**2.2. Neutron activation analysis procedure**

**•** Particle accelerators or neutron generators. The most common types are based on the ac‐ celeration of deuterium ions towards a target containing either deuterium or tritium, re‐ sulting in the reactions 2H(2H,n)3He and 3H(2H,n)4He, respectively. The first reaction, often denoted as (D,D), yields monoenergetic neutrons of 2.5 MeV and typical outputs in

146 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

**•** Nuclear research reactors. The neutron energy distribution depends on design of the reac‐ tor and its irradiation facilities. An example of an energy distribution in a light water moderated reactor is given in Fig. 2.3 from which it can be seen that the major part of the neutrons has a much lower energy distribution that in isotopic sources and neutron gen‐ erators. The neutron output of research reactors is often quoted as neutron fluence rate in an irradiation facility and varies, depending on reactor design and reactor power, be‐

Owing to the high neutron flux, experimental nuclear reactors operating in the maximum thermal power region of 100 kW -10 MW with a maximum thermal neutron flux of 1012-1014 neutrons cm-2 s-1 are the most efficient neutron sources for high sensitivity activation analy‐ sis induced by epithermal and thermal neutrons. The reason for the high sensitivity is that the cross section of neutron activation is high in the thermal region for the majority of the elements. There is a wide distribution of neutron energy in a reactor and, therefore, interfer‐ ing reactions must be considered. In order to take these reactions into account, the neutron spectrum in the channels of irradiation should be known exactly. E.g. if thermal neutron ir‐

Although there are several types of neutron sources (reactors, accelerators, and radioisotopic neutron emitters) one can use for NAA, nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes due to the materials used to moderate (or reduce the energies of) the primary fission neutrons. This is further elaborated in the title "Derivation of the measurement equation". In our case, the NAA method is based on the use of neutron flux in several irradiation channels of Es-Salam Research reactor. In 2011, Ha‐ midatou L et Al., reported "Experimental and MCNP calculations of neutron flux parame‐ ters in irradiation channel at Es-Salam reactor" the core modelling to calculate neutron spectra using experimental and MCNP approaches. The Es-Salam reactor was designed for a thermal power output of 15 Mw, with 72 cylindrical cluster fuel elements; each fuel element consists of 12 cylindrical rods of low enriched UO2. In addition the both of fuel throttle tube of the cluster and fuel element tube encloses heavy water as moderator and coolant. The fuel elements are arranged on a heavy water square lattice. The core of the reactor is constituted by a grid containing 72 fuel elements, 12 rods for reactivity control and two experimental

There is also a heavy water in the middle of the core including five experimental channels called inner reflector, In addition, all fuel elements have a reflector at each end called upper

–1011 s−1.

radiations are required, the most thermalized channels should be chosen.

–1010 s−1; the second reaction (D,T) results in monoenergetic neutrons of

the order of 108

14.7 MeV and outputs of 109

tween 1015 and 1018 m-2 s-1.

channels.

In the majority of INAA procedures thermal reactor neutrons are used for the activation: neutrons in thermal equilibrium with their environment. Sometimes activation with epither‐ mal reactor neutrons (neutrons in the process of slowing down after their formation from fission of 235U) is preferred to enhance the activation of elements with a high ratio of reso‐ nance neutron cross section over thermal neutron cross section relatively to the activation of elements with a lower such a ratio. In principle materials can be activated in any physical state, viz. solid, liquid or gaseous. There is no fundamental necessity to convert solid materi‐ al into a solution prior to activation; INAA is essentially considered to be a non-destructive method although under certain conditions some material damage may occur due to thermal heating, radiolysis and radiation tracks by e.g. fission fragments and α-radiation emitting nuclei. It is essential to have more than two or three qualified full-time member of the staff with responsibility for the NAA facilities. They should be able to control the counting equip‐ ment and have good knowledge of basic principles of the technique. In addition, the facility users and the operators must establish a good channel of communication. Other support staff will be required to maintain and improve the equipment and facility. It seems, there‐ fore, a multi-disciplinary team could run the NAA system well.

