- Email: [email protected]
GEANT4 simulations of a novel 3He-free thermalization neutron detector
Nuclear Inst. and Methods in Physics Research, A 889 (2018) 33–38
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
GEANT4 simulations of a novel 3 He-free thermalization neutron detector A. Mazzone a,d , P. Finocchiaro b , S. Lo Meo c , N. Colonna d, * a b c d
CNR - IC, Bari, Italy INFN - Laboratori Nazionali del Sud, Catania, Italy ENEA, Bologna and INFN, Sez. di Bologna, Bologna, Italy INFN - Sez. di Bari, Bari, Italy
a r t i c l e
i n f o
Keywords: Thermalization neutron detector Detection efficiency Stellar neutron source 3 He-free thermal neutron detector
a b s t r a c t A novel concept for3 He-free thermalization detector is here investigated by means of GEANT4 simulations. The detector is based on strips of solid-state detectors with6 Li deposit for neutron conversion. Various geometrical configurations have been investigated in order to find the optimal solution, in terms of value and energy dependence of the efficiency for neutron energies up to 10 MeV. The expected performance of the new detector are compared with those of an optimized thermalization detector based on standard3 He tubes. Although an3 Hebased detector is superior in terms of performance and simplicity, the proposed solution may become more appealing in terms of costs in case of shortage of3 He supply.
1. Introduction Neutrons are an important probe for studying nuclear reactions of interest for basic and applied Nuclear Physics [1]. In particular, the development of Radioactive Ion Beam (RIB) and neutron facilities is resulting in a growing interest in neutron studies, as reactions with neutron-rich nuclei could lead to a better understanding of the nuclear forces and of the nuclear structure far from stability. These studies could in turn provide crucial information of relevance for Nuclear Astrophysics, in particular for the rapid neutron capture process (the so-called r process), occurring in very high temperatures and neutron density environment, produced in Supernovae explosions or in neutron start mergers, like the one recently observed by multi-messenger astronomy [2]. Radioactive Ion Beams also offer the opportunity to study 𝛽-delayed neutron emission, a subject of high interest both for fundamental Nuclear Physics, for Nuclear Astrophysics and for its implication in Nuclear Technology, as delayed neutron emission plays an important role in the criticality and transient response of nuclear reactors and on the determination of the decay heat [3]. Finally, neutron studies are involved in another topic of great interest in Nuclear Astrophysics, i.e. the determination of the reaction rate of the two main neutron sources in stars: the 13 C(𝛼, 𝑛)16 O and 22 Ne(𝛼, 𝑛)25 Mg reactions [4,5]. Efforts are currently undergoing around the world aimed at measuring the cross section of these reactions down to the Gamow energy [6]. At the LUNA experiment in Italy [7], measurements are foreseen for both *
reactions, in the low-background underground laboratory of the Gran Sasso National Laboratory. In most cases mentioned above, neutrons are emitted with a wide energy spectrum, typically ranging from a few keV to several MeV. Furthermore, reaction rates are typically low, either because of the low reaction cross section, as in the case of the stellar neutron sources, or because of the low intensity of the incident particle beam, as in the case of studies at RIB facilities. As a consequence, in most cases these studies require a neutron detector with high efficiency, low sensitivity to other particles, in particular 𝛾-rays, and low intrinsic background. This implies the use of a 4𝜋 geometry, a neutron-converting reaction with high neutron cross section, and detectors and analysis technique optimized for low 𝛾-ray sensitivity. In this respect, the best choice is represented by the so-called thermalization detector, which consists of a large volume of neutron moderating material, typically polyethylene, and a thermal neutron detector. At present, the most commonly used solution is based on 3 He proportional counters. Both cylindrical and cubic thermalization detectors have been used in the past, with 3 He tubes mounted in one or more rings around the beam axis and target position. The size of the moderator and, especially, the number of thermal neutron detectors determines the detection efficiency and its behavior as a function of neutron energy. Efficiencies close to 50% are typically obtained with a relatively small number of 3 He tubes from thermal energy to several hundred keV. Above this energy the efficiency drops rapidly, unless a large number of tubes are used, at increasing distances from the center of the moderator.
Corresponding author. E-mail address: [email protected] (N. Colonna).
https://doi.org/10.1016/j.nima.2018.02.011 Received 12 January 2018; Received in revised form 30 January 2018; Accepted 1 February 2018 Available online 7 February 2018 0168-9002/© 2018 Elsevier B.V. All rights reserved.
A. Mazzone et al.
Nuclear Inst. and Methods in Physics Research, A 889 (2018) 33–38
Several thermalization detectors have been built and employed in the past, as the one specifically built for the measurement of the 22 Ne(𝛼,n) reaction [6]. The same detector, described in Ref. [8], was also used for the measurement of the 8 Li(𝛼, 𝑛)11 B reaction [9]. Other thermalization detectors are 3HEN at Oak Ridge [10], and NERO at NSCL-MSU [11]. A high-efficiency thermalization detector, BRIKEN, has recently been built by a large international collaboration [3]. Made of 160 3 He tubes, filling a large polyethylene cubic moderator, this device is capable of reaching an efficiency of 80% for neutron energies from thermal to 1 MeV, being still above 70% for 5 MeV neutron energy. Another large thermalization detector is TETRA [12], built by a joint JINR (Dubna) IPN (Orsay) collaboration, which uses a total of 122 3 He-tubes. While 3 He-based thermalization detectors are being extensively used in measurements of neutron emission in various fields of basic and applied Nuclear Physics, the increasing difficulty in 3 He procurement suggests that other, high efficiency and cost-effective thermal neutron detectors should be investigated [13]. Among the various possibilities being considered to substitute 3 He-tubes in thermalization detectors, Si detectors coated with a neutron converter, namely 6 Li or a 6 Licompound, seems to be one of the most promising solution, both in terms of efficiency and costs. More details on the R&D of such detectors, and their performance, can be found in [14–16] and references therein. In this paper, an 3 He-free thermalization detector based on a new type of thermal detector is presented. Monte Carlo simulations performed with the GEANT4 toolkit [17] have been performed in order to optimize the design and determine the efficiency. The results are compared with a relatively low-budget 3 He-based detector originally designed for the study of the 13 C(𝛼, 𝑛) reaction at LUNA. The paper is organized as follows: the physics list, geometry and material used in the simulations are described in Section 2, the results for the 3 He-based and 3 He-free detector are presented and compared in Section 3, while a discussion and conclusions are presented in Section 4.
