**1. Introduction**

Radiotherapy is one of the commonly used anticancer treatment methods, either applied alone or (what is more widely used) in conjunction with surgery and/or chemotherapy, as induction, supplement, or sensitizing agent. Therefore, it is administered as neoadjuvant, concurrent, or sequential (adjuvant) therapy [1–3]. In the first case, irradiation of a solid tumor may cause its shrinkage, making the subsequent surgery less extensive. In the second case, it is about to kill the cancer cell clusters too small to be seen and removed by the surgeon, limiting the risk of local recurrence or lymph node metastasis. The last issue is to gain the success of systemic chemotherapy, even in reducing treatment toxicity [1–3].

Generally, the distinction according to the localization of medically used radiation source relative to the patient's body divides radiotherapy into teletherapy—externally located radiation source, and internally located either sealed (brachytherapy) or unsealed (nuclear medicine) radiation source in the form of radioactive nuclide. The use of external beams in radiotherapy (EBT) requires increased penetration of radiation to reach deeply located tumors spearing healthy tissues at the same time. Therefore, EBT generally employs higher energies of radiation, whereas brachytherapy benefits from limited range of ionizing radiation, which helps to spear health tissues neighboring the tumor region.

In the connection with the above, external beam radiotherapy is of higher concern in terms of radiological protection and safety work around ionizing radiation devices. Special attention is paid to the construction of the treatment room, what will be discussed later.

Every ionizing radiation type could be used for the purpose of radiotherapy and each of them has its advantages and disadvantages, making their availability and applicability common or restricted to a specific cases. Limited range of electrons in the tissue makes their use practical for shallow location of tumors, especially when healthy radiosensitive tissues are located close below/behind. On the other hand, forming uniform dose distribution with high target conformity is hardly to achieve using electron beams. Neutron beam production and guidance is a task difficult enough to limit the usage of fast neutron EBT, despite of lower oxygen enhancement ratio (OER—dependence of tumor cell sensitivity on its oxygenation) and entrance/surface dose in comparison with electron beams [4]. Photon beams in the form of Bremsstrahlung radiation produce similar shape of depth dose distribution as electron and neutron beams, showing regions of build-up, dose maximum, and quasiexponential decrease; however, skin spearing effect is more pronounced and radiation is more susceptible on shaping with the use of collimation/absorption blocks. The major advantage of using photons with energies of the order of MeV is the high penetration property. Therefore, using multiple directions of incident beam, it is possible to reach with therapeutic dose the tumor surrounded by healthy tissues from the outside. However, exit dose is not negligible and region of low doses due to scattered radiation is extended. In contrary, heavy charged particles give quite different dose distribution, follow the Bragg peak shape. It means that energy transfer in case of hadrons increases with decreasing their energy, just the opposite to neutral particles and electrons [4]. Moreover, high degree of tissue spearing is achieved at localizations beneath the tumor, since physical dose behind the peak tends to be negligible, what is of high beneficial when therapy concerns structures near radiation-sensitive organs. Among the biggest disadvantages, expensiveness of accelerator technology, unsure radiobiological and physical interactions of highenergy heavy charged particles in tissues are usually mentioned.

Despite the diverse advantages of every radiation type, radiotherapy with the use of electron and photon beams remains the most widespread and widely available technology. Modern techniques for accelerating electrons are both miniaturized and efficient, what enables to generate beams with a wide range of energies, from several keV to tens of MeV. However, the trend in radiotherapy is to replace electron beams with photon (X-ray) beams when employing dynamic, intensity, and volumetric modulated delivery of therapeutic dose distribution. Electrons are more easily controlled and accelerated in short sections than heavy charged particles and as such, linear electron accelerators for medical purpose are the most common once. Even nowadays, the development of devices producing X-ray beams for radiotherapeutic use is taking place, although rather in terms of increasing the number of degree of freedom in beam delivery than in terms of new method of beam producing and controlling. This development has led X-ray machines in radiotherapy from orthovoltage Roentgen machines through classical linacs to devices such as Tomotherapy® or CyberKnife®. Among them, only linear accelerators in their

**159**

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room*

classical form are designed to produce X-rays at accelerating potentials from a quite wide range (4–25 MV). This feature enables to clinically use highly penetrating X-ray beams with mean energies of 1–6 MeV with long tail of high-energy photons up to the end-point values of 4–25 MeV, respectively. Extensive comparison of linac

