histomorphometric tests. However, the main disadvantage of these tests is that they are destructive (Meredith N. 1998). Therefore, it has been suggested that RFA can be used periodically to evaluate the established bond (osseointegration occurrence) between an implant and bone without sacrificing the object in

an *in vivo* study (Figure 10b) (Meredith N. 1998; Huang HM. et al. 2003).

magnification: 1000X).

greater stability).

**4.1 RFA** 

Dental caries are also known as tooth decay and result from demineralization of inorganic components of the outer layer (enamel) of the tooth structure. Released bacterial lactic acid will normally lead to tooth enamel demineralization. Detection of tooth caries at early stages is crucial because it only requires a non-costly simple treatment. However, the early stages of dental caries may not be easily detected by the naked eye during routine dental examinations. Therefore, fluorescence measurements have been recommended for early detection and diagnosis of enamel demineralization (Jablonski-Momeni et al. 2011; Lussi et al. 2003; Rodrigues et al. 2010). The structure of healthy and sound tooth enamel is characterized by a low baseline fluorescence level, while demineralized and infected enamel will have an increased fluorescence level. In addition, the fluorescence level increases as the caries process advances (Lussi et al. 2001). A recently developed device for detecting dental caries at its early stages based on fluorescence measurements is the DIAGNOdent device (KaVo, Biberach, Germany). The DIAGNOdent device emits red light at 655 nm and detects bacterial metabolites in the demineralized tooth structure (Lussi et al. 2003; Lussi et al. 2006a; Lussi et al. 2006b). The DIAGNOdent device then classifies the tested regions according to the calibrated fluorescence intensity as follows: scores 0–13 = no caries, scores 14–20 = early (incipient) enamel caries and scores above 20 = advanced dentine caries (Figure 10a) (Lussi & Hellwig 2006b).

Another recently developed fluorescence camera, VistaProof (Dürr Dental, Bietigheim-Bissingen, Germany), is used for early diagnosis of dental caries. The VistaProof emits blue light at 405 nm (Jablonski-Momeni et al. 2011) and records fluorescence from the probed tooth surfaces in the form of digital images (Rodrigues et al. 2008). Intact tooth structures show green fluorescent images, while infected and demineralized tooth structures show

Applications of Current Technologies for Nondestructive Testing of Dental Biomaterials 69

Al Jabbari YS., Fournelle R., Ziebert G., Toth J., Iacopino AM. (2007a). Mechanical behavior

Al Jabbari YS., Fournelle R., Ziebert G., Toth J., Iacopino AM. (2007b). Mechanical behavior

Alsaadi G., Quirynen M., Michiels K., Jacobs R., van Steenberghe D. (2007). A biomechanical

Bergmans L., Moisiadis P., Van Meerbeek B., Quirynen M., Lambrechts P. (2005).

Contreras E. F. R., Henriques G. E. P., Giolo S. R., Nobilo M. A. A. (2002). Fit of cast

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Dharmar S., Rathnasamy R. J., Swaminathan T. N. (1993). Radiographic and metallographic

Eisenburger M., Addy M. (2002). Radiological examination of dental castings -- a review of

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Elarbi E. A., Ismail Y. H., Azarbal M., Saini T. S. (1985). Radiographic detection of porosities

Elliopoulos D., Zinelis S., Papadopoulos T. (2004). Porosity of cpTi casting with four

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evaluation of porosity defects and grain structure of cast chromium cobalt removable partial dentures. *Journal of Prosthetic Dentistry*, Vol.69, No.4, pp. 369-73,

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**7. References** 

blue-violet fluorescent images. Infected areas with increased numbers of bacteria and bacterial byproducts show red fluorescent images (Figure 11b). The digital software utilized by the VistaProof quantifies the color components and provides scoring outcomes ranging from 0–4 that indicate the penetration depth of the dental caries within the tooth structure (Figure 11b). The scoring outcomes are good diagnostic values for the presence or absence and the severity of dental caries. The values are used as follows: 0–0.9 = sound and healthy tooth structure; 0.9–1.5 = initial (incipient) enamel caries; 1.5–2.0 = deep enamel caries; 2.0– 2.5 = dentine caries; and above 2.5 = deep dentine caries. It is important to mention that, despite the reported reliable applications of the DIAGOdent and VistaProof for noninvasive diagnosis of dental caries, dentists routinely verify their findings by taking radiographic X-rays to diagnose the occurrence of dental caries.

Fig. 11. Occlusal surface of a molar tooth with dental caries. The infected areas were diagnosed by DIAGNOdent (a) and VistaProof (b). Numbers in both images provide valuable information regarding the extent and penetration depth of dental caries within the tooth structure.

### **5. Conclusions**

NDT plays very important roles in dentistry, and in the dental biomaterials research field in particular. However, the application of NDT on a daily basis for dental diagnosis purposes and for assuring adequate therapeutic quality is limited, mainly because of the significant increases in relevant time and cost. Luckily, NDT along with its various applications in the dental biomaterials research field is performed routinely by scientific researchers. Consequently, this NDT has led to noticeable enhancements of the quality, performance and biocompatibility of dental biomaterials that are placed daily into patients' mouths.

### **6. Acknowledgments**

The author s would like to thank Prof. George Eliades, Dr. Triantaffilos Papadopoulos, Dr. Abdulaziz Al-Rasheed, Dr. Anil Sukumaran, Dr. Maysa Al-Marshood, Peter Tsakiridis, Dr. Alerxis Tagmatarxis and Maria Michalaki for their help in providing some of the pictures for this manuscript.

### **7. References**

68 Nondestructive Testing Methods and New Applications

blue-violet fluorescent images. Infected areas with increased numbers of bacteria and bacterial byproducts show red fluorescent images (Figure 11b). The digital software utilized by the VistaProof quantifies the color components and provides scoring outcomes ranging from 0–4 that indicate the penetration depth of the dental caries within the tooth structure (Figure 11b). The scoring outcomes are good diagnostic values for the presence or absence and the severity of dental caries. The values are used as follows: 0–0.9 = sound and healthy tooth structure; 0.9–1.5 = initial (incipient) enamel caries; 1.5–2.0 = deep enamel caries; 2.0– 2.5 = dentine caries; and above 2.5 = deep dentine caries. It is important to mention that, despite the reported reliable applications of the DIAGOdent and VistaProof for noninvasive diagnosis of dental caries, dentists routinely verify their findings by taking

radiographic X-rays to diagnose the occurrence of dental caries.

tooth structure.

**5. Conclusions** 

**6. Acknowledgments** 

this manuscript.

Fig. 11. Occlusal surface of a molar tooth with dental caries. The infected areas were diagnosed by DIAGNOdent (a) and VistaProof (b). Numbers in both images provide valuable information regarding the extent and penetration depth of dental caries within the

biocompatibility of dental biomaterials that are placed daily into patients' mouths.

NDT plays very important roles in dentistry, and in the dental biomaterials research field in particular. However, the application of NDT on a daily basis for dental diagnosis purposes and for assuring adequate therapeutic quality is limited, mainly because of the significant increases in relevant time and cost. Luckily, NDT along with its various applications in the dental biomaterials research field is performed routinely by scientific researchers. Consequently, this NDT has led to noticeable enhancements of the quality, performance and

The author s would like to thank Prof. George Eliades, Dr. Triantaffilos Papadopoulos, Dr. Abdulaziz Al-Rasheed, Dr. Anil Sukumaran, Dr. Maysa Al-Marshood, Peter Tsakiridis, Dr. Alerxis Tagmatarxis and Maria Michalaki for their help in providing some of the pictures for


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**4** 

**Neutron Radiography** 

*Department of Nuclear Engineering, Faculty of Engineering* 

A few years after the discovery of neutron by James Chadwick in 1932, H. Kallman and E. Kuhn started their work on neutron radiography in Germany using neutrons from a small neutron generator. Due to the second World War, their first publication was delayed until 1947. However, the first report on neutron radiography was published by Peters in 1946, a year before Kallman and Kuhn's. After research reactors were available, in 1956 Thewlis and Derbyshire in UK demonstrated that much better neutron radiographic images could be obtained by using intense thermal neutron beam from the reactor. Specific applications of neutron radiography were then started and expanded rapidly particularly where research

The radiographic technique was originally based on metallic neutron converter screen/film assembly. Neutron converter screen and film were gradually improved until early 1990's when computer technology became powerful and was available at low cost. Non-film neutron radiography was then possible to be used for routine inspection of specimens. After 2005, imaging plate specially designed for neutron radiography was available and could provide image quality comparable to the best image quality obtained from the gadolinium foil/film assembly with relative speed approximately 40 times faster. Nevertheless, neutron radiography has not been widely employed for routine inspection of specimen in industry like x-ray and gamma-ray radiography due to two main reasons. Firstly, excellent image quality still needs neutrons from nuclear reactor. Secondly, neutron radiography is only well-known among the academic but not industrial people. It is actually excellent for inspecting parts containing light elements in materials even when they are covered or enveloped by heavy elements. Nowadays, neutrons from small neutron generator and californium-252 source can give neutron intensity sufficient for modern image recording system such as the neutron imaging plate and the light-emitting neutron converter

Neutrons are fundamental particles which are bound together with protons within the atomic nucleus. Neutron is electrically neutral and has mass of nearly the same as a proton i.e. about 1 u. Once a neutron is emitted from the nucleus it becomes free neutron which is

not stable. It decays to a proton and an electron with a half-life of 12 minutes.

**1. Introduction** 

reactors were available.

screen/digital camera assembly.

**2. Principle of neutron radiography** 

Nares Chankow

*Thailand* 

*Chulalongkorn University* 


Nares Chankow

*Department of Nuclear Engineering, Faculty of Engineering Chulalongkorn University Thailand* 

### **1. Introduction**

72 Nondestructive Testing Methods and New Applications

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microstructure, and mechanical properties of commercially pure titanium castings.