2.2. Geometry and material The geometry implemented in the simulation consists of a polyethylene cuboid, acting as moderator, with an empty channel in the middle. The dimensions of the cuboid are 80 × 80 × 50 cm3 , while the empty channel has a section of 133 cm2 . The thermal neutron detectors are embedded in this polyethylene block. For low mass elements such as hydrogen the definition of materials in the simulation is crucial. For the scattering on polyethylene, the hydrogen has to be properly defined (as TS_H_of_Polyethylene), so that at low energies the binding energy of hydrogen in the polyethylene is not neglected. Analogously, the isotopic composition of elements that compose different materials can affect the results. For this reason, although the natural isotopic compositions are contained in the GEANT4 database, the isotopic composition of each material was specifically defined, for a better control of the simulations. For thermal neutron detection, two solutions were investigated. The first one consists of a variable number of 3 He cylindrical gas counters disposed concentrically around the empty channels. Tubes with both circular and elliptical section were used in the simulations. For the circular ones, two different dimensions were employed, corresponding to the most common commercially available, i.e. 1 and 2 cm radius. All tubes are 39 cm long and work at a pressure of 4 atm. Several geometric configurations were tested for the tubes, and their number and position were optimized in order to reduce the overall cost of the thermalization detector, without penalization of the efficiency in the neutron energy range up to a few MeV, typical of most measurements in which such detector is used. In particular, the simulations were performed keeping in mind the use of this detector for the measurement of the 13 C(𝛼, 𝑛) and 22 Ne(𝛼, 𝑛) reaction, of astrophysical interest. An analogous configuration is chosen for the solution based on the use of 6 LiF converter. In this case, the basic element is a 3 × 3 cm2 singlepad silicon detector, 300 μm thick. A 6 LiF layer is deposited on both sides of the detector. Such a sandwich-like configuration is chosen to increase the efficiency of the device. Several pads are aligned to form a 39 cm long strip, that represent the equivalent of an 3 He-tube. However, since the efficiency of the single strip is sensibly lower than that of an 3 Hetube, a much larger number of strips are typically necessary to achieve a similar overall efficiency. As in the case of the 3 He-based thermalization detector, the 6 Li-based solid state detectors are arranged in a cylindrical configuration around the empty channel of the polyethylene moderator, forming rings of different radius. In this case as well, the number of detectors and their position was chosen so to maximize the overall efficiency of the thermalization detector. A final element that was considered in the simulations to improve the overall detection efficiency is a 5 cm thick lead box surrounding the polyethylene moderator, acting as reflector, thus reducing the number of neutrons escaping the detector.
2. The GEANT4 simulations Monte Carlo simulations are a fundamental tool for the design and optimization of detector performances. Among the software most widely used in the scientific community to this aim, GEANT4, originally developed for high-energy physics, has proven to be very versatile for the geometric and material description and accurate in the description of the neutron scattering processes. In this work, the simulations were based on the version 10.03 of GEANT4. General physical processes for particles other than neutrons were described using standard packages, in particular the Bertini intra-nuclear cascade implementation was chosen. 2.1. The physics list In GEANT4, the neutrons interactions are considered hadronic processes and can be classified as elastic and inelastic scattering, radiative capture, and fission. Up to the previous GEANT4 version, the G4NeutronHPElastic package was set up for harmonic elastic scattering but starting with version 10.03, this package was removed and substituted with the G4ParticleHPElasticData one, a change transparent to user and other codes. Similar to elastic scattering, GEANT4 uses a high precision description of all other neutron reactions (capture, fission, inelastic) below 20 MeV, based on the use of evaluated data libraries for the reaction cross sections and other reaction-related quantities. This package allows using evaluated nuclear libraries in the G4NDL format whereas nuclear model fails in predicting with reasonable accuracy the neutron cross sections. More details on the evaluated neutron cross section libraries used in high precision neutron transport in GEANT4 can be found in Refs. [18,19]. When the free gas model is no longer effective, it is necessary to consider the interaction of thermal neutrons with nuclei of chemically bound atoms. This happens for neutrons with kinetic energy below 5 eV. In this case, the correct description of transport of neutrons requires, for elastic scattering process, a specific thermal neutron model with relative data libraries for this energy range. The G4ParticleHPThermalScattering model is used with the related data library.