From physics point of view, variety of interactions of therapeutic beams with accelerator elements, room equipment, air and human body should be taken into

• precise planning of dose distribution in patient's body requires advanced dosimetric modeling of treatment unit and taking into account majority of ionizing radiation interaction types in terms of dose deposition, scattering, and density

• there is usually a need to limit the ionizing radiation field for radiation protection purposes, which should take into account secondary radiation generated

The first one is crucial for radiotherapy beneficial outcome, whereas the second one is important as radiation safety issue. This second aspect is subjected in the

Several organizations have been releasing recommendations regarding shielding

• National Council on Radiation Protection and Measurements (NCRP) [11–14];

• American Association of Physicists in Medicine (AAPM) [15, 16];

• Institute of Physics and Engineering in Medicine (IPEM) [18, 19];

German Industrial Norms (DIN) [23–26] also serve as practical reference worldwide. The relevant reports are constantly updating to include most recent development of radiotherapy machines as well as treatment techniques, e.g., the usage of photon energies above 10 MV, dual or even triple photon energy machines, intensity modulated radiation therapy (IMRT), stereotactic body radiation therapy (SBRT), or total body irradiation (TBI), which in comparison with static simple geometry fields require more radiation in terms of linac monitor units (MUs) to be emitted to deposit therapeutic dose in planning target volume (PTV) or usage of nonstandard field dimensions. New modalities in photon radiation therapy include also more complicated irradiation geometry, i.e., more incident beam directions

• International Atomic Energy Agency (IAEA) [17];

• International Organization of Standards (ISO) [20];

• International Electrotechnical Commission (IEC) [21, 22].

The most frequently used radiotherapy machine set in oncological center includes medical electron linear accelerator (linac), brachytherapy unit (usually high dose rate (HDR) type), and CT scanner with virtual simulation option for therapy positioning purposes. Usually, several linacs are installed to secure nondisturbance of radiotherapy process. From a radiation protection point of view, shielding vault should be designed with respect to the most penetrative radiation

present work studied with the use of gamma radiation spectrometry.

*DOI: http://dx.doi.org/10.5772/intechopen.82726*

beam spectra could be found in [5–10].

by highly penetrative ionizing radiation.

type among those used in particular room.

design and radiation safety issues since 1970s:

account for two general reasons:

corrections [4];

#### *Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room DOI: http://dx.doi.org/10.5772/intechopen.82726*

classical form are designed to produce X-rays at accelerating potentials from a quite wide range (4–25 MV). This feature enables to clinically use highly penetrating X-ray beams with mean energies of 1–6 MeV with long tail of high-energy photons up to the end-point values of 4–25 MeV, respectively. Extensive comparison of linac beam spectra could be found in [5–10].

From physics point of view, variety of interactions of therapeutic beams with accelerator elements, room equipment, air and human body should be taken into account for two general reasons:


The first one is crucial for radiotherapy beneficial outcome, whereas the second one is important as radiation safety issue. This second aspect is subjected in the present work studied with the use of gamma radiation spectrometry.

The most frequently used radiotherapy machine set in oncological center includes medical electron linear accelerator (linac), brachytherapy unit (usually high dose rate (HDR) type), and CT scanner with virtual simulation option for therapy positioning purposes. Usually, several linacs are installed to secure nondisturbance of radiotherapy process. From a radiation protection point of view, shielding vault should be designed with respect to the most penetrative radiation type among those used in particular room.

Several organizations have been releasing recommendations regarding shielding design and radiation safety issues since 1970s:


German Industrial Norms (DIN) [23–26] also serve as practical reference worldwide. The relevant reports are constantly updating to include most recent development of radiotherapy machines as well as treatment techniques, e.g., the usage of photon energies above 10 MV, dual or even triple photon energy machines, intensity modulated radiation therapy (IMRT), stereotactic body radiation therapy (SBRT), or total body irradiation (TBI), which in comparison with static simple geometry fields require more radiation in terms of linac monitor units (MUs) to be emitted to deposit therapeutic dose in planning target volume (PTV) or usage of nonstandard field dimensions. New modalities in photon radiation therapy include also more complicated irradiation geometry, i.e., more incident beam directions

*Use of Gamma Radiation Techniques in Peaceful Applications*

will be discussed later.

