Measurement of Internal Conversion Electrons from Gd Neutron Capture

Nuclear Instruments and Methods in Physics Research A 705 (2013) 36–41

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Measurement of internal conversion electrons from Gd neutron capture

P. Kandlakunta, L.R. Cao n, P. Mulligan

Nuclear Engineering Program, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210, USA article info abstract

Article history: Gadolinium (Gd) is a suitable material for neutron conversion because of its superior neutron Received 21 September 2012 absorption cross-section. However, the principal secondary particles that generate electron-hole pairs Accepted 10 December 2012 in a semiconductor detector after Gd neutron capture are low-energy internal conversion (IC) electrons. Available online 20 December 2012 We measured the IC electron spectrum due to Gd neutron capture by using a thermal neutron beam Keywords: and a digitizer-based multidetector spectroscopy. We also discussed the effective use of the IC electrons Gadolinium neutron absorption in the context of a twin-detector design and the associated gamma-ray rejection issues. Extensive Gamma-ray rejection simulations of the spectra of IC electrons and gamma rays agreed well with the experimental results; Internal conversion electrons both types of results support the feasibility of the proposed n–g separation method. Semiconductor & 2012 Elsevier B.V. All rights reserved.

1. Introduction release of an average of 3.288 photons. These photons have a wide range of energies with a mean of 2.394 MeV [5]. A large number Compressed 3He gas is widely used as the standard medium of internal conversion (IC) electrons are emitted owing to the for detecting neutrons, primarily because of its large thermal large change in the angular momentum of the low-lying excited neutron cross-section and insensitivity to gamma radiation. The states of 158Gd*. Following IC, the Gd atom relaxes to the ground high demand for 3He in homeland security and nonproliferation state by emitting Auger electrons and characteristic X-rays. In this applications, and its widespread use in areas such as cryogenics study, we focus on the use of Si detectors to capture IC electrons [1], spallation neutron sources [2], and magnetic resonance created by neutron capture in Gd. imaging [3] have caused a long-standing shortage of 3He. Thus, Although abundant prompt gamma rays are emitted from an a replacement is urgently needed. Alternatives in which 10Bor6Li excited Gd* nucleus [6], they are mostly of high energy and compounds are applied as coatings are fundamentally unable to therefore almost transparent to a thin-film semiconductor detec- achieve a high intrinsic efficiency because of the conflict between tor. Thus, IC electrons are the principal neutron signal generator the comparatively long neutron mean free paths relative to the in the detector. In this study, we measured the IC electron energy short ranges of charged particles in the neutron conversion spectrum using a newly built 10 mm diameter thermal neutron media. As a low-density gas, 3He is less constrained by this beam at the Ohio State University Research Reactor (OSURR) conflict. This problem could be overcome by using gadolinium and digitizer-based multidetector spectroscopy system [7].We (Gd) as a neutron convertor because of its extremely high cross- also discuss the effective use of the IC electrons in the context of a section and Q value [4].The two most abundant isotopes of Gd and twin-detector design and the associated gamma-ray rejection their corresponding nuclear reactions induced by thermal neu- issues. trons are described in the following equations: 157 -158 n-158 nþ 64 Gd 64 Gd 64 Gdþgþconversion electronþ7:9 MeV, 2. IC electrons and range s0 ¼ 253,929 b ð1Þ

n The most intense IC electrons emitted during de-excitation of nþ 155Gd-156Gd -156Gdþgþconversion electronþ8:5 MeV, 64 64 64 158Gd* and 156Gd* are listed in Table 1. The total yield of IC

s0 ¼ 60,800 b ð2Þ electrons per neutron capture in Gd is 0.5991, as suggested by Harms and McCormack [8].The IC electrons do not have a wide 157 Following the absorption of a neutron by the Gd nucleus, for spectrum of energies and come primarily from the lowest two example, several isomeric transitions occur, which result in the energy-level transitions of the excited 158Gd* nucleus. Depending on whether the IC is on the K, L, or M shell, electrons are emitted n Corresponding author. Tel.: þ1 614 247 8701. in the energy range of 29–246 keV, with the most intense electron E-mail address: [email protected] (L.R. Cao). emission occurring at 71 keV.

