**3. Nuclear reactions induced by neutrons**

22 Gamma Radiation

centers for holes and electrons, and may create new donor and acceptor states, thus gradually changing the charge collection efficiency, the resolution, and the pulse timing characteristics of the detector. The n-type HPGe detectors are preferable in applications that involve neutron irradiation. They have been shown to be more resistant to damage by fast neutrons (Pehl et al., 1979). The neutron damage problem requires special attention and treatment (Fourches et al., 1991). The speed of the HPGe charge collection is another parameter to be considered in high count rate conditions and applications that require

The comparison of selected gamma ray detectors used in neutron-based material analysis

The shielding is required to protect the gamma ray detector from direct hit by the neutrons. Shielding size defines the geometry of the system since a neutron source and a gamma ray detector are separated by the shielding column. The combination of materials with large scattering cross sections for fast neutrons and large low energy neutron capture cross sections, and high Z materials with high stopping power for gamma rays is used. The goal is to keep fast neutrons away from the detector volume either by redirecting their path or moderating them with the subsequent capture. The d-d or d-t targets are in general of the "point source" type, thus the shielding may have a conical shape to minimize the weight. For 108-n/s d-t source, the simplest "shadow" shielding is a layered conical structure of ~50 cm length; the 30-40 cm borated polyethylene layer near the source, and the 10-20 cm lead layer near the gamma ray detector (Womble et al., 2003). The more complex shielding designs are possible using layers of other materials, but the size / weight / cost considerations add design limitations. In addition, the detector may be also shielded from lower energy neutrons scattered from surrounding materials. The two-layer shielding can reduce spectral noise due to low energy neutron interactions with the detector crystal. The outer layer of borated resin is effective as a thermal neutron shielding; the inner lead layer attenuates photons emitted from thermal neutron capture reactions in the outer layer. The lead also attenuates low energy photons that are not of interest in material analysis thus helping to reduce dead time of the gamma ray

%FWHM @ 662 keV Efficiency Cooling Neutron

NaI(Tl) ~7% Fair Medium No Activated,

LaBr3(Ce) 2.8% Good Medium No Activated,

interrogation. Standard analog and digital spectroscopy solutions are typically used.

Data acquisition electronics used with the gamma ray detectors in such systems should be appropriate for the detector's signal processing and count rates attainable in neutron

HPGe 0.4% Excellent Medium LN2 Temp. No Table 1. Gamma ray detectors used in neutron-based material analysis applications

Bi4Ge3O12 ~10% Fair High No, temp.

activation issues

beta-decay

beta-decay

shifts No

good timing resolution (Cooper & Koltick, 2001).

applications is shown in Table 1.

spectroscopy system.

Detector Energy resolution,

Neutrons emitted in d-d (En=2.45 MeV) and d-t (En=14.1 MeV) fusion reactions are highly penetrating particles. The typical range is several feet into materials commonly utilized in industry and commerce. Nuclear reactions energetically possible under 14.1-MeV fusion neutron's action in the volume of the irradiated object are the following: (n,n'), (n,), (n,), (n,p), (n,d), (n,t), (n,2p), (n,n'p), (n,n'), (n,3He), and (n,2n). If the sample contains heavy nuclei, (n,3n) and nuclear fission reactions may be induced with the low probability. Production of charged particles is prevailing for light nuclei; neutron production is favourable for heavier nuclei. The reactions (n,d) and (n,t) have noticeable cross-section for light mass isotopes, but products produced in such reactions are stable. The (n,d) and (n,t) reaction cross sections for medium and heavier mass nuclei are low.

Widely used in material analysis neutron induced nuclear reactions are inelastic neutron scattering (n,n'), thermal neutron capture (n,), and neutron activation (n,) and (n,p). The only source of fast neutrons is a fusion neutron source. Thermal neutrons are created by slowing down the fast source neutrons in collisions with low Z materials within the sample itself or within the environment around the sample, or by using neutron moderating materials.