(brachytherapy) or unsealed (nuclear medicine) radiation source in the form of radioactive nuclide. The use of external beams in radiotherapy (EBT) requires increased penetration of radiation to reach deeply located tumors spearing healthy tissues at the same time. Therefore, EBT generally employs higher energies of radiation, whereas brachytherapy benefits from limited range of ionizing radiation,

In the connection with the above, external beam radiotherapy is of higher concern in terms of radiological protection and safety work around ionizing radiation devices. Special attention is paid to the construction of the treatment room, what

Every ionizing radiation type could be used for the purpose of radiotherapy and each of them has its advantages and disadvantages, making their availability and applicability common or restricted to a specific cases. Limited range of electrons in the tissue makes their use practical for shallow location of tumors, especially when healthy radiosensitive tissues are located close below/behind. On the other hand, forming uniform dose distribution with high target conformity is hardly to achieve using electron beams. Neutron beam production and guidance is a task difficult enough to limit the usage of fast neutron EBT, despite of lower oxygen enhancement ratio (OER—dependence of tumor cell sensitivity on its oxygenation) and entrance/surface dose in comparison with electron beams [4]. Photon beams in the form of Bremsstrahlung radiation produce similar shape of depth dose distribution as electron and neutron beams, showing regions of build-up, dose maximum, and quasiexponential decrease; however, skin spearing effect is more pronounced and radiation is more susceptible on shaping with the use of collimation/absorption blocks. The major advantage of using photons with energies of the order of MeV is the high penetration property. Therefore, using multiple directions of incident beam, it is possible to reach with therapeutic dose the tumor surrounded by healthy tissues from the outside. However, exit dose is not negligible and region of low doses due to scattered radiation is extended. In contrary, heavy charged particles give quite different dose distribution, follow the Bragg peak shape. It means that energy transfer in case of hadrons increases with decreasing their energy, just the opposite to neutral particles and electrons [4]. Moreover, high degree of tissue spearing is achieved at localizations beneath the tumor, since physical dose behind the peak tends to be negligible, what is of high beneficial when therapy concerns structures near radiation-sensitive organs. Among the biggest disadvantages, expensiveness of accelerator technology, unsure radiobiological and physical interactions of high-

which helps to spear health tissues neighboring the tumor region.

energy heavy charged particles in tissues are usually mentioned.

Despite the diverse advantages of every radiation type, radiotherapy with the use of electron and photon beams remains the most widespread and widely available technology. Modern techniques for accelerating electrons are both miniaturized and efficient, what enables to generate beams with a wide range of energies, from several keV to tens of MeV. However, the trend in radiotherapy is to replace electron beams with photon (X-ray) beams when employing dynamic, intensity, and volumetric modulated delivery of therapeutic dose distribution. Electrons are more easily controlled and accelerated in short sections than heavy charged particles and as such, linear electron accelerators for medical purpose are the most common once. Even nowadays, the development of devices producing X-ray beams for radiotherapeutic use is taking place, although rather in terms of increasing the number of degree of freedom in beam delivery than in terms of new method of beam producing and controlling. This development has led X-ray machines in radiotherapy from orthovoltage Roentgen machines through classical linacs to devices such as Tomotherapy® or CyberKnife®. Among them, only linear accelerators in their

**158**

### *Use of Gamma Radiation Techniques in Peaceful Applications*

and thus, more complicated scattered radiation patterns (see: CyberKnife, Gamma Knife, or Tomotherapy).

Shielding considerations of medical linear accelerator room distinguish the following radiation groups, schematically presented in **Figure 1a**:


The characteristic design of the therapeutic room, which is schematically presented in **Figure 1b**, meets the protective requirements against all of the abovementioned radiation groups and contains:


The use of high-energy radiotherapeutic beams (E > 10 MeV) is additionally accompanied by the aspect of generation of secondary radiation, which will be widely addressed below.