0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.12.077 P. Kandlakunta et al. / Nuclear Instruments and Methods in Physics Research A 705 (2013) 36–41 37

Table 1 literature [10–13]. The continuous slowing down approximation IC electrons emitted during the de-excitation of Gd* nucleus [8]. (CSDA) estimates the electron range as the average of the total path length traveled by an electron as it moves through the IC electron energy (keV) Yield per IC electron energy (keV) Yield per from 158Gd* neutron from 156Gd* neutron material. The maximum penetration depth (MPD) of an electron is the straight-line distance between the beginning of the electron’s 29 0.0982 39 0.0419 track and its end. We evaluated the ranges of IC electrons from 71 0.268 81 0.0497 158Gd*, in various definitions, in Gd, Si, and polyethylene, both 78 0.0617 88 0.0116 theoretically and by Monte Carlo simulations. The CSDA was 131 0.0341 149 0.0084 173 0.0146 191 0.003 computed by the integral of the inverse stopping power [11] and 180 0.0031 198 0.0006 two Monte Carlo codes were used, i.e., the Penetration and Energy 228 0.004 246 0.0002 Loss of Positrons and Electrons (PENELOPE) [14] for the average Total yield 0.4837 Total yield 0.1154 path length and the Monte Carlo Simulation of Electrons Trajec- tory in Solids (CASINO) [15] for the MPD. All the values are tabulated in Table 2. The theoretical CSDA values of the electron range in these materials were computed using an effective atomic number and effective atomic weight for the material. The analytical expres- sions for both were extracted from Tabata et al [12]. As shown in Table 2, the average path lengths of electrons in the material are always larger than the MPD owing to multiple scattering deflec- tions [16]. The theoretical values and the average path length derived from PENELOPE agree because both deal with the total path length. As an example, the distribution of the MPD and the average path length of 71 keV electrons (from 158Gd*)ina50mm-thick Si detector are shown in Fig. 2. Si was chosen because the range of IC electrons in the detector material is the most important Fig. 1. Gamma-ray rejection scheme with a twin-detector configuration, a Gd foil, parameter. In contrast to a detector in which 10Bor6Li is applied, and a polyethylene layer. where the neutron-induced charged particles follow a straight, short path, the effective path length of electrons in a detector (e.g. Although the small volume and low Z of a thin-film semi- Si) is twice as long as its average MPD. This indicates a longer conductor detector make it relatively insensitive to external gamma rays, those low-energy external gamma rays that fall Table 2 within the IC electron energy range could lead to false positive Ranges of IC electrons in Si, Gd, and polyethylene emitted in Gd neutron capture neutron detection. One such application where this could present according to various range definitions (all units in mm). an issue is in special nuclear material detection, which typically occurs in environments that are ‘‘neutron signal starved’’, but Energy Gadolinium Silicon Polyethylene ‘‘gamma signal abundant’’ [9]. To mitigate the likelihood of a false (keV) Av. CSDA Av. Av. CSDA Av. Av. CSDA Av. detection due to gamma rays, a twin-detector scheme is proposed path MPD path MPD path MPD using Gd as a neutron convertor, and two detectors to identify length length length and reject external gamma rays, as shown in Fig. 1. Rejection is achieved by introducing a layer of Gd foil and another layer of an 29 4.47 4.95 1.2 9.33 9.80 4.13 16.3 18.6 10.7 71 20.1 23.4 5.03 43.8 49.2 21.0 79.1 95.3 54.8 electron separator material (polyethylene) into the composite 78 23.5 27.6 5.90 51.4 58.4 24.7 93.1 113 64.8 detector scheme. Detector 1 is placed in direct contact with a Gd foil, allowing the detection of IC and Auger electrons, as well as gamma rays produced by neutron absorption in Gd. A second detector is placed on the back side of the Gd layer, but uses a polyethylene layer of appropriate thickness to stop all Gd gener- ated IC and Auger electrons from reaching detector 2. In this configuration, detector 2 is sensitive only to gamma rays. Hence, detector 1 generates a combined signal induced by both neutrons and gamma rays, whereas detector 2 produces a signal induced only by gamma rays. Subtracting the two detector signals would yield a net signal induced solely by neutrons. Silicon (Si) detectors were used in the following experiments and simulations, but other semiconductor materials could be used, e.g., gallium nitride. We first discuss the electron ranges in the components of this twin-detector structure (Gd, Si, and polyethylene), which are closely related to the discussions presented in this paper. Electrons with energies in the kiloelectron- volts (keV) range traveling in a solid may be scattered elastically by atomic nuclei and also through large angles primarily by radiative collisions with nuclei. The electron trajectory in a solid is therefore not a straight line (as it is with heavy charged Fig. 2. Distribution of the maximum penetration depth of 71 keV IC electrons in Si particles), but is zigzagging and tortuous. Several definitions of obtained using the CASINO simulation, and average path length of these electrons the electron range in solid materials have been reported in the in Si computed using PENELOPE. 