2.3. The neutron source In all simulations performed in this study, neutrons were generated isotropically in the center of the empty channel of the thermalization detector. To study the efficiency as a function of neutron energy, neutrons were generated with energy from thermal (∼25 meV) to 10 MeV. In the case of a 3 He-based detector, the efficiency for neutrons as a function of energy was determined by counting the number of tritons produced in a tube for neutrons in a given energy range, divided by the number of neutrons generated in that range. For the 6 Li-based configuration, the detection condition was that a triton entered one of the silicon pads in the strip. 34
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Nuclear Inst. and Methods in Physics Research, A 889 (2018) 33–38
characterized by a constant efficiency in a wide energy range, possibly from thermal to several MeV neutron energy. Therefore, the main goal of the optimization performed in this work was to find the geometrical configuration for a fixed number of 3 He-tubes that leads to a higher overall efficiency and pushes to higher energy the region in which the efficiency remains constant. The best solution in this respect was found to consists of a configuration in which the tubes are disposed in three concentric cylindrical rings around the central axis of the detector. The radius of the concentric rings and the dimensions of the counters was varied in order to maximize the efficiency in terms of value and trend. A scheme of the final configuration is shown on the left panel of Fig. 2. The first ring is made of 12 counters of 1 cm radius, positioned at a distance of 11 cm from the detector’s central axis. Bigger tubes are needed in the second and third rings to efficiently detect higher energy neutrons that are thermalized at larger distance from the center. To this purpose, using tubes of 2 cm radius positioned at 22 and 27 cm distance (18 on each ring) ensures a reasonable coverage of the solid angle at those distances. The resulting efficiency is shown as a function of neutron energy in the right panel of Fig. 2. A flat behavior is obtained with this configuration all the way from thermal to 5 MeV, with a variation of less that 10%. As expected, the efficiency decreases at higher energy but at 10 MeV it is still within 80% of the maximum efficiency. While in absolute terms the efficiency is not very high, the geometrical configuration just described fulfills the condition of a flat behavior in a wide energy range. As a further check, simulations have also been performed with 3 He tubes of elliptical section. As in the case of the circular section, two different dimension were used in the simulations: for the first ring, elliptical tubes with semi-major and semi-minor axis of 1.5 and 0.5 cm, respectively, have been used in the simulations, while for the second and third rings the semi-axis were 2.5 cm and 1.5 cm, respectively. As shown in Fig. 3, this configuration yields results comparable to the previous case, in terms of efficiency, but with the advantage of a reduced amount of 3 He, by approximately 25%. A potential drawback of elliptical tubes could be related to a larger intrinsic background of the device, considering the larger surface of an elliptical tube, relative the circular one. Finally, simulations have been performed for circular 3 He tubes, but surrounded by a small layer of air. When in close contact with the tubes, the polyethylene acts both as moderator and absorber of thermal neutrons, through neutron capture by Hydrogen, in competition with the 3 He(𝑛, 𝑝)T reaction. This effect can be reduced by leaving a small gap of air around the tubes, where thermal neutrons can diffuse, and eventually enter the 3 He tube, before they are absorbed in the polyethylene. The detector layout and corresponding efficiency is shown in Fig. 4. The simulations were performed with the same circular tubes of Fig. 2. Comparing the efficiencies obtained in the two cases, it is evident that the presence of a small gap of air leads to a substantial increase of the efficiency at low energy, of up to 50%, while no effect is observed at high energy. While this solution is more convenient from the point of view of the overall absolute efficiency, the rapidly decreasing trend at high energy constitutes a problem when neutrons emitted in a wide, unknown spectrum, have to be detected.
Fig. 1. The efficiency of a 3 He-based thermalization detector simulated with GEANT4 as a function of the number of 3 He detectors, for different neutron energies. The slow increase of the efficiency above for a large number of tubes suggest that a relatively low-budget detector can be set up with less than 50 tubes, as simulated in the present work.
3. Results 3.1. Optimization of a 3 He-based detector Various geometrical configurations have been investigated by means of the GEANT4 simulations in order to find the best solution in terms of efficiency and costs. In particular, one of the requests considered in the simulations was to obtain an efficiency nearly constant as a function of neutron energy, in a wide range. Considering the spectrum of neutrons emitted in most nuclear reactions of interest for basic and applied nuclear physics, the aim of the optimization was to obtain an efficiency constant all the way from thermal to 10 MeV neutron energy. The first, basic consideration regards the number of 3 He-tubes to be used. Typically, the efficiency of a thermalization detector is constant from thermal to a few MeV, decreasing at higher energy due to incomplete thermalization and escape of high-energy neutrons from the sides of the polyethylene block, or their capture in the outer layers of the block. Increasing the number of 3 He tubes and positioning them at larger distance from the detector center has two major effects: on the one hand a higher overall efficiency is obtained, at all energies. On the other hand, and most importantly, the efficiency remains constant in a wider neutron energy range, i.e. up to higher energy. The increase in the number of tubes can partially be surrogated by an increase in the size of the tubes. In all cases, however, the higher efficiency is obtained at the expenses of a larger cost. Fig. 1 shows the relation between the efficiency and the number of 3 He tubes of the same dimension, arranged in concentric rings around the central channel. Since the total cost of the device is proportional to the number of 3 He detectors, the 𝑋-axis roughly represents the total cost of the device. It can be noticed that the efficiency increases with increasing number of tubes, as expected, but the trend is far from linearity, so that a small increase in efficiency can only be obtained at the expenses of the large increase in the cost. In this work we have focused on a detector for studies of astrophysical neutron sources, that could be built with a relatively low budget. In that case, the best compromise between efficiency and costs seems to be achieved with a total number of tubes around fifty, since above this number the efficiency tends to rapidly saturate. In other words, above this number there is no longer a significant increase in efficiency, compared to the rising cost. Having set the rough number of tubes to be used, the next step regards the optimization of their position in the polyethylene block. As already mentioned, an ideal thermalization detector should be
3.2. Design of a 3 He-free detector The first step in the design of a 3 He-free detector based on silicon detectors is the optimal choice of the thickness of the deposit of the neutron-converting material, in this case 6 LiF. To this end, a systematic study was performed by varying the thickness of the converting layer. More details on the detector are reported in Ref. [14]. In Fig. 5, the efficiency for a double-layer single silicon pad is shown as a function of the layer thickness, for neutrons of 10 keV energy. With increasing thickness, the efficiency initially increases up to a maximum value, and then remains constant or slightly decreases for thicker layers, depending on the neutron energy. 35
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Nuclear Inst. and Methods in Physics Research, A 889 (2018) 33–38
Fig. 2. GEANT4 simulations of an 3 He-based thermalization detector. The left panel shows the geometrical configuration used in the simulations, with cylindrical tubes arranged in three concentric rings. The right panel shows the resulting efficiency up to 10 MeV neutron energy.
Fig. 3. GEANT4 simulations of an 3 He-based thermalization detector, for tubes of elliptical section. The left panel shows the geometrical configuration used in the simulations, with the tubes arranged in three concentric rings. The right panel shows the resulting efficiency up to 10 MeV neutron energy.
Fig. 4. GEANT4 simulations of an 3 He-based thermalization detector, for circular tubes surrounded by a small gap of air. The effect of the gap is to increase the efficiency at low energy, leaving unchanged the high energy part.