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0109-5641

A few years after the discovery of neutron by James Chadwick in 1932, H. Kallman and E. Kuhn started their work on neutron radiography in Germany using neutrons from a small neutron generator. Due to the second World War, their first publication was delayed until 1947. However, the first report on neutron radiography was published by Peters in 1946, a year before Kallman and Kuhn's. After research reactors were available, in 1956 Thewlis and Derbyshire in UK demonstrated that much better neutron radiographic images could be obtained by using intense thermal neutron beam from the reactor. Specific applications of neutron radiography were then started and expanded rapidly particularly where research reactors were available.

The radiographic technique was originally based on metallic neutron converter screen/film assembly. Neutron converter screen and film were gradually improved until early 1990's when computer technology became powerful and was available at low cost. Non-film neutron radiography was then possible to be used for routine inspection of specimens. After 2005, imaging plate specially designed for neutron radiography was available and could provide image quality comparable to the best image quality obtained from the gadolinium foil/film assembly with relative speed approximately 40 times faster. Nevertheless, neutron radiography has not been widely employed for routine inspection of specimen in industry like x-ray and gamma-ray radiography due to two main reasons. Firstly, excellent image quality still needs neutrons from nuclear reactor. Secondly, neutron radiography is only well-known among the academic but not industrial people. It is actually excellent for inspecting parts containing light elements in materials even when they are covered or enveloped by heavy elements. Nowadays, neutrons from small neutron generator and californium-252 source can give neutron intensity sufficient for modern image recording system such as the neutron imaging plate and the light-emitting neutron converter screen/digital camera assembly.

### **2. Principle of neutron radiography**

Neutrons are fundamental particles which are bound together with protons within the atomic nucleus. Neutron is electrically neutral and has mass of nearly the same as a proton i.e. about 1 u. Once a neutron is emitted from the nucleus it becomes free neutron which is not stable. It decays to a proton and an electron with a half-life of 12 minutes.

Fig. 2. Mass attenuation coefficients for thermal neutrons (♦) and gamma-rays as a function

Similar to elastic scattering but when a neutron collides with the atomic nucleus it has enough kinetic energy to raise the nucleus into its excited state. After collision, the nucleus will give off gamma-ray(s) in returning to its ground state. Even inelastic scattering also reduces energy of fast neutron but this is not preferable in neutron radiography because it

A neutron can be absorbed by the atomic nucleus to form new nucleus with an additional neutron resulting in increasing of mass number by 1. For example, when cobalt-59 (59Co) captures a neutron will become radioactive cobalt-60 (60Co). The new nucleus mostly becomes radioactive and decays to beta-particle followed by emission of gamma-ray. Some of them are not radioactive such as 2H, 114Cd, 156Gd, and 158Gd. A few of them only decay by beta-particle without emission of gamma-ray such as 32P. This reaction plays a vital role in neutron radiography when metallic foil screen is used to convert neutrons into beta-

Most of charged particle emission occurs by fast neutrons except for the two important (n, α) reactions of lithium-6 (6Li) and boron-10 (10B). The 6Li(n, α)3H and 10B(n, α)7Li reactions play important roles in neutron detection and shielding. In neutron radiography, these two reactions are mainly employed to convert neutrons to alpha particles or to light. The (n, p) reaction is not important in neutron radiography but it may be useful when solid state track

of atomic number of elements (reproduced from [3] with some modifications)

ii. Inelastic scattering: (n, n') or (n, n'γ) reaction

iii. Neutron capture: (n, γ) reaction

particles and gamma-rays.

increases in contamination of gamma-rays to the system.

iv. Charged particle emission: (n, p) and (n, α) reactions

detector is selected as the image recorder.

Neutron radiography requires parallel beam or divergent beam of low energy neutrons having intensity in the range of only 104 – 106 neutrons/cm2-s to avoid formation of significant amount of long-lived radioactive isotope from neutron absorption within the specimen. The transmitted neutrons will then interact with neutron converter screen to generate particles or light photons which can be recorded by film or any other recording media. Free neutrons emitted from all sources are fast neutrons while neutron radiography prefers low energy neutrons. To reduce neutron energy, neutron sources are normally surrounded by large volume of hydrogeneous material such as water, polyethylene, transformer oil and paraffin. Neutron collimator is designed to bring low energy neutron beam to the test specimen. As illustrated in Figure 2, attenuation coefficient of gamma-ray increases with increasing of the atomic number of element while attenuation coefficients of neutron are high for light elements like hydrogen(H), lithium (Li) and boron(B) as well as some heavy elements such as gadolinium (Gd), cadmium(Cd) and dysprosium (Dy). In contrast, lead (Pb) has very high attenuation coefficient for gamma-ray but very low for neutron. Neutron radiography therefore can make parts containing light elements; such as polymer, plastic, rubber, chemical; visible even when they are covered or enveloped by heavy elements.

Neutrons may interact with matter in one or more of the following reactions.

i. Elastic scattering: (n, n) reaction

Neutron collides with the atomic nucleus, then loses its kinetic energy. It should be noted that neutron loses less kinetic energy when it collides with a heavy nucleus. In contrast, it loses more kinetic energy when collides with a light nucleus. Hydrogen(1H) is therefore the most effective neutron moderator because it is the lightest nucleus having mass almost the same as neutron (∼1u). Elastic scattering is most important in production of low energy or slow neutrons from fast neutrons emitted from the source for neutron radiography. Water, paraffin and polyethylene are common neutron moderators. In fact, hydrogen-2 (2H, so called "deuterium") is the best neutron moderator due to its extremely low neutron absorption probability. Heavy water (D2O) has neutron absorption cross section only about 1/500 that of light water (H2O) but heavy water is very costly.

Fig. 1. Major components of typical neutron radiography system

Neutron radiography requires parallel beam or divergent beam of low energy neutrons having intensity in the range of only 104 – 106 neutrons/cm2-s to avoid formation of significant amount of long-lived radioactive isotope from neutron absorption within the specimen. The transmitted neutrons will then interact with neutron converter screen to generate particles or light photons which can be recorded by film or any other recording media. Free neutrons emitted from all sources are fast neutrons while neutron radiography prefers low energy neutrons. To reduce neutron energy, neutron sources are normally surrounded by large volume of hydrogeneous material such as water, polyethylene, transformer oil and paraffin. Neutron collimator is designed to bring low energy neutron beam to the test specimen. As illustrated in Figure 2, attenuation coefficient of gamma-ray increases with increasing of the atomic number of element while attenuation coefficients of neutron are high for light elements like hydrogen(H), lithium (Li) and boron(B) as well as some heavy elements such as gadolinium (Gd), cadmium(Cd) and dysprosium (Dy). In contrast, lead (Pb) has very high attenuation coefficient for gamma-ray but very low for neutron. Neutron radiography therefore can make parts containing light elements; such as polymer, plastic, rubber, chemical; visible even when they are covered or enveloped by

Neutrons may interact with matter in one or more of the following reactions.

1/500 that of light water (H2O) but heavy water is very costly.

Fig. 1. Major components of typical neutron radiography system

Neutron collides with the atomic nucleus, then loses its kinetic energy. It should be noted that neutron loses less kinetic energy when it collides with a heavy nucleus. In contrast, it loses more kinetic energy when collides with a light nucleus. Hydrogen(1H) is therefore the most effective neutron moderator because it is the lightest nucleus having mass almost the same as neutron (∼1u). Elastic scattering is most important in production of low energy or slow neutrons from fast neutrons emitted from the source for neutron radiography. Water, paraffin and polyethylene are common neutron moderators. In fact, hydrogen-2 (2H, so called "deuterium") is the best neutron moderator due to its extremely low neutron absorption probability. Heavy water (D2O) has neutron absorption cross section only about

heavy elements.

i. Elastic scattering: (n, n) reaction

Fig. 2. Mass attenuation coefficients for thermal neutrons (♦) and gamma-rays as a function of atomic number of elements (reproduced from [3] with some modifications)

ii. Inelastic scattering: (n, n') or (n, n'γ) reaction

Similar to elastic scattering but when a neutron collides with the atomic nucleus it has enough kinetic energy to raise the nucleus into its excited state. After collision, the nucleus will give off gamma-ray(s) in returning to its ground state. Even inelastic scattering also reduces energy of fast neutron but this is not preferable in neutron radiography because it increases in contamination of gamma-rays to the system.

iii. Neutron capture: (n, γ) reaction

A neutron can be absorbed by the atomic nucleus to form new nucleus with an additional neutron resulting in increasing of mass number by 1. For example, when cobalt-59 (59Co) captures a neutron will become radioactive cobalt-60 (60Co). The new nucleus mostly becomes radioactive and decays to beta-particle followed by emission of gamma-ray. Some of them are not radioactive such as 2H, 114Cd, 156Gd, and 158Gd. A few of them only decay by beta-particle without emission of gamma-ray such as 32P. This reaction plays a vital role in neutron radiography when metallic foil screen is used to convert neutrons into betaparticles and gamma-rays.

iv. Charged particle emission: (n, p) and (n, α) reactions

Most of charged particle emission occurs by fast neutrons except for the two important (n, α) reactions of lithium-6 (6Li) and boron-10 (10B). The 6Li(n, α)3H and 10B(n, α)7Li reactions play important roles in neutron detection and shielding. In neutron radiography, these two reactions are mainly employed to convert neutrons to alpha particles or to light. The (n, p) reaction is not important in neutron radiography but it may be useful when solid state track detector is selected as the image recorder.