38 P. Kandlakunta et al. / Nuclear Instruments and Methods in Physics Research A 705 (2013) 36–41 effective ionizing path of electrons in detection medium that is in 5 kW for this measurement and delivered a thermal equivalent favor of generating more electron-hole pairs in the detector. neutron flux of about 8.6 104 cm2 s1 and a gamma dose rate of about 25 mR h1. A schematic illustration of the setup is shown in Fig. 4. A thin Gd foil (1.25 cm 1.25 cm 0.0025 cm) 3. Experiments in a mixed n–c radiation field was mounted on the sample holder (Fig. 3) inside the high- vacuum chamber. The multidetector setup provided an opportunity The instrument used in the experimental setup was designed to acquire multiple spectra during one experiment. Therefore, two of for neutron depth profiling and is shown in Fig. 3. An array of the eight Si charged particle detectors were covered with 350 mm eight Si detectors were placed on individually adjustable detector thick polyethylene caps to shield them from the IC electrons, while mounts and arranged to obtain annular views of the sample with the remaining six detectors were left unshielded. The detector the same solid angle. The specifications for the detectors are given signals were acquired from the eight independent detector channels in Table 3. The detectors and eight preamplifiers were placed in using digitizer-based data acquisition electronics. A programmable a stainless steel right circular cylinder 61 cm 61 cm in size, and trapezoidal energy filter was used to process the digitized pulses a vacuum of 106 Torr was maintained during the experiments. and enabled precise determination of the pulse height. Data were A thermal neutron beam (10 mm in diameter) facility [17] acquired in list mode, in which the time stamp and pulse height provided neutrons at the OSURR, which is a 500 kW thermal, light corresponding to each detection event are recorded and stored on a water, and pool-type research reactor. The reactor operated at host PC. The data were analyzed offline to generate histograms of the detector pulse height with appropriate bin widths. As expected, the energy spectra of the six uncovered (i.e., unshielded from IC electrons) Si detectors were identical, and the energy spectra of the two covered (i.e., shielded from IC electrons) Si detectors were also identical to each other but differed from that of the six uncovered detectors. Representative energy spectra from the set of uncovered detectors and the set of covered detectors are presented in Fig. 5. A broad peak with its centroid at 71 keV is clearly seen from the two bare detectors and two much less intense peaks at 131 keV and 173 keV are also visible. The peak broadening is attributed in part to the resolution of the detector, but more to the fact that IC electrons (29–78 keV) lose energy when escaping the Gd foil before reaching the Si detector. It is interesting to note that such peak broadening is indeed the electron depth profiling Fig. 3. Experimental setup inside a high-vacuum chamber used to measure the that could be utilized to measure the thickness of a Gd thin film mixed n–g response in a Si detector. A thin Gd foil mounted on the sample holder is viewed by eight identical Si detectors; two of them are covered (shielded) with deposited on foreign substrate. This, however, is beyond the polyethylene, and the other six are left open (unshielded). discussion of this paper and will be presented in another publication. The energy spectrum from the covered detectors indicates that the 350 mm thick polyethylene layer completely Table 3 Specifications of the Si charged particle detectors used in the measurements. blocked the IC electrons while still permitting some low-energy gamma rays into the detector. A lower-level threshold of 23.3 keV Model ULTRA ion implanted (model: U016-300-100) was set in the pulse height analysis for all Si detectors, which Contact 500 A˚ boron implantation rejected radiations with energies of 23.3 keV and below. 2 Active area 300 mm The energy scale of the Si detectors and the spectroscopy system Minimum depletion depth 100 mm were carefully calibrated using button-sized 241Am and 57Co sources. Resolution (FWHM) 16 keV @ 5.486 MeV alpha The energy spectra of 241Am and 57Co obtained from one Si detector