An important remark on the use of silicon pads with 6 LiF deposits for thermal neutron detection regards the 𝛾-ray sensitivity of the device. As shown in Refs. [14] and [20] a 6 LiF-Si thermal neutron detector is characterized by a small sensitivity to 𝛾-rays, of the same order of magnitude (typically 10−3 ÷ 10−4 ) of 3 He tubes and could therefore be considered adequate for an 3 He-free thermalization detector. Several geometrical configurations of the thermalization detector were simulated. In principle, both the number and the position of the
The saturation behavior is related to the absorption of the emitted products inside the layer, while the slight decrease for low-energy neutrons is an effect of the absorption of incident neutrons inside the deposit. As it can be deduced from the plot, a thickness of the converter between 15 and 20 microns seems to be the best choice. In the present simulations a thickness of 18 microns was used for the 6 LiF layer, consistently with the conclusions of Ref. [14]. The deposit was assumed to be in contact with the silicon detector on both sides of the pad. 36
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Nuclear Inst. and Methods in Physics Research, A 889 (2018) 33–38
is the slight increase of the efficiency above 1 MeV, just before the rapid decrease at higher energy. We have verified that this effect is not related to reactions induced by direct neutrons, i.e. neutrons that have not undergone a collision within the polyethylene. Indeed, simulations indicate that the contribution of reactions induced by MeV neutrons on fluorine (of the 6 LiF deposit) or on Si (of the detectors) is negligible. Rather, the bump in the efficiency above 1 MeV is related to the resonance at 250 keV in the 6 Li(𝑛, 𝑡) reaction cross section, with the effect showing up at higher neutron energy due to partial moderation (i.e. very few collisions) neutrons undergo before falling in the energy region of the resonance. A final case considered in the simulations is the presence of an air gap surrounding the single 6 LiF-silicon strip. As in the case of the 3 Hebased solution, the air gap has the effect of increasing the efficiency to low-energy neutrons, an obvious consequence of the smaller absorption of thermal neutrons by hydrogen capture in the polyethylene layers surrounding the thermal neutron detectors. However, since the efficiency for neutrons of higher energy is not affected by the presence of the air gap, a relatively strong energy dependence is produced, with a flat behavior up to a few hundred keV and a steep decrease above this energy, with an efficiency at 10 MeV less than half the maximum one. Such a behavior may not be suitable for measurements in which neutrons are emitted with a wide and unknown energy spectrum.
Fig. 5. The efficiency of a double-sided 6 LiF deposit on a silicon pad detector is shown as a function of the thickness of the single layer. The saturation and slight decrease of the efficiency for increasingly thick layers is related to the absorption in the layer of thermal neutrons and of the emitted products of the 6 Li+n reaction.
silicon strips inside the polyethylene moderator block could be varied in order to find the configuration that ensured the highest efficiency and a flat behavior as a function of neutron energy. However, as in the 3 He-based case, a reasonable number of strips was used, for a meaningful comparison between the performance of a 3 He-based and 3 He-free detector. The optimal configuration was found to consists of a total of five concentric cylindrical surfaces (or rings). Considering that the dimension of the strip is fixed, the number of silicon strips in each ring increases with the distance from the detector central axis. The radius of the rings were 7, 12, 16, 22 and 25 cm, while the number of silicon strips in each ring are 10, 16, 18, 27 and 29 respectively. A scheme of the detector is shown in Fig. 6, together with the simulated efficiency. Similarly to the 3 He-based thermalization detector discussed in the previous section, the position of the silicon strip, and in particular the radius of the five rings, has been optimized so to obtain a nearly flat efficiency from thermal to 10 MeV. Clearly, a higher efficiency can be obtained at low energy with the same number of detectors, more closely packed at shorter radii. The results shown in Fig. 6 indicate that a device based on 6 LiFsilicon strips for thermal neutron detection is characterized by an efficiency about half that of the corresponding optimized 3 He-based solution. In this sense, a 3 He-based thermalization detector still represents at present the best option for this kind of device, but a 6 LiF-based solution (or similar) could become more appealing in case of insufficient availability of 3 He. An interesting feature that can be observed in Fig. 6
4. Conclusions A new concept of an 3 He-free neutron thermalization detector has been here investigated. The proposed design is based on a polyethylene block in which 3 He tubes are substituted by a series of solid state strips, made of the recently proposed 6 LiF-Si-6 LiF sandwich device for thermal neutron detection. The response of the 3 He-free detector to neutrons of energy up to 10 MeV has been compared with the one of a 3 He-based thermalization neutron detector. In both cases, the geometry has been optimized so to obtain a close-to-flat efficiency as a function of neutron energy all the way from thermal to 10 MeV energy. The simulations have been performed with the GEANT4 toolkit. While a relatively high and energy-independent efficiency can be obtained with a small number of 3 He tubes, an 3 He-free detector based on the 6 Li(𝑛, 𝑡) reaction and solid state detectors requires a large number of units to reach efficiencies comparable (i.e. within a factor of two) with the 3 He-based solution. Even considering the simplification in electronics and high-voltage supply associated with the use of solid-state detectors, at present the 3 Hefree solution does not seem competitive when compared with the 3 Hebased counterpart. Nevertheless, the present work represents a first attempt to identify a possible substitute of 3 He tubes in thermalization neutron detectors for Nuclear Physics studies. Such a solution may
Fig. 6. A 3 He-free thermalization detector and its efficiency as a function of neutron energy. The thermal neutron detector unit is a strip of Silicon pads, with a 6 LiF deposit on both sides. The number of strips has been chosen as a compromise between increase in efficiency in costs. The corresponding efficiency is shown in the right panel. 37
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become more appealing in case of shortage of 3 He, and corresponding increase in price. Furthermore, future development of higher efficiency solid-state thermal neutron detectors cannot be excluded, such as for example the use of pure 6 Li instead of 6 LiF in the Silicon sandwich, leading in this case to a decrease in the needed number of thermal neutron units, thus considerably reducing the costs and complexity of 3 He-free thermalization neutron detectors.