#### **Figure 1.**

*(a) Schematic presentation of different radiation groups the linac room shielding must face with, available from [27]; (b) scheme of typical radiotherapy treatment vault. Medical accelerator head is able to rotationally move on 360° around the axis perpendicular to the beam (gray-line cone) central axis. The entrance door as the localization relevant for the presented study is also marked.*

**161**

**Figure 2.**

*of energy overlapping regions.*

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room*

Radiotherapeutic photon beams in the form of Bremsstrahlung radiation generated on conversion target by electrons accelerated at potentials of the order of MV have wide-energy spectrum up to energy determined by the nominal accelerating potential used. Therefore, in high-energy therapeutic beam (10–20 MV) even up to about 20% of photons could have energies above 8 MeV, which is the approximate threshold energy for photonuclear reactions. However, for tungsten (the main component of linac collimation system)—nuclear photo effect starts approximately from 5 MeV. Generally, the higher mass number of nuclide, the lower threshold energy, and for defined energy—the higher cross section for photon absorption by the nucleus is observed, what is presented in **Figure 2**. Among the products of such reactions are secondary gamma rays (when inelastic scattering occurs) or nucleons (protons, neutrons, or their groups: deuterons, alphas), when a reaction through a stage of compound nucleus has occurred. The final nucleus either nucleon-deficient (photonuclear reaction product) or nucleon-excess (neutron absorption product) could be unstable and undergo radioactive decay. From occupational radiation protection point of view, high-penetrative radiation, i.e., prompt and decay gamma rays as well as neutrons are of importance, since (1) they are able to reach entrance to the treatment room, (2) they form a significant part of radiation leakage from the

The energy range of photons and electrons used in linac radiotherapy is sufficient

to trigger a nuclear reaction via (γ/X,n), (γ/X,p), and (e,e'n) mechanisms and to observe subsequent nuclear reactions of secondary generated particles, among

*The comparison of photon beam spectra (modeled by us in commercial dose verification system) with cross sections of photon absorption nuclear reactions [28] for commonly observed activation target nuclides, in terms* 

*DOI: http://dx.doi.org/10.5772/intechopen.82726*

treatment room through the door.

#### *Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room DOI: http://dx.doi.org/10.5772/intechopen.82726*

Radiotherapeutic photon beams in the form of Bremsstrahlung radiation generated on conversion target by electrons accelerated at potentials of the order of MV have wide-energy spectrum up to energy determined by the nominal accelerating potential used. Therefore, in high-energy therapeutic beam (10–20 MV) even up to about 20% of photons could have energies above 8 MeV, which is the approximate threshold energy for photonuclear reactions. However, for tungsten (the main component of linac collimation system)—nuclear photo effect starts approximately from 5 MeV. Generally, the higher mass number of nuclide, the lower threshold energy, and for defined energy—the higher cross section for photon absorption by the nucleus is observed, what is presented in **Figure 2**. Among the products of such reactions are secondary gamma rays (when inelastic scattering occurs) or nucleons (protons, neutrons, or their groups: deuterons, alphas), when a reaction through a stage of compound nucleus has occurred. The final nucleus either nucleon-deficient (photonuclear reaction product) or nucleon-excess (neutron absorption product) could be unstable and undergo radioactive decay. From occupational radiation protection point of view, high-penetrative radiation, i.e., prompt and decay gamma rays as well as neutrons are of importance, since (1) they are able to reach entrance to the treatment room, (2) they form a significant part of radiation leakage from the treatment room through the door.

The energy range of photons and electrons used in linac radiotherapy is sufficient to trigger a nuclear reaction via (γ/X,n), (γ/X,p), and (e,e'n) mechanisms and to observe subsequent nuclear reactions of secondary generated particles, among

#### **Figure 2.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

Knife, or Tomotherapy).

in 10 × 10 cm2

rial, e.g., barite;

widely addressed below.

mentioned radiation groups and contains:

thinner than the primary barrier;

*localization relevant for the presented study is also marked.*

and thus, more complicated scattered radiation patterns (see: CyberKnife, Gamma

Shielding considerations of medical linear accelerator room distinguish the

• primary beam—therapeutically useful radiation always directed to the linac isocenter (point of gantry rotation), what means when using rotational technique, this beam might incident on four out of six walls of treatment room;

• leakage radiation—ionizing radiation of the treatment beam type leaving the linac head through unattended ways—directed from the source (e.g., Bremsstrahlung conversion target) to outside; at the linac construction stage, it should be limited to a maximum of 0.2% and an average of 0.1% of the maximum absorbed dose [22]

• scattered radiation originating from the patient body, room walls, and equip-

The characteristic design of the therapeutic room, which is schematically presented in **Figure 1b**, meets the protective requirements against all of the above-

• primary barrier, the thickest or made of concrete enriched with heavy mate-

• secondary barrier against leakage and scattered radiation, which is usually of lower intensity and/or energy than primary beam; therefore, the barrier is

• maze, as a construction protecting the entrance from direct incidence of unattenuated beam, with the purpose of lengthening the scattering radiation path.