Σ is equivalent to the linear attenuation coefficient of gamma-ray (μ) and is the characteristic of elements in the specimen. Σ and μ are the product of atom density of elements contained in the specimen (in atoms/ cm3) and their effective microscopic cross sections (σ) to the reactions of interest (in cm2). σ is the effective cross section, not the actual physical cross section of the nucleus. It indicates probability of occurrence for each neutron interaction. For examples, σ(n, γ) indicates the probability of (n, γ) reaction and σs indicates the probability of scattering reaction which combines elastic (n, n) and inelastic (n, n') scattering cross sections. Σ and σ of pure elements and common compounds or mixtures (such as water, heavy water and concrete) can be found in literatures. Figure 4 illustrates percentage of 0.0253 eV

neutron transmission through different kinds of material having thickness of 1 cm.

Boron 5 2.3 759 97.23 Carbon(graphite) 6 1.60 3.4×10-3 2.728×10-4 Heavy water 1.105 1.33×10-3 4.42×10-5 Water 1.0 0.664 0.02220 Aluminum 13 2.699 0.230 0.01386 Iron 26 7.87 2.55 0.2164 Copper 29 8.96 3.79 0.3219 Silver 47 10.49 63.6 3.725 Gadolinium 64 7.95 49000 1492 Dysprosium 66 8.56 930 29.50 Gold 79 19.32 98.8 5.836 Lead 82 11.34 0.170 5.603×10-3 Table 1. Microscopic and macroscopic cross sections of some elements and compounds [4]

Fig. 4. Comparison of 0.0253 eV neutron and 0.5 MeV gamma-ray transmission through

Nominal Density

(g/cm3) <sup>σ</sup>a, barns a, cm-1

number

Element or Molecular Atomic

materials having thickness of 1 cm

v. Neutron producing reaction: (n, 2n) and (n, 3n) reactions

These reactions occur only with fast neutrons which require a threshold energy to trigger. They may be useful in neutron radiography particularly when utilizing 14-MeV neutrons produced from a neutron generator. By inserting blocks of heavy metal like lead (Pb) or uranium (U) in neutron moderator, low energy neutron intensity can be increased by a factor of 2 – 3 or higher from (n, 2n) and (n, 3n) reactions.

vi. Fission: (n, f) reaction

Fission reaction is well-known for energy production in nuclear power plant and neutron production in nuclear research reactor. A heavy nucleus like uranium-235 (235U), plutonium-239(239Pu) undergoes fission after absorption of neutron. The nucleus splits into 2 nuclei of mass approximately one-half of the original nucleus with emission of 2 – 3 neutrons. When uranium (U) is used to increase neutron intensity by the above (n, 2n) and (n, 3n) reactions, fission reaction also contributes additional neutrons to the system. The degree of contribution depends on the ratio of uranium-235 to uranium-238 in uranium.

Attenuation of neutron by the specimen depends on thickness and the attenuation coefficient similar to gamma-ray as follows.

$$\mathbf{I}\_{\rm t} = \mathbf{I}\_{\rm 0} \exp\left(\mathbf{-}\Sigma\mathbf{t}\right) \tag{1}$$

Where


Fig. 3. Attenuation of neutrons by specimen

These reactions occur only with fast neutrons which require a threshold energy to trigger. They may be useful in neutron radiography particularly when utilizing 14-MeV neutrons produced from a neutron generator. By inserting blocks of heavy metal like lead (Pb) or uranium (U) in neutron moderator, low energy neutron intensity can be increased by a

Fission reaction is well-known for energy production in nuclear power plant and neutron production in nuclear research reactor. A heavy nucleus like uranium-235 (235U), plutonium-239(239Pu) undergoes fission after absorption of neutron. The nucleus splits into 2 nuclei of mass approximately one-half of the original nucleus with emission of 2 – 3 neutrons. When uranium (U) is used to increase neutron intensity by the above (n, 2n) and (n, 3n) reactions, fission reaction also contributes additional neutrons to the system. The degree of

Attenuation of neutron by the specimen depends on thickness and the attenuation

It = I0 exp (-Σt) (1)

contribution depends on the ratio of uranium-235 to uranium-238 in uranium.

v. Neutron producing reaction: (n, 2n) and (n, 3n) reactions

factor of 2 – 3 or higher from (n, 2n) and (n, 3n) reactions.

coefficient similar to gamma-ray as follows.

It is the transmitted neutron intensity (n/cm2-s) I0 is the incident neutron intensity (n/cm2-s)

Σ is the macroscopic cross section (cm-1)

Fig. 3. Attenuation of neutrons by specimen

t is the specimen thickness (cm)

vi. Fission: (n, f) reaction

Where

Σ is equivalent to the linear attenuation coefficient of gamma-ray (μ) and is the characteristic of elements in the specimen. Σ and μ are the product of atom density of elements contained in the specimen (in atoms/ cm3) and their effective microscopic cross sections (σ) to the reactions of interest (in cm2). σ is the effective cross section, not the actual physical cross section of the nucleus. It indicates probability of occurrence for each neutron interaction. For examples, σ(n, γ) indicates the probability of (n, γ) reaction and σs indicates the probability of scattering reaction which combines elastic (n, n) and inelastic (n, n') scattering cross sections. Σ and σ of pure elements and common compounds or mixtures (such as water, heavy water and concrete) can be found in literatures. Figure 4 illustrates percentage of 0.0253 eV neutron transmission through different kinds of material having thickness of 1 cm.


Table 1. Microscopic and macroscopic cross sections of some elements and compounds [4]

Fig. 4. Comparison of 0.0253 eV neutron and 0.5 MeV gamma-ray transmission through materials having thickness of 1 cm

neutrons per second per square centimeter, cm-2 s-1) in moderator. Neutron flux in water moderator per a neutron emitted from the neutron source at any distances can be obtained from Figures 6 and 7. For example, the thermalization factor of 252Cf obtained from Table 3 is 100. The neutron emission rate of 252Cf is 2.3 x 106 neutrons per microgram. If a 500 mg 252Cf is used, the maximum flux in water can be calculated from 500 x 2.3 x 106/100 = 1.15 x 107 cm-2 s-1. From a graph in Figure 6 for 252Cf, the maximum flux is at 1 cm distance from the source which indicates the neutron flux of about 1 x 10-2 per a neutron emission from 252Cf. Thus, the maximum neutron flux can be calculated from (500 x 2.3 x 106) x (1 x 10-2) = 1.15 x 107 cm-2 s-1. Neutron flux at other distances can also be obtained from Figure 6. It should be noted that the thermalization factor increases with increasing emitted neutron energy from

As mentioned earlier, neutron radiography requires low energy neutrons. The lower neutron energy gives better image contrast. Fast neutron or high energy neutrons emitted from the source are slowed down by moderator such as water to produce slow or low energy neutrons. The slow neutron energy in moderator is dependent of moderator temperature and the energy distribution follows Maxwellian's for gas molecules and particles. The slow neutron is therefore called "thermal neutron". "Cold neutrons" can be produced by cooling the moderator/collimator down, such as with liquid helium, to obtain better image contrast. The cadmium ratio of cold neutrons is indicated in Table 2 where

> constant neutron output, low cost, maintenance free, no operating cost, low neutron flux/long exposure time, acceptable image

moderate cost, moderate operating and maintenance cost, medium neutron flux/medium exposure time, good image quality, mobile

constant neutron output, high cost, high maintenance and operating cost, high neutron flux/short exposure time, excellent image

infinity (∞) means there is no epicadmium neutron (energy > 0.5 eV) in the beam.

Source Comments

quality, mobile unit is possible

quality, mobile unit is impossible

Table 2. Neutron sources and their general characteristics

unit is possible for small neutron generator

the source.

Radioisotope

Accelerator

Nuclear Reactor

### **3. Neutron sources**

Neutron sources for neutron radiography can be divided into 3 groups. These are radioisotope source, electronic source and nuclear reactor.

i. Radioisotope neutron source

Nowadays, two radioisotope sources are appropriate and available for neutron radiography i.e. americium-241/beryllium (241Am/Be) and californium-252 (252Cf). 241Am/Be produces neutron from (α, n) reaction by bombardment of beryllium (Be) nucleus with alpha particles from 241Am. The average neutron energy and the neutron emission rate are approximately 4.5 MeV and 2.2 x 106 neutrons/second per 1 curie (Ci) of 241Am with a half-life of 432 years. 241Am/Be can be available up to several tens curies of 241Am. 252Cf emits neutrons from spontaneous fission with average neutron energy of 2 MeV and the emission rate of 4.3 x 109 neutrons/second per curie or 2.3 x 106 neutrons/second per microgram of 252Cf. 252Cf has a half-life of 2.6 years and is the best radioisotope source for neutron radiography due to its extremely high neutron output, low average emitted neutron energy and small size.

ii. Electronic neutron source

Particle accelerator and neutron generator are neutron emitting sources produced by nuclear reactions. Particles are accelerated to a sufficient energy and brought to hit target nuclei to produce neutrons. Compact neutron generators are now available for field use with neutron emission rate of 109 to 1012 neutrons per second. The reactions below are commonly used to produce neutrons.

$$\begin{array}{ccccccccc} \text{\$^1\$D^2\$} & + & \text{\$^1\$D^2\$} & \rightarrow & \text{\$^0\$E^3\$} & + & \text{\$^2\$He^3\$} & + & \text{\$^3\$.28 MeV}\$, & \text{so called \$^0\$DD}\$ & \text{Reaction} ""\\\\ \text{\$^1\$T^3\$} & + & \text{\$^1\$D^2\$} & \rightarrow & \text{\$^0\$n^1\$} & + & \text{\$^2\$He^4\$} & + & \text{\$^1\$7.6 MeV}\$, & \text{so called \$^0\$DT}\$ & \text{Reaction} "" \\\\ & & & \text{\$^1\$D^2\$} & + & \text{\$^4\$Be^9\$} & \rightarrow & \text{\$^0\$n^1\$} & + & \text{\$^5\$B^{10}\$} & + & \text{4.35 MeV} \\\\ & & & \text{\$^1\$H^1\$} & + & \text{\$^4\$Be^9\$} & \rightarrow & \text{\$^0\$n^1\$} & + & \text{\$^5\$B^9\$} & - & \text{\$1.85 MeV} \\\\ \end{array}$$

Energy of fast neurons produced from the above reactions is monoenergetic and depends on the incoming particle i.e. 1H and 2D. The DT reaction is a well-known fusion reaction for generating 14 MeV neutrons.

iii. Nuclear reactor

Nuclear reactor generally produces neutrons from fission reaction of uranium-235 (235U). The fission neutron energy is in the range of 0 – 10 MeV with the most probable and the average energy of 0.7 and 2 MeV respectively. Fission reactions take place in the nuclear reactor fuel rods which are surrounded by neutron moderator. The moderator reduces the neutron energy to thermal energy or slow neutron. Due to its high slow neutron intensity in the reactor core of about 1012 – 1014 neutrons/cm2 per second, good collimation of neutron beam can be easily obtained to give excellent image quality for neutron radiography.