The analytical procedure is based on four steps:

**Step 1:** sample preparation (Figure 3) means in most cases only heating or freeze drying, crush‐ ing or pulverization, fractionating or pelletizing, evaporation or pre-concentration, put through a sieve, homogenising, weighing, washing, check of impurities (blank test), encapsu‐ lation and sealing irradiation vial, as well as the selection of the best analytical process and the preparation of the standards. The laboratory ambiance is also important for preservation and storage of the samples. Standardization is the basis for good accuracy of analytical tools and of‐ ten depends on particular technology, facility and personnel. For production of accurate data, careful attention to all possible errors in preparing single or multi-element standards is impor‐ tant, and standards must be well chosen depending on the nature of the samples.

**Step 2:** irradiation of samples can be taken from the various types of neutron sources ac‐ cording to need and availability. For the INAA, one pneumatic transfer system installed in the horizontal channel at Es-Salam research reactor for short irradiation of samples (Figure 4). In addition, two vertical channels located in different sites of the heavy water moderator and the graphite reflector have been used for long irradiations. The neutron spectrum pa‐ rameters at different irradiation channels such as alpha, f, Tn, etc are experimentally deter‐ mined using cadmium ratio, cadmium cover, bare triple monitor and bi-isotopic methods using HΦgdhal convention and Westcott formalism Table 1 and Table 2. The calibration of the irradiation positions has been carried out to implement the k0-NAA in our laboratory.

**Figure 3.** Some instruments and materials used for the sample preparation.


**Table 1.** The parameters α, f and *r*(α) *Tn* / *T*0obtained by different methods.


**Figure 4.** Pneumatic system for short irradiations using a thermal neutron flux at Es-Salam research reactor.

Most NAA labs operate one or more hyper-pure germanium (HPGe) detectors, which oper‐ ate at liquid nitrogen temperature (77 K). Although HPGe detectors come in many different shapes and sizes, the most common shape is coaxial. These detectors are very useful for measurement of gamma rays with energies in the range from about 60 keV to 3.0 MeV. The two most important characteristics a HPGe detector are its resolution and efficiency. Other characteristics to consider are peak shape, peak-to-Compton ratio, pulse rise time, crystal di‐ mensions or shape, and price. The detector's resolution is a measure of its ability to separate closely spaced peaks in the spectrum, and, in general, the resolution is specified in terms of the full width at half maximum (FWHM) of the 122 keV photopeak of 57Co and the 1,332 keV photopeak of 60Co. For most NAA applications, a detector with 0.5 keV resolution or less at 122 keV and 1.8 keV or less at 1,332 keV is sufficient. Detector efficiency for a given

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**Table 2.** Neutron spectrum parameters in the irradiation site at es-Salam research reactor.

**Step 3:** after the irradiation the measurement is performed after a suitable cooling time (tc). In NAA, nearly exclusively the (energy of the) gamma radiation is measured because of its higher penetrating power of this type of radiation, and the selectivity that can be obtained from distinct energies of the photons - differently from beta radiation which is a continuous energy distribution. The interaction of gamma- and X-radiation with matter results, among others, in ionization processes and subsequent generation of electrical signals (currents) that can be detected and recorded.

The instrumentation used to measure gamma rays from radioactive samples generally con‐ sists of a semiconductor detector, associated electronics, and a computer-based multi-chan‐ nel analyzer (MCA/computer).