Table 2. 14.1-MeV neutron induced nuclear reaction cross sections (in millibarns): tot – the total neutron cross-section; inl – the inelastic neutron cross-section; n-n' 1st level – the (n,n') cross-section which excites the nucleons to the first nuclear level; n-n' 2nd level – the (n,n') cross-section which excites the nucleons to the second nuclear level; n-n' 3rd level – the (n,n') cross-section which excites the nucleons to the third nuclear level; n,– the (n,) crosssection; and n,p – the (n,p) cross-section


Table 3. 2.45-MeV neutron induced nuclear reaction cross sections (in millibarns)

Material Analysis Using Characteristic Gamma Rays Induced by Neutrons 25

The representative set of elements (H, C, N, O, F, P, S, Cl, and As) is selected as an example of isotopes found in explosive, chemical threats, coal and other materials. The parameters of the d-t neutron induced nuclear reactions are shown in Table 2. Table 3 shows the d-d neutron induced nuclear reactions at En=2.45 MeV for the same set of elements. The (n,) thermal neutron capture reaction's cross-section at En=0.025 eV are shown in Table 4. The total neutron cross-sections tot for C, N, O, F, P, S, Cl, and As are shown in Fig.2. The neutron data for other isotopes are available from website of the Nuclear Information Service of the Los Alamos National Laboratory at http://t2.lanl.gov/data/data.html.

The (n,2n) reaction is also utilized to produce the excited energy states causing the delayed beta decay with associated photon emission that may be non-fingerprint in nature, but it may assist to identify the amount of a parent isotope in the sample. The fast neutron activation of this type can be used for example to measure amount of nitrogen in a sample via reaction 14N(n,2n)13N. The produced 13N isotope has t1/2=10.1 minutes emitting positrons. They annihilate immediately with electrons in the sample matrix emitting 511 keV gamma rays. Although it is not characteristic photon energy, it indicates the presence of a positron emitter. If measured correctly in time and associated with t1/2 of 13N isotope, this signature can be used in material analysis. The issue of such approach is the possibility of the beta annihilation photon's emission by other 13N-producing parent nuclei. For example, elements that may cause neutron based production of 13N are boron and oxygen. The 1.47- MeV alpha particles may be initiated by thermal neutrons via 10B(n,)7Li reaction, producing 511-keV photons through the 10B(,n)13N reaction. Knockout protons of high energy produced by fast neutrons may initiate the 5.5-MeV-threshold 16O(p,)13N reaction. The 63Cu(n,2n)62Cu reaction can produce the positron emitter 62Cu → 62Ni + e+ (t1/2=9.8 minutes). So, the 511-keV annihilation photons emitted from copper and nitrogen nuclei have a close half-life values. Thus the use of other gamma ray signatures utilizing other reactions in conjunction with the positron annihilation would be beneficial in the material

As a result of nuclear reactions involving the isotopes contained in the object under scrutiny, exited nuclei emit gamma rays with specific energies in the de-excitation process. They act as the "fingerprints" of these isotopes. Most -rays are emitted promptly after the reaction. The "prompt" photon emission from excited nucleus occurs within approximately 10-9 seconds after initial excitation. However, in some cases, a nucleus with a half-life of a few seconds to a couple of minutes is formed. This radioactive nucleus decays to a daughter nucleus emitting various particles (, β+, β-, etc.) and delayed photons. The prompt gamma ray emission occurs either in the single transition as it happens in the case of hydrogen 2.223-MeV gamma rays, or through several transitions emitting many prompt -rays of lower energy. The examples of energy level schemes for

analysis.

**4. Characteristic gamma radiation** 

12C and 16O nuclei are shown in Fig.3.

Isotope 1H 12C 14N 16O 19F 31P 32S 35Cl 75As th 332.7 3.5 79.8 0.2 9.7 172.7 548.1 33070.2 4528.3 Table 4. Thermal neutron capture reaction cross sections at En=0.025 eV (in millibarns)

Fig. 2. Neutron cross sections (n, total) for C, N, O, F, P, S, Cl, and As

Fig. 2. Neutron cross sections (n, total) for C, N, O, F, P, S, Cl, and As


Table 4. Thermal neutron capture reaction cross sections at En=0.025 eV (in millibarns)

The representative set of elements (H, C, N, O, F, P, S, Cl, and As) is selected as an example of isotopes found in explosive, chemical threats, coal and other materials. The parameters of the d-t neutron induced nuclear reactions are shown in Table 2. Table 3 shows the d-d neutron induced nuclear reactions at En=2.45 MeV for the same set of elements. The (n,) thermal neutron capture reaction's cross-section at En=0.025 eV are shown in Table 4. The total neutron cross-sections tot for C, N, O, F, P, S, Cl, and As are shown in Fig.2. The neutron data for other isotopes are available from website of the Nuclear Information Service of the Los Alamos National Laboratory at http://t2.lanl.gov/data/data.html.