Maximum neutron flux in moderator is a function of neutron emission rate from neutron source and neutron energy. Thermalization factor, as shown in Table 3, is the ratio of the neutron emission rate (in neutrons per second, s-1) to the maximum neutron flux (in

Neutron sources for neutron radiography can be divided into 3 groups. These are

Nowadays, two radioisotope sources are appropriate and available for neutron radiography i.e. americium-241/beryllium (241Am/Be) and californium-252 (252Cf). 241Am/Be produces neutron from (α, n) reaction by bombardment of beryllium (Be) nucleus with alpha particles from 241Am. The average neutron energy and the neutron emission rate are approximately 4.5 MeV and 2.2 x 106 neutrons/second per 1 curie (Ci) of 241Am with a half-life of 432 years. 241Am/Be can be available up to several tens curies of 241Am. 252Cf emits neutrons from spontaneous fission with average neutron energy of 2 MeV and the emission rate of 4.3 x 109 neutrons/second per curie or 2.3 x 106 neutrons/second per microgram of 252Cf. 252Cf has a half-life of 2.6 years and is the best radioisotope source for neutron radiography due to its

extremely high neutron output, low average emitted neutron energy and small size.

Particle accelerator and neutron generator are neutron emitting sources produced by nuclear reactions. Particles are accelerated to a sufficient energy and brought to hit target nuclei to produce neutrons. Compact neutron generators are now available for field use with neutron emission rate of 109 to 1012 neutrons per second. The reactions below are

1D2 + 1D2 0n1 + 2He3 + 3.28 MeV, *so called "DD Reaction"*

1T3 + 1D2 0n1 + 2He4 + 17.6 MeV, *so called "DT Reaction"*

1D2 + 4Be9 0n1 + 5B10 + 4.35 MeV

1H1 + 4Be9 0n1 + 5B9 - 1.85 MeV Energy of fast neurons produced from the above reactions is monoenergetic and depends on the incoming particle i.e. 1H and 2D. The DT reaction is a well-known fusion reaction for

Nuclear reactor generally produces neutrons from fission reaction of uranium-235 (235U). The fission neutron energy is in the range of 0 – 10 MeV with the most probable and the average energy of 0.7 and 2 MeV respectively. Fission reactions take place in the nuclear reactor fuel rods which are surrounded by neutron moderator. The moderator reduces the neutron energy to thermal energy or slow neutron. Due to its high slow neutron intensity in the reactor core of about 1012 – 1014 neutrons/cm2 per second, good collimation of neutron

Maximum neutron flux in moderator is a function of neutron emission rate from neutron source and neutron energy. Thermalization factor, as shown in Table 3, is the ratio of the neutron emission rate (in neutrons per second, s-1) to the maximum neutron flux (in

beam can be easily obtained to give excellent image quality for neutron radiography.

radioisotope source, electronic source and nuclear reactor.

**3. Neutron sources** 

i. Radioisotope neutron source

ii. Electronic neutron source

generating 14 MeV neutrons.

iii. Nuclear reactor

commonly used to produce neutrons.

neutrons per second per square centimeter, cm-2 s-1) in moderator. Neutron flux in water moderator per a neutron emitted from the neutron source at any distances can be obtained from Figures 6 and 7. For example, the thermalization factor of 252Cf obtained from Table 3 is 100. The neutron emission rate of 252Cf is 2.3 x 106 neutrons per microgram. If a 500 mg 252Cf is used, the maximum flux in water can be calculated from 500 x 2.3 x 106/100 = 1.15 x 107 cm-2 s-1. From a graph in Figure 6 for 252Cf, the maximum flux is at 1 cm distance from the source which indicates the neutron flux of about 1 x 10-2 per a neutron emission from 252Cf. Thus, the maximum neutron flux can be calculated from (500 x 2.3 x 106) x (1 x 10-2) = 1.15 x 107 cm-2 s-1. Neutron flux at other distances can also be obtained from Figure 6. It should be noted that the thermalization factor increases with increasing emitted neutron energy from the source.

As mentioned earlier, neutron radiography requires low energy neutrons. The lower neutron energy gives better image contrast. Fast neutron or high energy neutrons emitted from the source are slowed down by moderator such as water to produce slow or low energy neutrons. The slow neutron energy in moderator is dependent of moderator temperature and the energy distribution follows Maxwellian's for gas molecules and particles. The slow neutron is therefore called "thermal neutron". "Cold neutrons" can be produced by cooling the moderator/collimator down, such as with liquid helium, to obtain better image contrast. The cadmium ratio of cold neutrons is indicated in Table 2 where infinity (∞) means there is no epicadmium neutron (energy > 0.5 eV) in the beam.


Table 2. Neutron sources and their general characteristics

Fig. 6. Neutron flux in water per source neutron emitted from radioisotope neutron

sources [6]


Table 3. Neutron sources and their technical characteristics [2, 6]

Fig. 5. Energy spectra of neutrons from neutron sources (reproduced from [5])

Range Mean in moderator at specimen

108

109

Nuclear Reactor 0-10 2.0 1011 - 1014 106 – 108 100 100-300 100-∞

Normal thermal neutron flux (cm-2 s)

103 - 104

103 – 105

Thermalization factor

400

100

L/D ratio Cd ratio

5-20

5-20

5-20 5-20 5-20 5-20

30-50

50-100

20-30 20-30 20-30 20-30

Source

Radioisotope - 241Am/Be (50 Ci) - 252Cf (1 mg)

Accelerator - 2D(d, n)3He - 3T(d, n)4He - 9Be(p, n)9B - 9Be(d, n)10B Emitted neutron energy (MeV)

4.5

2.0

2.7 14.1 1.15 3.96

Table 3. Neutron sources and their technical characteristics [2, 6]

Fig. 5. Energy spectra of neutrons from neutron sources (reproduced from [5])

0 – 10

0 - 10

2.7 14.1 1.15 3.96

Fig. 6. Neutron flux in water per source neutron emitted from radioisotope neutron sources [6]

Neutrons in moderator are scattered in all directions which are not suitable for radiography. Neutron collimator is a structure designed to extract slow neutron beam from the moderator to the specimen. Ideally, parallel neutron beam is preferred because it gives best image sharpness. If this is the case, Soller or multitube collimator is used. However, divergent collimator is easier to construct and gives good image sharpness depending on the

i. *Soller or multitube collimator*: This collimator is constructed with neutron absorbing material; such as boron, cadmium and gadolinium; as illustrated in Figure 8 so as to bring parallel neutron beam to the test specimen. Neutrons can only get into the collimator from one end which is in the moderator then get out to the other end. Neutrons those are not travel in parallel with the collimator axis will hit the side of the tube or plate and are then absorbed allowing only neutrons travelling in parallel with the tube axis to reach the test specimen. This type of collimator is applicable to nuclear reactor where input neutron intensity to the collimator is high. The drawbacks are that the pattern of parallel plates or tubes may be seen on the image and it is more costly to

ii. *Divergent collimator*: Divergent collimator is designed in the way that neutrons are allowed to get into the collimator only through a small hole from one end then diverge at the other end. The collimator is lined with neutron absorber to absorb unwanted scattered neutrons. It is easy to construct and can be used with non-reactor neutron source like radioisotope and accelerator where slow neutron input is low. The drawback is that image sharpness may not be as good as the Soller collimator. For low neutron intensity as in radioisotope system, neutron output at the specimen position can still be increased by making part of the collimator on the input or source side free from neutron absorber as shown in Figure 10. Neutrons can thus enter the collimator through this part resulting in increasing of neutron intensity. From experience with 241Am/Be and 252Cf sources, neutron intensity can be increased approximately by 10 - 60 % and the cadmium ratio can also be increased from about 5 to 20. In doing so, the image contrast is significantly improved while the image sharpness is a little poorer.

**4. Neutron collimators** 

geometrical parameters as will be discussed later.

construct in comparison to the divergent collimator.

Fig. 8. Multitube neutron collimator

Fig. 7. Neutron flux in water per source neutron emitted from neutron producing accelerators [6]

Fig. 7. Neutron flux in water per source neutron emitted from neutron producing

accelerators [6]

### **4. Neutron collimators**

Neutrons in moderator are scattered in all directions which are not suitable for radiography. Neutron collimator is a structure designed to extract slow neutron beam from the moderator to the specimen. Ideally, parallel neutron beam is preferred because it gives best image sharpness. If this is the case, Soller or multitube collimator is used. However, divergent collimator is easier to construct and gives good image sharpness depending on the geometrical parameters as will be discussed later.