[7] http://luna.lngs.infn.it. [8] A. Del Zoppo, et al., Nucl. Instrum. Methods A 581 (2007) 783. [9] M. La Cognata, et al., Phys. Lett. B 664 (2008) 157. http://dx.doi.org/10.1016/j. physletb.2008.05.026. [10] R. Grzywacz, et al., Acta Phys. Polon. B 45 (2014) 217. [11] J. Pereira, et al., Nucl. Instrum. Methods A 618 (2010) 275. [12] D. Testov, et al., Nucl. Instrum. Methods A 815 (2016) 96. [13] N. Colonna, et al., Eur. Phys. Journal Plus 130 (3) (2015) 53. [14] S. Lo Meo, L. Cosentino, A. Mazzone, P. Bartolomei, P. Finocchiaro, Nucl. Instrum. Methods A 866 (2017) 48. [15] A. Pappalardo, et al., Nucl. Instrum. Methods A 810 (2016) 6. [16] A. Pappalardo, C. Vasi, P. Finocchiaro, Results Phys. 6 (2016) 12. [17] J. Allison, et al., Nucl. Instrum. Methods A 835 (2016) 186. [18] E. Mendoza, et al., IAEA technical report INDC(NDS)-0612, Vienna, 2012. Data available online at http://www-nds.iaea.org/geant4. [19] E. Mendoza, et al., IEEE Trans. Nucl. Sci. 61 (2014) 2357. [20] A. Kaplanov, et al., J. Instr. 8 (2013) P10025.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
GEANT4 simulations of a novel 3 He-free thermalization neutron detector A. Mazzone a,d , P. Finocchiaro b , S. Lo Meo c , N. Colonna d, * a b c d
CNR - IC, Bari, Italy INFN - Laboratori Nazionali del Sud, Catania, Italy ENEA, Bologna and INFN, Sez. di Bologna, Bologna, Italy INFN - Sez. di Bari, Bari, Italy
a r t i c l e
i n f o
Keywords: Thermalization neutron detector Detection efficiency Stellar neutron source 3 He-free thermal neutron detector
a b s t r a c t A novel concept for3 He-free thermalization detector is here investigated by means of GEANT4 simulations. The detector is based on strips of solid-state detectors with6 Li deposit for neutron conversion. Various geometrical configurations have been investigated in order to find the optimal solution, in terms of value and energy dependence of the efficiency for neutron energies up to 10 MeV. The expected performance of the new detector are compared with those of an optimized thermalization detector based on standard3 He tubes. Although an3 Hebased detector is superior in terms of performance and simplicity, the proposed solution may become more appealing in terms of costs in case of shortage of3 He supply.
1. Introduction Neutrons are an important probe for studying nuclear reactions of interest for basic and applied Nuclear Physics [1]. In particular, the development of Radioactive Ion Beam (RIB) and neutron facilities is resulting in a growing interest in neutron studies, as reactions with neutron-rich nuclei could lead to a better understanding of the nuclear forces and of the nuclear structure far from stability. These studies could in turn provide crucial information of relevance for Nuclear Astrophysics, in particular for the rapid neutron capture process (the so-called r process), occurring in very high temperatures and neutron density environment, produced in Supernovae explosions or in neutron start mergers, like the one recently observed by multi-messenger astronomy [2]. Radioactive Ion Beams also offer the opportunity to study 𝛽-delayed neutron emission, a subject of high interest both for fundamental Nuclear Physics, for Nuclear Astrophysics and for its implication in Nuclear Technology, as delayed neutron emission plays an important role in the criticality and transient response of nuclear reactors and on the determination of the decay heat [3]. Finally, neutron studies are involved in another topic of great interest in Nuclear Astrophysics, i.e. the determination of the reaction rate of the two main neutron sources in stars: the 13 C(𝛼, 𝑛)16 O and 22 Ne(𝛼, 𝑛)25 Mg reactions [4,5]. Efforts are currently undergoing around the world aimed at measuring the cross section of these reactions down to the Gamow energy [6]. At the LUNA experiment in Italy [7], measurements are foreseen for both *
reactions, in the low-background underground laboratory of the Gran Sasso National Laboratory. In most cases mentioned above, neutrons are emitted with a wide energy spectrum, typically ranging from a few keV to several MeV. Furthermore, reaction rates are typically low, either because of the low reaction cross section, as in the case of the stellar neutron sources, or because of the low intensity of the incident particle beam, as in the case of studies at RIB facilities. As a consequence, in most cases these studies require a neutron detector with high efficiency, low sensitivity to other particles, in particular 𝛾-rays, and low intrinsic background. This implies the use of a 4𝜋 geometry, a neutron-converting reaction with high neutron cross section, and detectors and analysis technique optimized for low 𝛾-ray sensitivity. In this respect, the best choice is represented by the so-called thermalization detector, which consists of a large volume of neutron moderating material, typically polyethylene, and a thermal neutron detector. At present, the most commonly used solution is based on 3 He proportional counters. Both cylindrical and cubic thermalization detectors have been used in the past, with 3 He tubes mounted in one or more rings around the beam axis and target position. The size of the moderator and, especially, the number of thermal neutron detectors determines the detection efficiency and its behavior as a function of neutron energy. Efficiencies close to 50% are typically obtained with a relatively small number of 3 He tubes from thermal energy to several hundred keV. Above this energy the efficiency drops rapidly, unless a large number of tubes are used, at increasing distances from the center of the moderator.
Corresponding author. E-mail address: [email protected] (N. Colonna).
https://doi.org/10.1016/j.nima.2018.02.011 Received 12 January 2018; Received in revised form 30 January 2018; Accepted 1 February 2018 Available online 7 February 2018 0168-9002/© 2018 Elsevier B.V. All rights reserved.
A. Mazzone et al.
Nuclear Inst. and Methods in Physics Research, A 889 (2018) 33–38
Several thermalization detectors have been built and employed in the past, as the one specifically built for the measurement of the 22 Ne(𝛼,n) reaction [6]. The same detector, described in Ref. [8], was also used for the measurement of the 8 Li(𝛼, 𝑛)11 B reaction [9]. Other thermalization detectors are 3HEN at Oak Ridge [10], and NERO at NSCL-MSU [11]. A high-efficiency thermalization detector, BRIKEN, has recently been built by a large international collaboration [3]. Made of 160 3 He tubes, filling a large polyethylene cubic moderator, this device is capable of reaching an efficiency of 80% for neutron energies from thermal to 1 MeV, being still above 70% for 5 MeV neutron energy. Another large thermalization detector is TETRA [12], built by a joint JINR (Dubna) IPN (Orsay) collaboration, which uses a total of 122 3 He-tubes. While 3 He-based thermalization detectors are being extensively used in measurements of neutron emission in various fields of basic and applied Nuclear Physics, the increasing difficulty in 3 He procurement suggests that other, high efficiency and cost-effective thermal neutron detectors should be investigated [13]. Among the various possibilities being considered to substitute 3 He-tubes in thermalization detectors, Si detectors coated with a neutron converter, namely 6 Li or a 6 Licompound, seems to be one of the most promising solution, both in terms of efficiency and costs. More details on the R&D of such detectors, and their performance, can be found in [14–16] and references therein. In this paper, an 3 He-free thermalization detector based on a new type of thermal detector is presented. Monte Carlo simulations performed with the GEANT4 toolkit [17] have been performed in order to optimize the design and determine the efficiency. The results are compared with a relatively low-budget 3 He-based detector originally designed for the study of the 13 C(𝛼, 𝑛) reaction at LUNA. The paper is organized as follows: the physics list, geometry and material used in the simulations are described in Section 2, the results for the 3 He-based and 3 He-free detector are presented and compared in Section 3, while a discussion and conclusions are presented in Section 4.