The (n,2n) reaction is also utilized to produce the excited energy states causing the delayed beta decay with associated photon emission that may be non-fingerprint in nature, but it may assist to identify the amount of a parent isotope in the sample. The fast neutron activation of this type can be used for example to measure amount of nitrogen in a sample via reaction 14N(n,2n)13N. The produced 13N isotope has t1/2=10.1 minutes emitting positrons. They annihilate immediately with electrons in the sample matrix emitting 511 keV gamma rays. Although it is not characteristic photon energy, it indicates the presence of a positron emitter. If measured correctly in time and associated with t1/2 of 13N isotope, this signature can be used in material analysis. The issue of such approach is the possibility of the beta annihilation photon's emission by other 13N-producing parent nuclei. For example, elements that may cause neutron based production of 13N are boron and oxygen. The 1.47- MeV alpha particles may be initiated by thermal neutrons via 10B(n,)7Li reaction, producing 511-keV photons through the 10B(,n)13N reaction. Knockout protons of high energy produced by fast neutrons may initiate the 5.5-MeV-threshold 16O(p,)13N reaction. The 63Cu(n,2n)62Cu reaction can produce the positron emitter 62Cu → 62Ni + e+ (t1/2=9.8 minutes). So, the 511-keV annihilation photons emitted from copper and nitrogen nuclei have a close half-life values. Thus the use of other gamma ray signatures utilizing other reactions in conjunction with the positron annihilation would be beneficial in the material analysis.

#### **4. Characteristic gamma radiation**

As a result of nuclear reactions involving the isotopes contained in the object under scrutiny, exited nuclei emit gamma rays with specific energies in the de-excitation process. They act as the "fingerprints" of these isotopes. Most -rays are emitted promptly after the reaction. The "prompt" photon emission from excited nucleus occurs within approximately 10-9 seconds after initial excitation. However, in some cases, a nucleus with a half-life of a few seconds to a couple of minutes is formed. This radioactive nucleus decays to a daughter nucleus emitting various particles (, β+, β-, etc.) and delayed photons. The prompt gamma ray emission occurs either in the single transition as it happens in the case of hydrogen 2.223-MeV gamma rays, or through several transitions emitting many prompt -rays of lower energy. The examples of energy level schemes for 12C and 16O nuclei are shown in Fig.3.

Material Analysis Using Characteristic Gamma Rays Induced by Neutrons 27

Demidov and colleagues in the IAEA document INDC-CCP-120 (Demidov et al., 1978). The gamma ray spectra from inelastic scattering were measured for all elements except unstable

The gamma spectrum obtained from ammonium nitrate sample irradiated with a d-t source is shown in Fig.4. Spectrum was measured using HPGe detector. We would like to note the spectral feature of Doppler broadening that is specific for photons induced by 14.1-MeV neutrons on light nuclei. The two inset expanded spectra in Fig.4 show the gamma ray from 12C(n,n')12C, 4.438 MeV and 16O(n,n')16O, 6.13 MeV. It is readily apparent that the gamma ray peak from 12C is much wider than the gamma ray peak from 16O. Other causes of this widening such as electronic noise, crystal damage due to neutron irradiation can be dismissed since 16O does not have any evidence of the broadening. The broadening of the gamma ray peaks for light nuclei was studied in (Womble et al., 2009). The energy levels of the nucleus have different spins and parities, and the state's life times. For example, in 16O, the 2nd excited state with the energy of 6.13 MeV has a half-life of 18.4 ps (see Fig.3). The 3rd excited state has a half-life nearly 2000 times shorter (8.3 fs). Energies of these two states are close to each other, but the 2nd excited state to ground state transition is 3-0+ and the 3rd excited state to ground state transition is 2+0+. Thus the difference in half-life is due to the transition probability of producing E3 radiation versus E2 radiation. The half-life time of 4.43-MeV level in 12C is 42 fs. Carbon and oxygen nuclei recoiling in inelastic neutron scattering reactions under 14.1-MeV neutrons have similar stopping times moving in the matrix of the sample; for example, approximately 1800 fs for the NH4NO3 sample. Therefore 12C nucleus may emit photon while in motion exhibiting the Doppler broadening effect for the 4.43-MeV peak, but 16O nucleus is stopped before the emission of the 6.13-MeV gamma ray and therefore does not experience the peak broadening in the measured spectrum.