Fig. 8. Multitube neutron collimator

After neutrons pass through the specimen they interact with the converter screen to produce radioisotope, alpha particle or light which can be recorded by film, imaging plate, optical camera or video camera. Image recording medium must be selected to match with the particles or light emitted from the neutron converter screen so as to obtain the maximum efficiency. The neutron converter screen/image recording device assemblies commonly

i. *Metallic foil screen/film*: Metallic foil with high neutron cross section is employed to convert slow neutrons to beta-particles, gamma-rays and/or conversion electron while industrial x-ray film is normally used as the image recorder. Gadolinium (Gd) foil is the best metallic screen for neutron radiography in terms of having extremely high neutron absorption cross section, giving the best image resolution and not becoming radioisotope after neutron absorption. 155Gd and 157Gd are found 14.9 and 15.7 percent of natural Gd isotopes with neutron absorption cross sections of 61,000 and 254,000 barns respectively. 155Gd and 157Gd absorb neutrons then become 156Gd and 158Gd correspondingly which are not radioactive. Prompt captured gamma-rays emitted during neutron absorption can cause film blackening. More importantly, prompt gamma-rays may hit atomic electrons resulting in ejection of electrons from the atoms (so called "conversion electron") which are more effective to cause film blackening. It should be noted that less than a few percentage of gamma-ray photons cause film blackening. Electrons and beta-particles are

preferred because they interact with film much more than gamma-rays.

Formation of radioisotope from neutron irradiation follows the equation below.

A = nσφ (1 – e-

λΤ) (2)

Film may be replaced by imaging plate (IP) which has more than 10 times faster speed than the x-ray film. Gd foil/x-ray film requires relatively high neutron exposure thus it is not possible to carry out neutron radiography with low neutron flux system using radioisotope. About 5 years ago, Fuji started to produce neutron imaging plate by adding Gd into the imaging plate which can give the image quality comparable to that from the Gd foil/x-ray film assembly with approximately 50 times reduction of neutron exposure. It is therefore

Other metallic foil screens can also be used (as listed in Table 4) but the image quality is not as good as that obtained from Gd. This is mainly because low energy electrons emitted from Gd have very short ranges resulting in much better image sharpness. In case of having large gamma-ray contamination in the neutron beam and specimen containing gamma-ray emitting radioisotopes, dysprosium (Dy) is often used. To avoid gamma-ray exposure to xray film, the transfer method must be applied by exposing only the Dy screen with transmitted neutrons from the specimen. During exposure, radioisotopes 165mDy and 165Dy are formed with half-lives of 1.26 minutes and 2.3 hours respectively. The Dy foil is then removed from the neutron beam and placed in close contact with an x-ray film to produce a latent image. The film density or film darkness is corresponding to the activity of Dy

**5. Neutron radiographic techniques** 

used in neutron radiography are described below.

possible to be used with low neutron flux system.

radioisotopes formed in each part of the Dy foil.

Fig. 9. Divergent neutron collimator allowing neutrons to get into the collimator only through the hole of diameter "D"

Fig. 10. Divergent neutron collimator with part of the source side contains no neutron absorber allowing more neutrons to get into the collimator

Fig. 9. Divergent neutron collimator allowing neutrons to get into the collimator only

Fig. 10. Divergent neutron collimator with part of the source side contains no neutron

absorber allowing more neutrons to get into the collimator

through the hole of diameter "D"

### **5. Neutron radiographic techniques**

After neutrons pass through the specimen they interact with the converter screen to produce radioisotope, alpha particle or light which can be recorded by film, imaging plate, optical camera or video camera. Image recording medium must be selected to match with the particles or light emitted from the neutron converter screen so as to obtain the maximum efficiency. The neutron converter screen/image recording device assemblies commonly used in neutron radiography are described below.

i. *Metallic foil screen/film*: Metallic foil with high neutron cross section is employed to convert slow neutrons to beta-particles, gamma-rays and/or conversion electron while industrial x-ray film is normally used as the image recorder. Gadolinium (Gd) foil is the best metallic screen for neutron radiography in terms of having extremely high neutron absorption cross section, giving the best image resolution and not becoming radioisotope after neutron absorption. 155Gd and 157Gd are found 14.9 and 15.7 percent of natural Gd isotopes with neutron absorption cross sections of 61,000 and 254,000 barns respectively. 155Gd and 157Gd absorb neutrons then become 156Gd and 158Gd correspondingly which are not radioactive. Prompt captured gamma-rays emitted during neutron absorption can cause film blackening. More importantly, prompt gamma-rays may hit atomic electrons resulting in ejection of electrons from the atoms (so called "conversion electron") which are more effective to cause film blackening. It should be noted that less than a few percentage of gamma-ray photons cause film blackening. Electrons and beta-particles are preferred because they interact with film much more than gamma-rays.

Film may be replaced by imaging plate (IP) which has more than 10 times faster speed than the x-ray film. Gd foil/x-ray film requires relatively high neutron exposure thus it is not possible to carry out neutron radiography with low neutron flux system using radioisotope. About 5 years ago, Fuji started to produce neutron imaging plate by adding Gd into the imaging plate which can give the image quality comparable to that from the Gd foil/x-ray film assembly with approximately 50 times reduction of neutron exposure. It is therefore possible to be used with low neutron flux system.

Other metallic foil screens can also be used (as listed in Table 4) but the image quality is not as good as that obtained from Gd. This is mainly because low energy electrons emitted from Gd have very short ranges resulting in much better image sharpness. In case of having large gamma-ray contamination in the neutron beam and specimen containing gamma-ray emitting radioisotopes, dysprosium (Dy) is often used. To avoid gamma-ray exposure to xray film, the transfer method must be applied by exposing only the Dy screen with transmitted neutrons from the specimen. During exposure, radioisotopes 165mDy and 165Dy are formed with half-lives of 1.26 minutes and 2.3 hours respectively. The Dy foil is then removed from the neutron beam and placed in close contact with an x-ray film to produce a latent image. The film density or film darkness is corresponding to the activity of Dy radioisotopes formed in each part of the Dy foil.

Formation of radioisotope from neutron irradiation follows the equation below.

$$\mathbf{A} \quad = \begin{array}{c} \mathbf{n} \mathfrak{o} \mathfrak{ϕ} \left( \mathbf{1} - \mathbf{e}^{\lambda \mathbf{T}} \right) \end{array} \tag{2}$$

**Half-life** 

**Major particle emitted**

*<sup>t</sup> e* (3)

**Direct or Transfer technique**

**Crosssection (barns)** 

Lithium Li6(n, α)H3 935 Stable α Direct Boron B10(n, α)Li7 3,837 Stable α Direct Rhodium Rh103(n, γ)Rh104 144 43 s β- Direct Rh103(n, n)Rh103m 57 min x-ray Rh103(n, γ)Rh104m 11 4.4 min β-Cadmium Cd113(n, γ)Cd114 20,000 Stable γ Direct Indium In115(n, γ)In116 45 14 s β- Transfer In115(n, γ)In116m 154 54 min β-Samarium Sm149(n, γ)Sm150 41,500 Stable γ Direct Sm152(n, γ)Sm153 210 46.7 h β-Gadolinium Gd155(n, γ)Gd156 58,000 Stable e- Direct Gd157(n, γ)Gd158 240,000 Stable e-

Dysprosium Dy164(n, γ)Dy165 800 2.3 h β- Transfer Dy164(n, γ)Dy165 2,000 1.26 min β-

Table 4. Characteristics of Some Possible Neutron Radiography Converter Materials [2, 7]

Radioisotope decays exponentially according to its half-life. If A0 is the radioactivity of the radioisotope after completion of neutron irradiation, the radioactivity at any time t can be

For Gd foil, no radioisotope is formed during neutron irradiation. Emission of prompt gamma-rays and conversion electrons follows neutron absorption by 155Gd and 157Gd at the rate of nσφ per second. In case of Dy foil, 165mDy and 165Dy are formed with the radioactivity following equation (2). After removal from the neutron facility, 165mDy and 165Dy will decay with half-lives of 1.26 minutes and 2.3 hours respectively. Film is exposed to emitted radiation while placing in close contact with the radioactive foil. Build-up and decay of a

ii. *Light emitting screen/film*: Light-emitting screen is a mixture of scintillator or phosphor with lithium-6 (6Li) and/or boron-10 (10B). Neutrons interact with 6Li or 10B to produce alpha-particles via (n, α) reaction. Light is then emitted from energy loss of alphaparticles in scintillator or phosphor. Light sensitive film, digital camera or video camera can be used to record image. This makes real-time and near real-time radiography possible. The most common light-emitting screen is NE426 available from NE Technology which is composed of ZnS(Ag) scintillator and boron compound. Gadolinium oxysulfide (terbium) [Gd2O2S (Tb), GOS] and lithium loaded glass scintillator are also common in neutron radiography. GOS itself is a scintillator. Conversion electrons as well as low energy prompt gamma-rays emitted from interaction of neutrons with Gd cause light emission. Glass scintillator is sensitive to

λ

**Material** 

calculated from

**Mode of Production of active Isotope** 

At= A0 <sup>−</sup>

radioisotope is illustrated graphically in Figure 13.

Where A is the radioactivity of radioisotope formed in disintegration per second (dps) after completion of neutron irradiation. n is the number of original stable isotope atoms. σ is the neutron absorption cross section of the original stable isotope in cm2. φ is the neutron flux in cm-2 s-1. λ is the decay constant of the radioisotope formed in s-1 and T is the irradiation time in second (s).