The use of high-energy radiotherapeutic beams (E > 10 MeV) is additionally accompanied by the aspect of generation of secondary radiation, which will be

*(a) Schematic presentation of different radiation groups the linac room shielding must face with, available from [27]; (b) scheme of typical radiotherapy treatment vault. Medical accelerator head is able to rotationally move on 360° around the axis perpendicular to the beam (gray-line cone) central axis. The entrance door as the* 

ment, directed in full solid angle, with wide range of energies.

radiation field by, e.g., using beam stoppers or lead shielding;

following radiation groups, schematically presented in **Figure 1a**:

**160**

**Figure 1.**

*The comparison of photon beam spectra (modeled by us in commercial dose verification system) with cross sections of photon absorption nuclear reactions [28] for commonly observed activation target nuclides, in terms of energy overlapping regions.*

which (n,n'γ) and (n,γ) are the most commonly observed, for fast and thermalized neutrons, respectively. Every mechanism mentioned above could activate radionuclides. Nevertheless, the majority of induced radioactivity is found in construction materials of the accelerator head, mostly in heavy elements of collimation and beam shaping system. The contribution of particular elements of linac head in overall induced radioactivity is studied mostly with Monte Carlo simulations, as in: [29, 30]. However, gamma radiation spectrometry is a good tool for identification of particular radionuclides and their contribution in this phenomenon, for example, see: [31]. The apparent linac radioactivity depends on the localization of measuring point; therefore, the radiation hazard due to this phenomenon is different for patients and for the staff, with the dominant contribution of tungsten collimator or head casing, respectively [32]. Induced radioactivity has been also observed and investigated in tissues [33–36], air [37], treatment couch [38], and treatment accessories stored inside the linac room [39]. Moreover, the dependence of induced activity on the therapeutic dose rate could be observed in some cases, i.e., when half-life of radioisotope is comparable with the time of beam emission, and is more pronounced for higher nominal accelerating potentials [35].

Among the mechanisms of radionuclide activation outside the field of irradiation, neutron capture contributes the most. Linacs used nowadays are not routinely equipped with shielding constructions dedicated for neutrons; therefore, neutron fluence all over the treatment room is reported [32, 40–44] in the amount sufficient for inducing radioactivity at measurable level. Therefore, medical linear accelerators are often characterized in terms of neutron source strength Q [14, 44], which depends on beam nominal potential, as presented in **Figure 3**.

The spectrum of neutron flux undergoes changes via scattering mechanisms. Leaving the linac head, the mean energy of neutrons is of the order of 1 MeV, on treatment couch, an additional peak at thermal energies is already observed and neutrons impinging the door have an average energy of ~0.2 MeV [45, 46].

Neutron radiation weighting factor for effective dose calculation strongly depends on energy, having maximal values around 1 MeV [47]. The cross section of (n,γ) nuclear reaction follows the 1/E dependence with some resonance peaks at intermediate energies [28]. Therefore, high-energy neutrons contribute mostly to the dose, whereas slow neutrons to the phenomenon of induced radioactivity.

It is of high importance to be aware of the physical mechanisms of radiation absorption and removal from the beam. These are in principle different for various radiation types. Nevertheless, similar mechanisms might be observed for various radiation types but occurring with different efficiency.

The readily used in diagnostic radiology heavy metal shielding is no longer valid in high-energy radiotherapy rooms due to the generation of secondary

**Figure 3.**

*The comparison of neutron source strength values reported in [14]* (0)*, [44]* (□)*, and obtained by us* (Δ)*.*

**163**

Inc.).

instead.