Fig. 4. Schematic illustration of the experimental setup and the data acquisition electronics used in the mixed n–g measurements. P. Kandlakunta et al. / Nuclear Instruments and Methods in Physics Research A 705 (2013) 36–41 39

were overlain together, as shown in Fig. 6, for illustration. The 59.5 keV gamma rays from 241Am and 122 keV gamma rays from 57Co were used in the calibration. The other two peaks in the 241Am energy spectrum correspond in part to the 33.2 keV and 26.3 keV X- ray photons. The energy calibration of the detectors yielded a conversion factor of 0.932 keV/ADC channel. The experimental measurement indicates that the spectra from the bare detectors can be attributed to mixed IC electrons and gamma rays, whereas those from the polyethylene-covered detectors are caused by gamma rays only. Thus, the neutron signal may be effectively separated from the gamma-ray background by using the proposed gamma-ray rejection scheme in Fig. 1.

4. Simulations of IC electrons resulting from neutron capture

Although the experimental results are convincing, extensive Monte Carlo simulations were performed to enhance our under- Fig. 5. IC electron energy spectra measured by Si detectors from a Gd thin foil standing of the results and to demonstrate the principle of the placed at the center of the neutron beam. gamma-ray rejection scheme. Simulations were performed using Monte Carlo N-Particle Transport Code (MCNP5) [18] and Soft- ware for Optimization of Radiation Detectors (SWORD) 3.2 [19] in order to observe the IC electron energy spectrum following thermal neutron absorption in Gd. In the geometry used for this model (Fig. 7), a beam of 0.0253 eV thermal neutrons is perpen- dicularly incident on a natural Gd foil (10 mm thick), and a Si detector (200 mm thick) is used to tally the energy deposition of the Gd(n,g)Gd* reaction products. An IC electron source was created in the Gd foil for MCNP5 simulation. A comparison of the spectra obtained by MCNP5 and SWORD reveals a close resemblance, and both are similar to the experimental results shown in Fig. 5. However, the proposed twin-detector structure differs from the structures used in the experiments as well as the simulation model shown in Fig. 7. Therefore, the simulations in SWORD were extended to a model in which a thermal neutron beam is incident on twin-detector structure (Fig. 8,left),andthe energy spectra were tallied and presented (Fig. 8,right). The IC electron results clearly resemble both the experimental results and those of the simulation with a different geometry. This shows that the proposed scheme is insensitive to the direction of

241 57 the incident neutrons. Again, the energy spectrum from detector Fig. 6. Energy spectra of button-size Am and Co sources used to calibrate the Si detectors. Measurements were made separately, but the spectra are overlaid 2 illustrates that all the IC electrons are fully stopped by the together. polyethylene layer before reaching the detector. This conﬁrms that

Fig. 7. Left: geometry used in the two simulations to observe IC electron energy spectrum in a Si detector. Right: energy spectrum of IC electrons in a Si detector showing the three main IC electron peaks obtained with two simulation codes. 40 P. Kandlakunta et al. / Nuclear Instruments and Methods in Physics Research A 705 (2013) 36–41

Fig. 8. Left: geometry used in SWORD simulation to model the twin-detector structure and thermal neutron (0.0253 eV) source beam. Right: energy spectra of electrons in the two detectors obtained using SWORD simulation; Detector 1’s response includes that from IC electrons, indicated by the three main IC electron peaks. IC electron response is totally absent in detector 2’s energy spectrum.