The decay constant (λ) can be obtained from :

$$
\lambda = 0.693/\text{T}\_{1/2} \tag{3}
$$

where T1/2 is half-life of the radioisotope. Form equation (1), more than 96 % of the maximum radioactivity can be obtained if the irradiation time is greater than 5 times of the half-life.

Fig. 11. Illustration of direct exposure method

Fig. 12. Illustration of two steps of indirect or transfer exposure method

Where A is the radioactivity of radioisotope formed in disintegration per second (dps) after completion of neutron irradiation. n is the number of original stable isotope atoms. σ is the neutron absorption cross section of the original stable isotope in cm2. φ is the neutron flux in cm-2 s-1. λ is the decay constant of the radioisotope formed in s-1 and T is the irradiation time

 λ = 0.693/T1/2 (3) where T1/2 is half-life of the radioisotope. Form equation (1), more than 96 % of the maximum radioactivity can be obtained if the irradiation time is greater than 5 times of the

in second (s).

half-life.

The decay constant (λ) can be obtained from :

Fig. 11. Illustration of direct exposure method

Fig. 12. Illustration of two steps of indirect or transfer exposure method


Table 4. Characteristics of Some Possible Neutron Radiography Converter Materials [2, 7]

Radioisotope decays exponentially according to its half-life. If A0 is the radioactivity of the radioisotope after completion of neutron irradiation, the radioactivity at any time t can be calculated from

$$\mathbf{A} = \mathbf{A}\_0 \ e^{-\lambda t} \tag{3}$$

For Gd foil, no radioisotope is formed during neutron irradiation. Emission of prompt gamma-rays and conversion electrons follows neutron absorption by 155Gd and 157Gd at the rate of nσφ per second. In case of Dy foil, 165mDy and 165Dy are formed with the radioactivity following equation (2). After removal from the neutron facility, 165mDy and 165Dy will decay with half-lives of 1.26 minutes and 2.3 hours respectively. Film is exposed to emitted radiation while placing in close contact with the radioactive foil. Build-up and decay of a radioisotope is illustrated graphically in Figure 13.

ii. *Light emitting screen/film*: Light-emitting screen is a mixture of scintillator or phosphor with lithium-6 (6Li) and/or boron-10 (10B). Neutrons interact with 6Li or 10B to produce alpha-particles via (n, α) reaction. Light is then emitted from energy loss of alphaparticles in scintillator or phosphor. Light sensitive film, digital camera or video camera can be used to record image. This makes real-time and near real-time radiography possible. The most common light-emitting screen is NE426 available from NE Technology which is composed of ZnS(Ag) scintillator and boron compound. Gadolinium oxysulfide (terbium) [Gd2O2S (Tb), GOS] and lithium loaded glass scintillator are also common in neutron radiography. GOS itself is a scintillator. Conversion electrons as well as low energy prompt gamma-rays emitted from interaction of neutrons with Gd cause light emission. Glass scintillator is sensitive to

Light-emitting screen offers highest speed but gives poorest image sharpness comparing to other screen/film assemblies. This is the only type of screen that can be used with low neutron flux system using radioisotope neutron source. From experience, photographic film is more suitable with the light-emitting screen than industrial x-ray film by the following two main reasons. Firstly, photographic film is less sensitive to gamma-ray. As a result, it gives better image contrast particularly when neutron beam is contaminated by large

iii. *Alpha-emitting screen/track-etch film*: Alpha-emitting screen is made of lithium and/or boron compound. Particles emitted from 6Li(n, α)3H and 10B(n, α)7Li reactions interact with track-etch film (or so called "solid state track detector, SSTD)") to produce damage tracks along their trajectories. The detector is later put into hot chemical solution to enlarge or "etch" the damage tracks. After etching, the damage tracks can be made visible under an optical microscope with a magnification of x 100 up. Radiation dose and/or neutron intensity can be evaluated by counting number of tracks per unit area. The area where track density is so large becomes translucent while the area with low track density is more transparent. The degree of translucence depends on track density resulting in formation of visible image on the film. However, contrast of the image is poor while sharpness is comparable to the Gd foil/x-ray film assembly. Methods for viewing the image is needed to improve image contrast such as reprinting the image on a high contrast film. It has been reported that the simplest method is to scan image on the track-etch film using a desktop scanner [8]. Track-etch film is not sensitive to light, beta-particle and gamma-ray. The alpha-emitting screen/track-etch film assembly can therefore be used to radiograph radioactive specimens by the direct method. No darkroom is needed for film processing. Kodak LR115 Type II, CA80-15 Type II and CN85 Type B have been widely used during the past two decades. They are cellulose nitrate films coated with lithium metaborate (Li2B4O7). The optimum etching condition is 10 - 40 % sodium hydroxide (NaOH) at 60 ⁰C for a duration of 30 – 40 minutes. Later, BE-10 screen of 93 % enriched boron-10 in boron carbide (B4C) form manufactured by Kodak became available and has been widely used since then due to its highest neutron conversion efficiency. Kodak LR115, CA8015 and CN85 cellulose nitrate film without lithium metaborate are used with the BE-10 screen. CR39 plastic or poly(alyl diglycol) carbonate is also available and is used extensively for alpha detection due to its higher track registration efficiency. The CR39/BE-10 assembly will probably become the most

fraction of gamma-rays. Secondly, photographic film is cheaper and easily available.

common in track-etch neutron radiography.

**6. Applications of neutron radiography: Facilities and sample images** 

Neutron radiography has been employed for non-destructive testing of specimens. Parts of test specimen containing light elements; such as rubber, plastic, chemicals; can be made visible even when they are covered or enveloped by heavy elements. Nuclear reactor gives the best thermal or cold neutron beam for neutron radiography as can be seen in Tables 3 and 6. The cadmium ratio is normally greater than 10 and can be as high as 300 or even infinity if required. The L/D ratio is always greater than 100 which indicate excellent image sharpness. Nuclear research reactor generally provides excellent beam ports for neutron experiments including for neutron radiography. All neutron converter screen/image

charged particles such as alpha- and beta-particles. Lithium is added into the glass scintillator so that alpha-particle will be emitted from 6Li(n, α)3H reaction resulting in emission of light.

Fig. 13. Graphical illustration of build-up and decay of radioactivity of a radioisotopein neutron converter screen using indirect or transfer method

Fig. 14. Real-time or near real-time neutron imaging system using light-emitting neutron converter screen

Fig. 13. Graphical illustration of build-up and decay of radioactivity of a radioisotopein

Fig. 14. Real-time or near real-time neutron imaging system using light-emitting neutron

neutron converter screen using indirect or transfer method

emission of light.

converter screen

charged particles such as alpha- and beta-particles. Lithium is added into the glass scintillator so that alpha-particle will be emitted from 6Li(n, α)3H reaction resulting in Light-emitting screen offers highest speed but gives poorest image sharpness comparing to other screen/film assemblies. This is the only type of screen that can be used with low neutron flux system using radioisotope neutron source. From experience, photographic film is more suitable with the light-emitting screen than industrial x-ray film by the following two main reasons. Firstly, photographic film is less sensitive to gamma-ray. As a result, it gives better image contrast particularly when neutron beam is contaminated by large fraction of gamma-rays. Secondly, photographic film is cheaper and easily available.

iii. *Alpha-emitting screen/track-etch film*: Alpha-emitting screen is made of lithium and/or boron compound. Particles emitted from 6Li(n, α)3H and 10B(n, α)7Li reactions interact with track-etch film (or so called "solid state track detector, SSTD)") to produce damage tracks along their trajectories. The detector is later put into hot chemical solution to enlarge or "etch" the damage tracks. After etching, the damage tracks can be made visible under an optical microscope with a magnification of x 100 up. Radiation dose and/or neutron intensity can be evaluated by counting number of tracks per unit area. The area where track density is so large becomes translucent while the area with low track density is more transparent. The degree of translucence depends on track density resulting in formation of visible image on the film. However, contrast of the image is poor while sharpness is comparable to the Gd foil/x-ray film assembly. Methods for viewing the image is needed to improve image contrast such as reprinting the image on a high contrast film. It has been reported that the simplest method is to scan image on the track-etch film using a desktop scanner [8]. Track-etch film is not sensitive to light, beta-particle and gamma-ray. The alpha-emitting screen/track-etch film assembly can therefore be used to radiograph radioactive specimens by the direct method. No darkroom is needed for film processing. Kodak LR115 Type II, CA80-15 Type II and CN85 Type B have been widely used during the past two decades. They are cellulose nitrate films coated with lithium metaborate (Li2B4O7). The optimum etching condition is 10 - 40 % sodium hydroxide (NaOH) at 60 ⁰C for a duration of 30 – 40 minutes. Later, BE-10 screen of 93 % enriched boron-10 in boron carbide (B4C) form manufactured by Kodak became available and has been widely used since then due to its highest neutron conversion efficiency. Kodak LR115, CA8015 and CN85 cellulose nitrate film without lithium metaborate are used with the BE-10 screen. CR39 plastic or poly(alyl diglycol) carbonate is also available and is used extensively for alpha detection due to its higher track registration efficiency. The CR39/BE-10 assembly will probably become the most common in track-etch neutron radiography.