**method α f** *r***(α)** *Tn* **/** *T***<sup>0</sup>**

148 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

Cd-ratio 0.026±0.012 28.4±1.6 0.038±0.004

Bare bi-isotopic - 29.5±2.5 0.036±0.003

Average 0.027±0.010 28.8±2.0 0.037±0.003

**parameter α f Tn(°C) Rcd(Au)** *r***(α)** *Tn* **/** *T***<sup>0</sup>**

Measured value 0.027±0.010 28.8±2.0 34±1.8 2.93±0.32 0.037±0.003

**Step 3:** after the irradiation the measurement is performed after a suitable cooling time (tc). In NAA, nearly exclusively the (energy of the) gamma radiation is measured because of its higher penetrating power of this type of radiation, and the selectivity that can be obtained from distinct energies of the photons - differently from beta radiation which is a continuous energy distribution. The interaction of gamma- and X-radiation with matter results, among others, in ionization processes and subsequent generation of electrical signals (currents) that

The instrumentation used to measure gamma rays from radioactive samples generally con‐ sists of a semiconductor detector, associated electronics, and a computer-based multi-chan‐

**Table 2.** Neutron spectrum parameters in the irradiation site at es-Salam research reactor.

Cd-covered 0.024±0.010 28.7±2.1 -

**Figure 3.** Some instruments and materials used for the sample preparation.

Bare triple monitor 0.030±0.008 28.6±1.8 -

**Table 1.** The parameters α, f and *r*(α) *Tn* / *T*0obtained by different methods.

can be detected and recorded.

nel analyzer (MCA/computer).

**Figure 4.** Pneumatic system for short irradiations using a thermal neutron flux at Es-Salam research reactor.

Most NAA labs operate one or more hyper-pure germanium (HPGe) detectors, which oper‐ ate at liquid nitrogen temperature (77 K). Although HPGe detectors come in many different shapes and sizes, the most common shape is coaxial. These detectors are very useful for measurement of gamma rays with energies in the range from about 60 keV to 3.0 MeV. The two most important characteristics a HPGe detector are its resolution and efficiency. Other characteristics to consider are peak shape, peak-to-Compton ratio, pulse rise time, crystal di‐ mensions or shape, and price. The detector's resolution is a measure of its ability to separate closely spaced peaks in the spectrum, and, in general, the resolution is specified in terms of the full width at half maximum (FWHM) of the 122 keV photopeak of 57Co and the 1,332 keV photopeak of 60Co. For most NAA applications, a detector with 0.5 keV resolution or less at 122 keV and 1.8 keV or less at 1,332 keV is sufficient. Detector efficiency for a given detector depends on gamma-ray energy and the sample and detector geometry, i.e. subtend‐ ed solid angle. Of course, a larger volume detector will have a higher efficiency.

At Es-Salam NAA Lab, four gamma-ray spectrometers of Canberra for which one of them consists of a HPGe detector 35% relative efficiency connected with Genie 2k Inspector and the three other spectrometers are composed of detectors (30, 35 and 45 % relative efficiency) connected with a three Lynx® Digital Signal Analyser, It is a 32K channel integrated signal analyzer based on advanced digital signal processing (DSP) techniques. All spectrometers operate with Genie™2000 spectroscopy software. A radiation detector therefore consists of an absorbing material in which at least part of the radiation energy is converted into detecta‐ ble products, and a system for the detection of these products. Figure 5 illustrates Gammaray spectroscopy systems. The detectors are kept at liquid nitrogen temperatures (dewers under cave). The boxes in the left and in the right of the computer are the Lynx Digital Spec‐ trometer Processing.

**Figure 6.** Gamma-ray spectrum showing several short-lived elements measured in a CRM-DSD-12 standard irradiat‐ ed at Es-salam research reactor for 30 seconds, decayed for 30.7 minutes, and counted for 5 minutes with an

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The positions – often expressed as channel numbers of the memory of a multi-channel pulse height analyzer – can be converted into the energies of the radiation emitted; this is the basis for the identification of the radioactive nuclei. On basis of knowledge of possible nuclear re‐ actions upon neutron activation, the (stable) element composition is derived. The values of the net peak areas can be used to calculate the amounts of radioactivity of the radionuclides