2.2. Geometry and material The geometry implemented in the simulation consists of a polyethylene cuboid, acting as moderator, with an empty channel in the middle. The dimensions of the cuboid are 80 × 80 × 50 cm3 , while the empty channel has a section of 133 cm2 . The thermal neutron detectors are embedded in this polyethylene block. For low mass elements such as hydrogen the definition of materials in the simulation is crucial. For the scattering on polyethylene, the hydrogen has to be properly defined (as TS_H_of_Polyethylene), so that at low energies the binding energy of hydrogen in the polyethylene is not neglected. Analogously, the isotopic composition of elements that compose different materials can affect the results. For this reason, although the natural isotopic compositions are contained in the GEANT4 database, the isotopic composition of each material was specifically defined, for a better control of the simulations. For thermal neutron detection, two solutions were investigated. The first one consists of a variable number of 3 He cylindrical gas counters disposed concentrically around the empty channels. Tubes with both circular and elliptical section were used in the simulations. For the circular ones, two different dimensions were employed, corresponding to the most common commercially available, i.e. 1 and 2 cm radius. All tubes are 39 cm long and work at a pressure of 4 atm. Several geometric configurations were tested for the tubes, and their number and position were optimized in order to reduce the overall cost of the thermalization detector, without penalization of the efficiency in the neutron energy range up to a few MeV, typical of most measurements in which such detector is used. In particular, the simulations were performed keeping in mind the use of this detector for the measurement of the 13 C(𝛼, 𝑛) and 22 Ne(𝛼, 𝑛) reaction, of astrophysical interest. An analogous configuration is chosen for the solution based on the use of 6 LiF converter. In this case, the basic element is a 3 × 3 cm2 singlepad silicon detector, 300 μm thick. A 6 LiF layer is deposited on both sides of the detector. Such a sandwich-like configuration is chosen to increase the efficiency of the device. Several pads are aligned to form a 39 cm long strip, that represent the equivalent of an 3 He-tube. However, since the efficiency of the single strip is sensibly lower than that of an 3 Hetube, a much larger number of strips are typically necessary to achieve a similar overall efficiency. As in the case of the 3 He-based thermalization detector, the 6 Li-based solid state detectors are arranged in a cylindrical configuration around the empty channel of the polyethylene moderator, forming rings of different radius. In this case as well, the number of detectors and their position was chosen so to maximize the overall efficiency of the thermalization detector. A final element that was considered in the simulations to improve the overall detection efficiency is a 5 cm thick lead box surrounding the polyethylene moderator, acting as reflector, thus reducing the number of neutrons escaping the detector.
2. The GEANT4 simulations Monte Carlo simulations are a fundamental tool for the design and optimization of detector performances. Among the software most widely used in the scientific community to this aim, GEANT4, originally developed for high-energy physics, has proven to be very versatile for the geometric and material description and accurate in the description of the neutron scattering processes. In this work, the simulations were based on the version 10.03 of GEANT4. General physical processes for particles other than neutrons were described using standard packages, in particular the Bertini intra-nuclear cascade implementation was chosen. 2.1. The physics list In GEANT4, the neutrons interactions are considered hadronic processes and can be classified as elastic and inelastic scattering, radiative capture, and fission. Up to the previous GEANT4 version, the G4NeutronHPElastic package was set up for harmonic elastic scattering but starting with version 10.03, this package was removed and substituted with the G4ParticleHPElasticData one, a change transparent to user and other codes. Similar to elastic scattering, GEANT4 uses a high precision description of all other neutron reactions (capture, fission, inelastic) below 20 MeV, based on the use of evaluated data libraries for the reaction cross sections and other reaction-related quantities. This package allows using evaluated nuclear libraries in the G4NDL format whereas nuclear model fails in predicting with reasonable accuracy the neutron cross sections. More details on the evaluated neutron cross section libraries used in high precision neutron transport in GEANT4 can be found in Refs. [18,19]. When the free gas model is no longer effective, it is necessary to consider the interaction of thermal neutrons with nuclei of chemically bound atoms. This happens for neutrons with kinetic energy below 5 eV. In this case, the correct description of transport of neutrons requires, for elastic scattering process, a specific thermal neutron model with relative data libraries for this energy range. The G4ParticleHPThermalScattering model is used with the related data library.