### **6. Applications of neutron radiography: Facilities and sample images**

Neutron radiography has been employed for non-destructive testing of specimens. Parts of test specimen containing light elements; such as rubber, plastic, chemicals; can be made visible even when they are covered or enveloped by heavy elements. Nuclear reactor gives the best thermal or cold neutron beam for neutron radiography as can be seen in Tables 3 and 6. The cadmium ratio is normally greater than 10 and can be as high as 300 or even infinity if required. The L/D ratio is always greater than 100 which indicate excellent image sharpness. Nuclear research reactor generally provides excellent beam ports for neutron experiments including for neutron radiography. All neutron converter screen/image

Fig. 15. Thermal neutron exposure required film for radiography [9]

respectively.

can be calculated from:

For example, total thermal neutrons per square centimeter required for Gd metallic foil screen/film and NE-426 light emitting screen/film assemblies to make a density of 1.5 on film are approximately 5.5 x 108 and 5 x 106 respectively. If neutron flux at the specimen position is 106 cm-2 s-1, the exposure time needed for the two screens are 550 and 5 seconds

When a 1 mg (1000 μg) Cf-252 is used as in Table 6, the maximum neutron flux in water will be 1000 μg x (2.3 x 106 s-1 μg-1)/100 = 2.3 x 107 cm-2 s-1. The neutron flux at the specimen position for a circular cross-section, divergent collimator with an L/D of 12 (as in Table 6)

Φexit = 2.3 x 107 (1/12)2/16 = 9.98 x 103 ≈ 104 cm-2 s-1 Where Φsource and Φexit are neutron fluxes at the source side and the specimen position respectively. The neutron flux obtained from calculation agrees with the value in Table 6. Thus, the exposure time required for the Gd metallic foil screen/film and NE-426 light emitting screen/film assemblies to make a density of 2.0 on film will be 5.5 x 104 and 500 seconds respectively. It is therefore impossible to use the Gd foil screen/film assembly with Cf-252 source. However, the neutron flux can still be increased by leaving part of the collimator on the source side without neutron absorber. The neutron flux will then be increased by a factor of (1 + 2a/L), where a is the length of the collimator without neutron absorber. For example, if the length of the total collimator (L) is 30 cm and the part without neutron absorber (a) is 10 cm, the neutron flux is increased by a factor of [1 + (2 x 10)/30] =

Φexit = Φsource (D/L)2/16 (4)

recorder assemblies mentioned above can be employed for inspection of specimens but the exposure times vary considerably. Neutron exposure for some converter screen/film assemblies can be estimated by using the curves in Figure 15.


Table 5. Common neutron converter screens and image recording media

recorder assemblies mentioned above can be employed for inspection of specimens but the exposure times vary considerably. Neutron exposure for some converter screen/film


Photographic film, Industrial x-ray film, Optical or video camera

Track-etch film e.g. CR39, cellulose nitrate

Recording Medium Comment

Industrial x-ray film Best for transfer method, good

high neutron exposure

image quality, needs high exposure

Fastest speed, needs low exposure, acceptable image quality, allows real-time or near real-time imaging

Good image quality, needs high exposure, low contrast, requires image viewing technique, does not require a darkroom for film loading and processing

assemblies can be estimated by using the curves in Figure 15.

Particles

Beta-particles, Gamma-rays

Light

Alpha-particles

Table 5. Common neutron converter screens and image recording media

Converter Screen Emitted

*Metallic foil screen* 

*Light-emitting screen* 


*Alpha-emitting screen* 






Fig. 15. Thermal neutron exposure required film for radiography [9]

For example, total thermal neutrons per square centimeter required for Gd metallic foil screen/film and NE-426 light emitting screen/film assemblies to make a density of 1.5 on film are approximately 5.5 x 108 and 5 x 106 respectively. If neutron flux at the specimen position is 106 cm-2 s-1, the exposure time needed for the two screens are 550 and 5 seconds respectively.

When a 1 mg (1000 μg) Cf-252 is used as in Table 6, the maximum neutron flux in water will be 1000 μg x (2.3 x 106 s-1 μg-1)/100 = 2.3 x 107 cm-2 s-1. The neutron flux at the specimen position for a circular cross-section, divergent collimator with an L/D of 12 (as in Table 6) can be calculated from:

$$
\Phi\_{\text{exit}} = \Phi\_{\text{source}} \text{ (D/L)}^2/16 \tag{4}
$$

$$
\Phi\_{\text{exit}} = 2.3 \times 10^7 \text{ (1/12)}^2/16 \qquad = 9.98 \times 10^3 = 10^4 \text{ cm}^2\text{s}^{-1}
$$

Where Φsource and Φexit are neutron fluxes at the source side and the specimen position respectively. The neutron flux obtained from calculation agrees with the value in Table 6. Thus, the exposure time required for the Gd metallic foil screen/film and NE-426 light emitting screen/film assemblies to make a density of 2.0 on film will be 5.5 x 104 and 500 seconds respectively. It is therefore impossible to use the Gd foil screen/film assembly with Cf-252 source. However, the neutron flux can still be increased by leaving part of the collimator on the source side without neutron absorber. The neutron flux will then be increased by a factor of (1 + 2a/L), where a is the length of the collimator without neutron absorber. For example, if the length of the total collimator (L) is 30 cm and the part without neutron absorber (a) is 10 cm, the neutron flux is increased by a factor of [1 + (2 x 10)/30] =

Converter Foil Technique Foil Thickness (<sup>μ</sup>m) Relative Speed Front Back Rh/Gd Direct 250 50 5.3 Rh/Rh Direct 250 250 4.7 Gd/Gd Direct 25 50 3.7 In/In Direct 500 750 3.7 Dy/Dy Direct 150 250 3.7 Cd/Cd Direct 250 500 3.3 Ag/Ag Direct 450 450 2.7 Dy Direct - 250 2.5 Gd Direct - 25 2.4 Cd Direct - 250 2.2 Rh Direct - 250 2.1 In Direct 500 - 1.7 Dy Transfer 250 - 16.4 In Transfer 50 - 11.2

Remark : The relative speed for the direct and transfer methods are not comparable

Table 7. Relative Speed of Neutron Converter Screens [2]

Fig. 16. An example of Cf-252 based neutron radiography system

1.67 or 67 %. The exposure time will then be reduced from 500 seconds to 300 seconds. In doing so, the cadmium ratio is increased from about 5 to 15 -20 resulting in significant improvement in image contrast but the image sharpness is gradually reduced [10]. Use of the second neutron converter screen can also decrease the exposure time by a factor of up to 2.2 as shown in Table 7. During the past decades, the image recording devices have been rapidly improved in speed as well as graininess including film, imaging plate (IP), digital optical camera, digital video camera, CCD and CMOS chips. The new devices allow radiographers to perform non-film neutron radiography with neutron generator and Cf-252 neutron sources. The Fuji neutron imaging plate offers speed several ten times faster than that of the Gd/film assembly with comparable image quality [11-13]. The light-emitting screen coupled with a digital camera with light sensitivity from ISO 1600 and time integration mode makes non-film neutron radiography by Cf-252 possible. An image intensifier or a microchannel plate (MCP) is useful for real-time or near real-time neutron imaging in low flux system. Examples of neutron radiographic images taken from different neutron facilities and by different techniques are illustrated in Figures 18 to 23.


Table 6. Examples of common neutron radiography facilities [2, 6]

1.67 or 67 %. The exposure time will then be reduced from 500 seconds to 300 seconds. In doing so, the cadmium ratio is increased from about 5 to 15 -20 resulting in significant improvement in image contrast but the image sharpness is gradually reduced [10]. Use of the second neutron converter screen can also decrease the exposure time by a factor of up to 2.2 as shown in Table 7. During the past decades, the image recording devices have been rapidly improved in speed as well as graininess including film, imaging plate (IP), digital optical camera, digital video camera, CCD and CMOS chips. The new devices allow radiographers to perform non-film neutron radiography with neutron generator and Cf-252 neutron sources. The Fuji neutron imaging plate offers speed several ten times faster than that of the Gd/film assembly with comparable image quality [11-13]. The light-emitting screen coupled with a digital camera with light sensitivity from ISO 1600 and time integration mode makes non-film neutron radiography by Cf-252 possible. An image intensifier or a microchannel plate (MCP) is useful for real-time or near real-time neutron imaging in low flux system. Examples of neutron radiographic images taken from different

neutron facilities and by different techniques are illustrated in Figures 18 to 23.

Position Base flux

Radiography reactor Radial 1012 106 250 2-5

Be(d, n); 3 MeV, 400 µA Radial 3×109 2×105 33 5-20

Be(γ, n); 5.5 MeV, 100 µA Radial 4×108 8×104 18 5-20

T(d, n)+ U; 120 keV, 7 mA Radial 108 2×104 18 5-20

252Cf ; 1 mg Radial 2×107 104 12 5-20

Table 6. Examples of common neutron radiography facilities [2, 6]

(cm-2·s-1)

Collimator Typical beam characteristics

Intensity (cm-2·s-1)

Radial 1014 108 250 2-5

Tangential 1013 107 250 10-50

Cold source 2×1011 106 100 ∞

Tangential 4×1011 2×106 100 10-50

Radial 3×109 2×105 18 5-20

L/D ratio

Cd ratio

Source

Multi-purpose research

252Cf ; 5 mg + sub-critical Reactor assembly

reactor


Remark : The relative speed for the direct and transfer methods are not comparable

Table 7. Relative Speed of Neutron Converter Screens [2]

Fig. 16. An example of Cf-252 based neutron radiography system

Neutron(Gd/film) Neutron (imaging plate) X-Ray

Fig. 19. Neutron radiographs with a floppy disk drive (above) and RS-232 connectors (below) using neutron beam from a TRIGA Mark III research reactor in comparison with

Neutron (imaging plate) X-Ray

Fig. 20. Neutron radiograph of a Buddha statue using neutron beam from a TRIGA Mark III research reactor in comparison with x-ray radiograph [13] (Clay can be clearly seen in the

Neutron(Gd/film) Neutron (imaging plate) X-Ray

middle part of the body in the neutron radiograph)

x-ray radiograph [12]

Fig. 17. An example of neutron generator based neutron radiography system

Fig. 18. A neutron radiograph of pistol bullets [14] (research reactor, Gd foil/film technique)