Fig. 4. d-t neutron induced gamma ray spectrum for ammonium nitrate

isotopes and noble gases.

Fig. 3. Energy level schemes for 12C and 16O


Table 5. Characteristic gamma rays

The intensities of the obtained specific gamma rays provide information about the number of atoms in the sample. Hence, the information on its chemical composition can be extracted from the measured gamma ray spectrum. The list of isotopes, nuclear reactions, and energies of most prominent characteristic gamma rays are shown in Table 5. Emitted due to neutron induced reactions photons are highly penetrating. For example, energy of gamma rays emitted from nuclear reactions on nuclei of carbon, oxygen, and nitrogen isotopes is between 4 and 11 MeV. Table 5 is not inclusive. The prompt gamma rays for other elements can be found in the following libraries. The prompt gamma rays from thermal neutron captures (n,) are catalogued in the library for natural elements (Lone et al., 1981). It lists the prompt gamma ray energies in the range from 23 keV to 10829 keV for all isotopes, in terms of gamma rays emitted per 100 neutron radiative captures. These data are also available online from the National Nuclear Data Center (Brookhaven National Laboratory) at http://www.nndc.bnl.gov/capgam/. Zhou Chunmei compiled the thermal neutron capture data for nuclides with A>190 (Chunmei, 2001) and new evaluation data for thermal neutron capture for elements A=1-25: level properties, prompt gamma rays, and decay scheme properties (Chunmei, 2000). The experimental data on the (n,n') photons are compiled by

730, 1634, 2313

197, 1236, 1348, 1357 582, 2453, 3589

841, 2380, 3221, 5420

199, 265, 280, 573 165, 472, 1534, 6810

1266, 2028, 2233 636, 2154, 3900, 6785

1273, 2230

35Cl (n,) 788, 1165, 1951, 1959, 6111, 7414

The intensities of the obtained specific gamma rays provide information about the number of atoms in the sample. Hence, the information on its chemical composition can be extracted from the measured gamma ray spectrum. The list of isotopes, nuclear reactions, and energies of most prominent characteristic gamma rays are shown in Table 5. Emitted due to neutron induced reactions photons are highly penetrating. For example, energy of gamma rays emitted from nuclear reactions on nuclei of carbon, oxygen, and nitrogen isotopes is between 4 and 11 MeV. Table 5 is not inclusive. The prompt gamma rays for other elements can be found in the following libraries. The prompt gamma rays from thermal neutron captures (n,) are catalogued in the library for natural elements (Lone et al., 1981). It lists the prompt gamma ray energies in the range from 23 keV to 10829 keV for all isotopes, in terms of gamma rays emitted per 100 neutron radiative captures. These data are also available online from the National Nuclear Data Center (Brookhaven National Laboratory) at http://www.nndc.bnl.gov/capgam/. Zhou Chunmei compiled the thermal neutron capture data for nuclides with A>190 (Chunmei, 2001) and new evaluation data for thermal neutron capture for elements A=1-25: level properties, prompt gamma rays, and decay scheme properties (Chunmei, 2000). The experimental data on the (n,n') photons are compiled by

1885, 5269, 5298, 10829, 10318

Fig. 3. Energy level schemes for 12C and 16O

14N (n,n')

19F (n,n')

31P (n,n')

32S (n,n')

75As (n,n')

Table 5. Characteristic gamma rays

Isotope Reaction E, keV 1H (n,) 2223 12C (n,n') 4438

16O (n,n') 5618, 6129

(n,)

(n,)

(n,)

(n,)

(n,)

Demidov and colleagues in the IAEA document INDC-CCP-120 (Demidov et al., 1978). The gamma ray spectra from inelastic scattering were measured for all elements except unstable isotopes and noble gases.