2.3. The neutron source In all simulations performed in this study, neutrons were generated isotropically in the center of the empty channel of the thermalization detector. To study the efficiency as a function of neutron energy, neutrons were generated with energy from thermal (∼25 meV) to 10 MeV. In the case of a 3 He-based detector, the efficiency for neutrons as a function of energy was determined by counting the number of tritons produced in a tube for neutrons in a given energy range, divided by the number of neutrons generated in that range. For the 6 Li-based configuration, the detection condition was that a triton entered one of the silicon pads in the strip. 34
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characterized by a constant efficiency in a wide energy range, possibly from thermal to several MeV neutron energy. Therefore, the main goal of the optimization performed in this work was to find the geometrical configuration for a fixed number of 3 He-tubes that leads to a higher overall efficiency and pushes to higher energy the region in which the efficiency remains constant. The best solution in this respect was found to consists of a configuration in which the tubes are disposed in three concentric cylindrical rings around the central axis of the detector. The radius of the concentric rings and the dimensions of the counters was varied in order to maximize the efficiency in terms of value and trend. A scheme of the final configuration is shown on the left panel of Fig. 2. The first ring is made of 12 counters of 1 cm radius, positioned at a distance of 11 cm from the detector’s central axis. Bigger tubes are needed in the second and third rings to efficiently detect higher energy neutrons that are thermalized at larger distance from the center. To this purpose, using tubes of 2 cm radius positioned at 22 and 27 cm distance (18 on each ring) ensures a reasonable coverage of the solid angle at those distances. The resulting efficiency is shown as a function of neutron energy in the right panel of Fig. 2. A flat behavior is obtained with this configuration all the way from thermal to 5 MeV, with a variation of less that 10%. As expected, the efficiency decreases at higher energy but at 10 MeV it is still within 80% of the maximum efficiency. While in absolute terms the efficiency is not very high, the geometrical configuration just described fulfills the condition of a flat behavior in a wide energy range. As a further check, simulations have also been performed with 3 He tubes of elliptical section. As in the case of the circular section, two different dimension were used in the simulations: for the first ring, elliptical tubes with semi-major and semi-minor axis of 1.5 and 0.5 cm, respectively, have been used in the simulations, while for the second and third rings the semi-axis were 2.5 cm and 1.5 cm, respectively. As shown in Fig. 3, this configuration yields results comparable to the previous case, in terms of efficiency, but with the advantage of a reduced amount of 3 He, by approximately 25%. A potential drawback of elliptical tubes could be related to a larger intrinsic background of the device, considering the larger surface of an elliptical tube, relative the circular one. Finally, simulations have been performed for circular 3 He tubes, but surrounded by a small layer of air. When in close contact with the tubes, the polyethylene acts both as moderator and absorber of thermal neutrons, through neutron capture by Hydrogen, in competition with the 3 He(𝑛, 𝑝)T reaction. This effect can be reduced by leaving a small gap of air around the tubes, where thermal neutrons can diffuse, and eventually enter the 3 He tube, before they are absorbed in the polyethylene. The detector layout and corresponding efficiency is shown in Fig. 4. The simulations were performed with the same circular tubes of Fig. 2. Comparing the efficiencies obtained in the two cases, it is evident that the presence of a small gap of air leads to a substantial increase of the efficiency at low energy, of up to 50%, while no effect is observed at high energy. While this solution is more convenient from the point of view of the overall absolute efficiency, the rapidly decreasing trend at high energy constitutes a problem when neutrons emitted in a wide, unknown spectrum, have to be detected.
Fig. 1. The efficiency of a 3 He-based thermalization detector simulated with GEANT4 as a function of the number of 3 He detectors, for different neutron energies. The slow increase of the efficiency above for a large number of tubes suggest that a relatively low-budget detector can be set up with less than 50 tubes, as simulated in the present work.
3. Results 3.1. Optimization of a 3 He-based detector Various geometrical configurations have been investigated by means of the GEANT4 simulations in order to find the best solution in terms of efficiency and costs. In particular, one of the requests considered in the simulations was to obtain an efficiency nearly constant as a function of neutron energy, in a wide range. Considering the spectrum of neutrons emitted in most nuclear reactions of interest for basic and applied nuclear physics, the aim of the optimization was to obtain an efficiency constant all the way from thermal to 10 MeV neutron energy. The first, basic consideration regards the number of 3 He-tubes to be used. Typically, the efficiency of a thermalization detector is constant from thermal to a few MeV, decreasing at higher energy due to incomplete thermalization and escape of high-energy neutrons from the sides of the polyethylene block, or their capture in the outer layers of the block. Increasing the number of 3 He tubes and positioning them at larger distance from the detector center has two major effects: on the one hand a higher overall efficiency is obtained, at all energies. On the other hand, and most importantly, the efficiency remains constant in a wider neutron energy range, i.e. up to higher energy. The increase in the number of tubes can partially be surrogated by an increase in the size of the tubes. In all cases, however, the higher efficiency is obtained at the expenses of a larger cost. Fig. 1 shows the relation between the efficiency and the number of 3 He tubes of the same dimension, arranged in concentric rings around the central channel. Since the total cost of the device is proportional to the number of 3 He detectors, the 𝑋-axis roughly represents the total cost of the device. It can be noticed that the efficiency increases with increasing number of tubes, as expected, but the trend is far from linearity, so that a small increase in efficiency can only be obtained at the expenses of the large increase in the cost. In this work we have focused on a detector for studies of astrophysical neutron sources, that could be built with a relatively low budget. In that case, the best compromise between efficiency and costs seems to be achieved with a total number of tubes around fifty, since above this number the efficiency tends to rapidly saturate. In other words, above this number there is no longer a significant increase in efficiency, compared to the rising cost. Having set the rough number of tubes to be used, the next step regards the optimization of their position in the polyethylene block. As already mentioned, an ideal thermalization detector should be
3.2. Design of a 3 He-free detector The first step in the design of a 3 He-free detector based on silicon detectors is the optimal choice of the thickness of the deposit of the neutron-converting material, in this case 6 LiF. To this end, a systematic study was performed by varying the thickness of the converting layer. More details on the detector are reported in Ref. [14]. In Fig. 5, the efficiency for a double-layer single silicon pad is shown as a function of the layer thickness, for neutrons of 10 keV energy. With increasing thickness, the efficiency initially increases up to a maximum value, and then remains constant or slightly decreases for thicker layers, depending on the neutron energy. 35
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Fig. 2. GEANT4 simulations of an 3 He-based thermalization detector. The left panel shows the geometrical configuration used in the simulations, with cylindrical tubes arranged in three concentric rings. The right panel shows the resulting efficiency up to 10 MeV neutron energy.
Fig. 3. GEANT4 simulations of an 3 He-based thermalization detector, for tubes of elliptical section. The left panel shows the geometrical configuration used in the simulations, with the tubes arranged in three concentric rings. The right panel shows the resulting efficiency up to 10 MeV neutron energy.
Fig. 4. GEANT4 simulations of an 3 He-based thermalization detector, for circular tubes surrounded by a small gap of air. The effect of the gap is to increase the efficiency at low energy, leaving unchanged the high energy part.