The amounts (mass) of the elements may then be determined if the neutron fluence rate and cross sections are known. In the practice, however, the masses of the elements are deter‐ mined from the net peak areas by comparison with the induced radioactivity of the same neutron activation produced radionuclides from known amounts of the element of interest. The combination of energy of emitted radiation, relative intensities if photons of different energies are emitted and the half life of the radionuclide is unique for each radionuclide, and forms the basis of the qualitative information in NAA. The amount of the radiation is directly proportional to the number of radioactive nuclei produced (and decaying), and thus with the number of nuclei of the stable isotope that underwent the nuclear reaction. It pro‐

using the full energy photopeak efficiency of the detector.

vides the quantitative information in NAA.

HPGe detector.

**Figure 5.** Gamma-ray spectroscopy systems in NAA/CRNB laboratory.

**Step 5:** Measurement, evaluation and calculation involve taking the gamma spectra and the calculating trace element concentrations of the sample and preparation of the NAA report.

In this part of work, Peter bode describes clearly in his paper [1] the analysis procedure of gamma-spectrum to the determination of the amount of element in sample. The acquisition of gamma spectrum Fig.6 and Fig.7 via the spectroscopy system Fig. 5 is analyzed to identi‐ fy the radionuclides produced and their amounts of radioactivity in order to derive the tar‐ get elements from which they have been produced and their masses in the activated sample. The spectrum analysis starts with the determination of the location of the (centroids of the) peaks. Secondly, the peaks are fitted to obtain their precise positions and net peak areas. The Analytical protocol adopted in our NAA laboratory is presented in Fig.8.

detector depends on gamma-ray energy and the sample and detector geometry, i.e. subtend‐

150 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

At Es-Salam NAA Lab, four gamma-ray spectrometers of Canberra for which one of them consists of a HPGe detector 35% relative efficiency connected with Genie 2k Inspector and the three other spectrometers are composed of detectors (30, 35 and 45 % relative efficiency) connected with a three Lynx® Digital Signal Analyser, It is a 32K channel integrated signal analyzer based on advanced digital signal processing (DSP) techniques. All spectrometers operate with Genie™2000 spectroscopy software. A radiation detector therefore consists of an absorbing material in which at least part of the radiation energy is converted into detecta‐ ble products, and a system for the detection of these products. Figure 5 illustrates Gammaray spectroscopy systems. The detectors are kept at liquid nitrogen temperatures (dewers under cave). The boxes in the left and in the right of the computer are the Lynx Digital Spec‐

**Step 5:** Measurement, evaluation and calculation involve taking the gamma spectra and the calculating trace element concentrations of the sample and preparation of the NAA report.

In this part of work, Peter bode describes clearly in his paper [1] the analysis procedure of gamma-spectrum to the determination of the amount of element in sample. The acquisition of gamma spectrum Fig.6 and Fig.7 via the spectroscopy system Fig. 5 is analyzed to identi‐ fy the radionuclides produced and their amounts of radioactivity in order to derive the tar‐ get elements from which they have been produced and their masses in the activated sample. The spectrum analysis starts with the determination of the location of the (centroids of the) peaks. Secondly, the peaks are fitted to obtain their precise positions and net peak areas. The

Analytical protocol adopted in our NAA laboratory is presented in Fig.8.

ed solid angle. Of course, a larger volume detector will have a higher efficiency.

trometer Processing.

**Figure 5.** Gamma-ray spectroscopy systems in NAA/CRNB laboratory.

**Figure 6.** Gamma-ray spectrum showing several short-lived elements measured in a CRM-DSD-12 standard irradiat‐ ed at Es-salam research reactor for 30 seconds, decayed for 30.7 minutes, and counted for 5 minutes with an HPGe detector.