Fig. 9. Left: geometry used in the simulation of gamma-ray interaction with the twin-detector structure using MCNP5. Right: energy spectra of the simulated 57Co gamma rays. subtracting the two spectra from the shielded and the unshielded twin-detector structure. 57Co was chosen to demonstrate the detectors yields the detector response due primarily to IC electrons. sensitivity of a thin semiconductor detector to low-energy gamma rays. The detector response includes the two 57Co gamma peaks (122.0 keV and 136.5 keV) and KX-ray peaks (42.3 keV and 5. Gamma-ray interactions with Gd 43.0 keV) from gamma activation of Gd. The features of detector 1 spectrum at 50–75 keV and 80–120 keV could be attributed A question remains regarding whether spectrum subtraction to energy deposition of the photoelectrons ejected from Gd works for external gamma rays. In other words, do the two following photoelectric absorption of 122 keV photons. The poly- detectors have the same response to gamma rays, originated both ethylene layer completely stops these electrons from reaching internally and externally? Owing to its high atomic number detector 2; hence, the corresponding features in the spectrum are (Z¼64), Gd has a very good probability of interaction with gamma absent. Nevertheless, subtracting the two spectra significantly rays. Because of the high stopping power of Gd, detectors that reduces the overall gamma-ray response. incorporate Gd as the neutron conversion material possess To validate the above simulations of gamma-ray rejection, an inherent gamma-ray sensitivity. The high gamma-ray interaction experiment was designed using two Si detectors (same as in probability of Gd leads to an enhanced gamma-ray background Table 3) and a 57Co gamma-ray button-size source. A 25 mm-thick due to the conversion of high-energy gamma rays into low- Gd foil and a 350 mm-thick polyethylene cap were used to separate energy KX-rays. Although a thin-film semiconductor is almost the two Si detectors and to reproduce the gamma-ray rejection transparent to high-energy gamma rays, it does react with low- scenario. The instrument and geometry used in this experiment are energy KX-rays [20]. Hence, the hypothesis is that the low-energy shown in Fig. 10 (left), and the vacuum cover is removed to display KX-rays emitted after the activation of Gd by external gamma the setup. A multichannel digitizer-based acquisition system was rays interfere with the low-energy IC electrons. The proposed used for simultaneous signal acquisition from two independent gamma-ray rejection scheme assumes that the gamma-ray detector channels. Energy spectra from both detectors were response in both detectors is identical, so subtracting the two acquired using the digitizer with a trapezoidal energy filter. detector signals effectively cancels out the gamma-ray interfer- The response of the two detectors to 57Co gamma rays is ence in the final detector response. Nevertheless, an accurate illustrated in Fig. 10 (right). The energy spectra encompass the simulation model is needed to substantiate this hypothesis. For resolved gamma peaks at 122 keV, a significant component from this purpose, a simulation and an experiment using external backscattered photons (83 keV), and the characteristic KX-rays gamma-ray were performed. The gamma-ray interactions in the from Gd. The gamma-ray response of detector 2 is lower than that twin-detector structure were modeled with MCNP5 (Fig. 9 left), of detector1 except in the low-energy region. This attenuation in and the detector responses were obtained (Fig. 9 right). response is attributed to the point-source-like geometry used in Gamma rays emitted by 57Co were used as the source particles the experiment, in other words, the detector that is close to the in this model, in which a beam of gamma rays is incident on the source has a higher solid angle. This is in contrast to the P. Kandlakunta et al. / Nuclear Instruments and Methods in Physics Research A 705 (2013) 36–41 41

Fig. 10. Left: experimental setup reproducing the twin-detector scenario for rejection of external gamma rays and a 57Co source was used. Right: energy spectra from two Si charged particle detectors.

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