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room*

radiation via nuclear reactions, as discussed above. Therefore, concrete, bariteconcrete, earth bricks, and similar materials are preferred during solid shielding construction. These, built of mostly light elements, for which neutron production threshold energies are relatively high, i.e., tens of MeV, should not gain the production of secondary radiation. Additional lamination of maze walls as well as using multibend geometries are the solutions advised for increasing the neutron absorption before reaching the entrance. These solutions help to slim down the room door or even built the door-less entrance, minimizing secondary radiation at the entrance [48]. Typical door construction contains the most inner layer of neutronabsorption material (polyethylene, paraffin, or borax), enclosed with heavy photon-absorption layer (lead, tungsten) coated with industrial material, typically of stainless steel or wood. Unfortunately, to maintain an acceptable mechanics/ kinetics of the door, the weakness of this radiation barrier must be accepted. Therefore, from radiation protection point of view, the vicinity of entrance to the treatment room is not an advised place for staying during radiotherapy beam emission as a location with increased occupational radiation hazard. The standard radiometric methods used in such case could seriously underestimate the radiation indications since they are calibrated on 60Co or 137Cs sources. Although average energy of leakage/scattered radiation reaching the entrance door is close to the energies of these radionuclide sources, prompt gamma rays produced during neutron capture are much more energetic (over a dozen of MeV), therefore, detected with very low efficiency by these devices. Moreover, standard spectrometric range of detected energies is aimed at measuring decay gamma rays up to about 3 MeV and therefore omits significant range of prompt gammas. That is the reason which makes gamma spectrometry with extended energy range to be adequate for more precise investigation of the occupational radiation hazard near the entrance door

**2. Semiconductor spectrometry and its application for radiation** 

Semiconductor high-purity germanium (HPGe) detectors are most suitable for the investigation of gamma radiation spectra of unknown origin since the excellent energy resolution enables the exact identification of any radionuclide, which contributes to the radiation field in measured localization. Their use is, however, limited due to the need of liquid nitrogen cooling; therefore, scintillation or roomtemperature semiconductor detectors, both with limited resolution, are used

The spectrometric system used in this study, as shown in **Figure 4**, consists of:

• coaxial HPGe detector with reversed electrodes (ReGe), manufactured by Canberra Inc., having 40% relative efficiency and characterized by the resolution of 2.1 keV FWHM @ 1332 keV; the use of a standard spectrometric gain of

• InSpector™ 2000 MultiChannel Analyzer (MCA) with 8194 channels;

• Genie™ 2000 v.3.2.1 Gamma Acquisition and Analysis Software (Canberra

The carbon-composite entrance window enables the registration of low-energy photons (above 7 keV). The end-point energy of measured spectra has been set

5.0 enables for spectra registration up to 3.2 MeV;

*DOI: http://dx.doi.org/10.5772/intechopen.82726*

to the high-energy medical linac room.

**characterization**

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room DOI: http://dx.doi.org/10.5772/intechopen.82726*

radiation via nuclear reactions, as discussed above. Therefore, concrete, bariteconcrete, earth bricks, and similar materials are preferred during solid shielding construction. These, built of mostly light elements, for which neutron production threshold energies are relatively high, i.e., tens of MeV, should not gain the production of secondary radiation. Additional lamination of maze walls as well as using multibend geometries are the solutions advised for increasing the neutron absorption before reaching the entrance. These solutions help to slim down the room door or even built the door-less entrance, minimizing secondary radiation at the entrance [48]. Typical door construction contains the most inner layer of neutronabsorption material (polyethylene, paraffin, or borax), enclosed with heavy photon-absorption layer (lead, tungsten) coated with industrial material, typically of stainless steel or wood. Unfortunately, to maintain an acceptable mechanics/ kinetics of the door, the weakness of this radiation barrier must be accepted. Therefore, from radiation protection point of view, the vicinity of entrance to the treatment room is not an advised place for staying during radiotherapy beam emission as a location with increased occupational radiation hazard. The standard radiometric methods used in such case could seriously underestimate the radiation indications since they are calibrated on 60Co or 137Cs sources. Although average energy of leakage/scattered radiation reaching the entrance door is close to the energies of these radionuclide sources, prompt gamma rays produced during neutron capture are much more energetic (over a dozen of MeV), therefore, detected with very low efficiency by these devices. Moreover, standard spectrometric range of detected energies is aimed at measuring decay gamma rays up to about 3 MeV and therefore omits significant range of prompt gammas. That is the reason which makes gamma spectrometry with extended energy range to be adequate for more precise investigation of the occupational radiation hazard near the entrance door to the high-energy medical linac room.