The positions – often expressed as channel numbers of the memory of a multi-channel pulse height analyzer – can be converted into the energies of the radiation emitted; this is the basis for the identification of the radioactive nuclei. On basis of knowledge of possible nuclear re‐ actions upon neutron activation, the (stable) element composition is derived. The values of the net peak areas can be used to calculate the amounts of radioactivity of the radionuclides using the full energy photopeak efficiency of the detector.

The amounts (mass) of the elements may then be determined if the neutron fluence rate and cross sections are known. In the practice, however, the masses of the elements are deter‐ mined from the net peak areas by comparison with the induced radioactivity of the same neutron activation produced radionuclides from known amounts of the element of interest. The combination of energy of emitted radiation, relative intensities if photons of different energies are emitted and the half life of the radionuclide is unique for each radionuclide, and forms the basis of the qualitative information in NAA. The amount of the radiation is directly proportional to the number of radioactive nuclei produced (and decaying), and thus with the number of nuclei of the stable isotope that underwent the nuclear reaction. It pro‐ vides the quantitative information in NAA.

**Figure 7.** Gamma-ray spectrum (**a**) from 0 to 450 keV, (**b**) from 450 to 1000 keV and (**c**) from 1000 to 2000 keV: showing medium- and long-lived elements measured in a sample of CRM-GSD-12 standard irradiated at Es-salam research reactor for 4 hours, decayed for 5 days, and counted for 90 minutes on a HPGe detector.

The measured in NAA – the quantity intended to be measured – is the total mass of a given element in a test portion of a sample of a given matrix in all physico-chemical states. The quantity 'subject to measurement' is the number of disintegrating nuclei of a radionuclide. The measurement results in the number of counts in a given period of time, from which the disintegration rate and the number of disintegrating nuclei is calculated; the latter number is directly proportional to the number of nuclei of the stable isotope subject to the nuclear reac‐ tion, and thus to the number of nuclei of the element, which finally provides information on the mass and amount of substance of that element (see Eq. 16). An example of typical ranges of experimental conditions is given in Table 3 [1].

**Figure 8.** Analytical protocol adopted in NAA/CRNB laboratory [13].

Irradiation Decay Measurement Analyzed element 5 – 30 seconds 5 – 600 seconds 15 – 300 seconds Short lived 1 – 8 hours 3 – 5 days 1 – 4 hours Medium lived

**Table 3.** Example of typical ranges of experimental conditions of an INAA procedure.

20 days 1 – 16 hours Long lived

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Test portion mass : 5-500 mg

Neutron fluence rates available 1016 – 1018 m-2 s-1

In practice, our laboratory proceeds in the treatment of spectra and calculation of elemental concentrations of analyzed samples according the approach illustrated in figure 8.

**Figure 8.** Analytical protocol adopted in NAA/CRNB laboratory [13].

(a) (b)

152 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

**Figure 7.** Gamma-ray spectrum (**a**) from 0 to 450 keV, (**b**) from 450 to 1000 keV and (**c**) from 1000 to 2000 keV: showing medium- and long-lived elements measured in a sample of CRM-GSD-12 standard irradiated at Es-salam

The measured in NAA – the quantity intended to be measured – is the total mass of a given element in a test portion of a sample of a given matrix in all physico-chemical states. The quantity 'subject to measurement' is the number of disintegrating nuclei of a radionuclide. The measurement results in the number of counts in a given period of time, from which the disintegration rate and the number of disintegrating nuclei is calculated; the latter number is directly proportional to the number of nuclei of the stable isotope subject to the nuclear reac‐ tion, and thus to the number of nuclei of the element, which finally provides information on the mass and amount of substance of that element (see Eq. 16). An example of typical ranges

In practice, our laboratory proceeds in the treatment of spectra and calculation of elemental

concentrations of analyzed samples according the approach illustrated in figure 8.

research reactor for 4 hours, decayed for 5 days, and counted for 90 minutes on a HPGe detector.

of experimental conditions is given in Table 3 [1].

(c)


**Table 3.** Example of typical ranges of experimental conditions of an INAA procedure.