94 Nondestructive Testing Methods and New Applications

Fig. 17. An example of neutron generator based neutron radiography system

Fig. 18. A neutron radiograph of pistol bullets [14] (research reactor, Gd foil/film technique)

### Neutron(Gd/film) Neutron (imaging plate) X-Ray

Neutron(Gd/film) Neutron (imaging plate) X-Ray

Fig. 19. Neutron radiographs with a floppy disk drive (above) and RS-232 connectors (below) using neutron beam from a TRIGA Mark III research reactor in comparison with x-ray radiograph [12]

Neutron (imaging plate) X-Ray

Fig. 20. Neutron radiograph of a Buddha statue using neutron beam from a TRIGA Mark III research reactor in comparison with x-ray radiograph [13] (Clay can be clearly seen in the middle part of the body in the neutron radiograph)

(a) (b) Fig. 23. Neutron radiograph (b) of a hard disk drive using neutrons from a research reactor in comparison with x-ray radiograph (a) [16] (The neutron radiograph from the track-etch film was scanned by using a desktop scanner with a shiny polished metal sheet used as the

Quality of neutron radiograph is affected by various factors not only the L/D ratio and the cadmium ratio as mentioned previously but also the gamma-ray content, the geometric unsharpness, type of converter screen, type of image recording medium and film processing. The image sharpness is improved with increasing of the L/D ratio while the image contrast is improved with increasing of the cadmium ratio. The gamma-ray content in neutron beam will deteriorate the image contrast. The other factors affect the neutron radiographs in the same way as in x-ray and gamma-ray radiography. The ASTM Beam Purity Indicator (BPI) and the ASTM Sensitivity Indicator (SI) are common neutron beam quality indicators as illustrated in Figures 24 and 25 respectively. Department of Nuclear Engineering of Chulalongkorn University also developed a neutron beam quality indicator, so called "CU-NIQI", as illustrated in Figure 26. The CU-NIQI is used with a 3 mm thick lead (Pb) plate to determine gamma-ray content. The quality indicators are based on the same principles. Teflon and polyethylene are hydrogeneous materials used for indicating proportion of slow neutrons to fast neutrons. Cadmium and boron are good neutron absorbers used for indicating proportion of neutrons of energy below 0.5 eV to beyond 0.5 eV. Lead strip or wire is the indicator for gamma-ray content. Film density readings at the positions corresponding to those materials

light reflecting surface.)

**7. Quality control of neutron radiographic image** 

can be used to evaluate quality of the neutron beam.

Neutron (imaging plate) X-Ray

Fig. 21. Neutron radiograph of an ancient lacquerware using neutron beam from a TRIGA Mark III research reactor in comparison of x-ray radiograph [13] (Pattern of embroidered bamboo thread can be seen clearer in the neutron radiograph)

X-Ray Neutron

Fig. 22. Neutron radiograph of an RS-232 connector using low intensity neutrons from Cf-252 and NE426 light-emitting screen/photographic film [15]

Neutron (imaging plate) X-Ray

bamboo thread can be seen clearer in the neutron radiograph)

252 and NE426 light-emitting screen/photographic film [15]

Fig. 21. Neutron radiograph of an ancient lacquerware using neutron beam from a TRIGA Mark III research reactor in comparison of x-ray radiograph [13] (Pattern of embroidered

X-Ray Neutron Fig. 22. Neutron radiograph of an RS-232 connector using low intensity neutrons from Cf-

Fig. 23. Neutron radiograph (b) of a hard disk drive using neutrons from a research reactor in comparison with x-ray radiograph (a) [16] (The neutron radiograph from the track-etch film was scanned by using a desktop scanner with a shiny polished metal sheet used as the light reflecting surface.)

### **7. Quality control of neutron radiographic image**

Quality of neutron radiograph is affected by various factors not only the L/D ratio and the cadmium ratio as mentioned previously but also the gamma-ray content, the geometric unsharpness, type of converter screen, type of image recording medium and film processing. The image sharpness is improved with increasing of the L/D ratio while the image contrast is improved with increasing of the cadmium ratio. The gamma-ray content in neutron beam will deteriorate the image contrast. The other factors affect the neutron radiographs in the same way as in x-ray and gamma-ray radiography. The ASTM Beam Purity Indicator (BPI) and the ASTM Sensitivity Indicator (SI) are common neutron beam quality indicators as illustrated in Figures 24 and 25 respectively. Department of Nuclear Engineering of Chulalongkorn University also developed a neutron beam quality indicator, so called "CU-NIQI", as illustrated in Figure 26. The CU-NIQI is used with a 3 mm thick lead (Pb) plate to determine gamma-ray content. The quality indicators are based on the same principles. Teflon and polyethylene are hydrogeneous materials used for indicating proportion of slow neutrons to fast neutrons. Cadmium and boron are good neutron absorbers used for indicating proportion of neutrons of energy below 0.5 eV to beyond 0.5 eV. Lead strip or wire is the indicator for gamma-ray content. Film density readings at the positions corresponding to those materials can be used to evaluate quality of the neutron beam.

Diameter of the central through hole = 3 mm, diameter of hole no. 1 - 4 = 2 mm.

Fig. 26. The CU-NIQI Neutron Beam Purity Indicator developed by the Department of

Fig. 27. Illustration of a method for determining the neutron exposure by measuring

transmitted neutron intensity with a small neutron detector

In practice there is no standard procedure for constructing an exposure curve for neutron radiography as in x-ray and gamma-ray radiography. For specific application, the radiographer can first begin with trial and error to choose the best exposure. Without knowing the specimen composition and thickness to determine the neutron attenuation, the exposure cannot be obtained. The best method, so far, is to measure transmitted neutron intensity by using a small neutron detector at a few positions behind the specimen before placing the screen/film assembly. The transmitted neutron intensity is inversely

Depth of hole no. 1, 2, 3 and 4 are 1, 2, 3 and 4 mm respectively.

Nuclear Engineering of Chulalongkorn University

**8. Methods for determining neutron exposure** 

Fig. 24. The ASTM Beam Purity Indicator [2]

Fig. 25. The ASTM Sensitivity Indicator [2]

Fig. 24. The ASTM Beam Purity Indicator [2]

Fig. 25. The ASTM Sensitivity Indicator [2]

Diameter of the central through hole = 3 mm, diameter of hole no. 1 - 4 = 2 mm. Depth of hole no. 1, 2, 3 and 4 are 1, 2, 3 and 4 mm respectively.

Fig. 26. The CU-NIQI Neutron Beam Purity Indicator developed by the Department of Nuclear Engineering of Chulalongkorn University

### **8. Methods for determining neutron exposure**

In practice there is no standard procedure for constructing an exposure curve for neutron radiography as in x-ray and gamma-ray radiography. For specific application, the radiographer can first begin with trial and error to choose the best exposure. Without knowing the specimen composition and thickness to determine the neutron attenuation, the exposure cannot be obtained. The best method, so far, is to measure transmitted neutron intensity by using a small neutron detector at a few positions behind the specimen before placing the screen/film assembly. The transmitted neutron intensity is inversely

Fig. 27. Illustration of a method for determining the neutron exposure by measuring transmitted neutron intensity with a small neutron detector

**5** 

*China* 

Qian Huang and Yuan Wu *South China University of Technology* 

**Flaw Simulation in Product Radiographs** 

X-ray inspection machines have been widely used in modern industry to explore the interior of products, and to control product quality. With the advancement of automatic product inspection, we need lots of flaw radiographs to validate the system's sensitivity and to help tune the automatic inspection parameters. Flaw simulation in product radiographs has been

Before implementing the program to inspect the work pieces, a large number of sample images are needed to tune the algorithm, examine its performance, and ensure its accuracy. Product radiographs with defects from the production line are the best, however, they are often not available in sufficient quantities or variety. Simulation of casting defects is an

Recently, the CAD model method, the Monte Carlo method and the generative image

1. The CAD model method enables simulations to be produced for complex three dimensional (3D) casting objects. Ray-tracing, together with calculation of the X-ray

3. Generative image model for flaw simulation of the product radiographs has been developed from the idea of the superimposition technique, and is based on defect

In this chapter, we will discuss flaw simulation in product radiographs. The chapter is organized as follows. Section 2 introduces the CAD models for simulation, the three main parts of CAD models will be discussed including the X-ray source, the geometric and material properties of the objects, and the imaging process. Section 3 presents the applications of the Monte Carlo method for X-ray image simulation, a brief introduction of the Monte Carlo method and how to combine it in the simulation application are discussed. Section 4 concludes with the generative image model for flaw simulation in radiographs, the generative models and simulation results are outlined, the authors themselves have produced significant results in this field. Section 5 concludes and offers suggestions for

attenuation, is the basis of this model to produce 3D casting defect simulations. 2. Monte Carlo simulation is a method for iteratively evaluating a deterministic model using sets of random numbers as inputs. The creation of a proper physical model is the

receiving an increasing attention for over two decades.

method have become the three main approaches for flaw simulation.

further research. Finally, a reference list is provided for further reading.

alternative approach to deal with this problem.

most critical task for the simulation.

**1. Introduction** 

analysis.

proportional to the exposure. In doing so, the radiographer do not need to perform decay and distance corrections. This method can also be applied in x-ray and gamma-ray radiography by changing the neutron detector to x-ray/gamma-ray detector.

### **9. Acknowledgements**

The author would like to express his deepest gratitude to Mr. JateChan Channuie and Mr. Kittiwin Iaemsumang for their assistance in the preparation of the figures and the tables in this chapter. Thanks are also extended to my eldest daughter, Miss Katriya Chankow, and my wife, Mrs. Julie Chankow, for their times spent in correcting the English translation.

### **10. References**