The gamma spectrum obtained from ammonium nitrate sample irradiated with a d-t source is shown in Fig.4. Spectrum was measured using HPGe detector. We would like to note the spectral feature of Doppler broadening that is specific for photons induced by 14.1-MeV neutrons on light nuclei. The two inset expanded spectra in Fig.4 show the gamma ray from 12C(n,n')12C, 4.438 MeV and 16O(n,n')16O, 6.13 MeV. It is readily apparent that the gamma ray peak from 12C is much wider than the gamma ray peak from 16O. Other causes of this widening such as electronic noise, crystal damage due to neutron irradiation can be dismissed since 16O does not have any evidence of the broadening. The broadening of the gamma ray peaks for light nuclei was studied in (Womble et al., 2009). The energy levels of the nucleus have different spins and parities, and the state's life times. For example, in 16O, the 2nd excited state with the energy of 6.13 MeV has a half-life of 18.4 ps (see Fig.3). The 3rd excited state has a half-life nearly 2000 times shorter (8.3 fs). Energies of these two states are close to each other, but the 2nd excited state to ground state transition is 3-0+ and the 3rd excited state to ground state transition is 2+0+. Thus the difference in half-life is due to the transition probability of producing E3 radiation versus E2 radiation. The half-life time of 4.43-MeV level in 12C is 42 fs. Carbon and oxygen nuclei recoiling in inelastic neutron scattering reactions under 14.1-MeV neutrons have similar stopping times moving in the matrix of the sample; for example, approximately 1800 fs for the NH4NO3 sample. Therefore 12C nucleus may emit photon while in motion exhibiting the Doppler broadening effect for the 4.43-MeV peak, but 16O nucleus is stopped before the emission of the 6.13-MeV gamma ray and therefore does not experience the peak broadening in the measured spectrum.

Fig. 4. d-t neutron induced gamma ray spectrum for ammonium nitrate

Material Analysis Using Characteristic Gamma Rays Induced by Neutrons 29

neutron beam also has a small angular divergence and estimates (Strellis, 2009) are that the

PFNA systems can be used to screen very large cargo shipments such as tractor-trailer shipping containers and airport shipping containers. A large, 2-dimensional array of NaI

The energies of the gamma rays emitted from the object are plotted against the TOF of the neutron. This creates a two-dimensional array of data that looks similar to a spectrogram in that the intensity of the gamma ray at a particular TOF is represented by a color using the RGB color scheme. In this array of data, color bands parallel to the TOF axis indicate constant gamma ray background such as from normally occurring radioactive materials (NORM). Color bands parallel to the energy axis represent the gamma ray spectra of volume elements ("voxels") within the object of interest. The volume element size is based on the time resolution of the system so the voxels are approximately 5-cm thick. For example, in the center of the container the voxel is 9 cm × 12 cm × 5 cm. The small voxel size increases the SNR of the system. Another benefit is that the lifetime of certain activation products can

At early development stage, price and size were the drawbacks of using PFNA. However, since the 9/11 attacks the main challenge is the system cost. The cost includes installation and maintenance of this complex system. In 2009, there was a single system working at the George Bush Intercontinental Airport (Strellis, 2009). As of this writing, we are aware of no

The Pulsed Fast/Thermal Neutron Analysis (PFTNA) is a technique used in conjunction with small, portable electronic neutron generators. It was originally developed by George Vourvopoulos, Phillip Womble, and Frederick Schultz and presented in (Womble et al., 1995). Unlike PFNA, which has pulse duration of approximately 2 ns, PFTNA employs pulses with a minimum duration of 5 s. Longer pulse duration significantly reduces cost of PFTNA systems. The PFNA system can be used in a "macro-pulse" mode, in which the neutron beam is turned off for a period of 100 s. This "macro-pulse" mode mimics the

The advantage of the PFTNA systems is an ability to separate the gamma ray spectrum of inelastic scattering reactions (n,n') from thermal neutron capture (n,) and activation reactions (e.g. (n,p)) gamma-ray spectra. The data acquisition system collects data during the neutron pulse at one memory address and then switches to another memory address to acquire data between pulses. The data collected during the pulse is primarily from (n,n') reactions and the data collected between pulses is primarily from (n, reactions. Often systems are designed to be shut off for a few minutes to collect short-lived activation products such as 16O(n,p) (t1/2 ≈ 16 s). It is a common misconception that the frequency and duration of the neutron pulses is chosen to maximize the data from the (n,n') reactions. In fact these parameters are chosen to maximize the (n,) reactions or more precisely the thermal neutron flux. The neutron pulse frequency determines whether the thermal neutron flux is kept near constant or if it is allowed to diffuse. Applications such as differential die-

gamma ray detectors covers the cross-section area of the object under scrutiny.

be measured and this gives more data upon which to identify the material.

other installations.

PFTNA system's mode.

**5.3 Pulsed fast / thermal neutron analysis** 

beam is 9 cm × 12 cm in the center of the object of interest.