An important remark on the use of silicon pads with 6 LiF deposits for thermal neutron detection regards the 𝛾-ray sensitivity of the device. As shown in Refs. [14] and [20] a 6 LiF-Si thermal neutron detector is characterized by a small sensitivity to 𝛾-rays, of the same order of magnitude (typically 10−3 ÷ 10−4 ) of 3 He tubes and could therefore be considered adequate for an 3 He-free thermalization detector. Several geometrical configurations of the thermalization detector were simulated. In principle, both the number and the position of the
The saturation behavior is related to the absorption of the emitted products inside the layer, while the slight decrease for low-energy neutrons is an effect of the absorption of incident neutrons inside the deposit. As it can be deduced from the plot, a thickness of the converter between 15 and 20 microns seems to be the best choice. In the present simulations a thickness of 18 microns was used for the 6 LiF layer, consistently with the conclusions of Ref. [14]. The deposit was assumed to be in contact with the silicon detector on both sides of the pad. 36
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is the slight increase of the efficiency above 1 MeV, just before the rapid decrease at higher energy. We have verified that this effect is not related to reactions induced by direct neutrons, i.e. neutrons that have not undergone a collision within the polyethylene. Indeed, simulations indicate that the contribution of reactions induced by MeV neutrons on fluorine (of the 6 LiF deposit) or on Si (of the detectors) is negligible. Rather, the bump in the efficiency above 1 MeV is related to the resonance at 250 keV in the 6 Li(𝑛, 𝑡) reaction cross section, with the effect showing up at higher neutron energy due to partial moderation (i.e. very few collisions) neutrons undergo before falling in the energy region of the resonance. A final case considered in the simulations is the presence of an air gap surrounding the single 6 LiF-silicon strip. As in the case of the 3 Hebased solution, the air gap has the effect of increasing the efficiency to low-energy neutrons, an obvious consequence of the smaller absorption of thermal neutrons by hydrogen capture in the polyethylene layers surrounding the thermal neutron detectors. However, since the efficiency for neutrons of higher energy is not affected by the presence of the air gap, a relatively strong energy dependence is produced, with a flat behavior up to a few hundred keV and a steep decrease above this energy, with an efficiency at 10 MeV less than half the maximum one. Such a behavior may not be suitable for measurements in which neutrons are emitted with a wide and unknown energy spectrum.
Fig. 5. The efficiency of a double-sided 6 LiF deposit on a silicon pad detector is shown as a function of the thickness of the single layer. The saturation and slight decrease of the efficiency for increasingly thick layers is related to the absorption in the layer of thermal neutrons and of the emitted products of the 6 Li+n reaction.
silicon strips inside the polyethylene moderator block could be varied in order to find the configuration that ensured the highest efficiency and a flat behavior as a function of neutron energy. However, as in the 3 He-based case, a reasonable number of strips was used, for a meaningful comparison between the performance of a 3 He-based and 3 He-free detector. The optimal configuration was found to consists of a total of five concentric cylindrical surfaces (or rings). Considering that the dimension of the strip is fixed, the number of silicon strips in each ring increases with the distance from the detector central axis. The radius of the rings were 7, 12, 16, 22 and 25 cm, while the number of silicon strips in each ring are 10, 16, 18, 27 and 29 respectively. A scheme of the detector is shown in Fig. 6, together with the simulated efficiency. Similarly to the 3 He-based thermalization detector discussed in the previous section, the position of the silicon strip, and in particular the radius of the five rings, has been optimized so to obtain a nearly flat efficiency from thermal to 10 MeV. Clearly, a higher efficiency can be obtained at low energy with the same number of detectors, more closely packed at shorter radii. The results shown in Fig. 6 indicate that a device based on 6 LiFsilicon strips for thermal neutron detection is characterized by an efficiency about half that of the corresponding optimized 3 He-based solution. In this sense, a 3 He-based thermalization detector still represents at present the best option for this kind of device, but a 6 LiF-based solution (or similar) could become more appealing in case of insufficient availability of 3 He. An interesting feature that can be observed in Fig. 6
4. Conclusions A new concept of an 3 He-free neutron thermalization detector has been here investigated. The proposed design is based on a polyethylene block in which 3 He tubes are substituted by a series of solid state strips, made of the recently proposed 6 LiF-Si-6 LiF sandwich device for thermal neutron detection. The response of the 3 He-free detector to neutrons of energy up to 10 MeV has been compared with the one of a 3 He-based thermalization neutron detector. In both cases, the geometry has been optimized so to obtain a close-to-flat efficiency as a function of neutron energy all the way from thermal to 10 MeV energy. The simulations have been performed with the GEANT4 toolkit. While a relatively high and energy-independent efficiency can be obtained with a small number of 3 He tubes, an 3 He-free detector based on the 6 Li(𝑛, 𝑡) reaction and solid state detectors requires a large number of units to reach efficiencies comparable (i.e. within a factor of two) with the 3 He-based solution. Even considering the simplification in electronics and high-voltage supply associated with the use of solid-state detectors, at present the 3 Hefree solution does not seem competitive when compared with the 3 Hebased counterpart. Nevertheless, the present work represents a first attempt to identify a possible substitute of 3 He tubes in thermalization neutron detectors for Nuclear Physics studies. Such a solution may
Fig. 6. A 3 He-free thermalization detector and its efficiency as a function of neutron energy. The thermal neutron detector unit is a strip of Silicon pads, with a 6 LiF deposit on both sides. The number of strips has been chosen as a compromise between increase in efficiency in costs. The corresponding efficiency is shown in the right panel. 37
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become more appealing in case of shortage of 3 He, and corresponding increase in price. Furthermore, future development of higher efficiency solid-state thermal neutron detectors cannot be excluded, such as for example the use of pure 6 Li instead of 6 LiF in the Silicon sandwich, leading in this case to a decrease in the needed number of thermal neutron units, thus considerably reducing the costs and complexity of 3 He-free thermalization neutron detectors.
[7] http://luna.lngs.infn.it. [8] A. Del Zoppo, et al., Nucl. Instrum. Methods A 581 (2007) 783. [9] M. La Cognata, et al., Phys. Lett. B 664 (2008) 157. http://dx.doi.org/10.1016/j. physletb.2008.05.026. [10] R. Grzywacz, et al., Acta Phys. Polon. B 45 (2014) 217. [11] J. Pereira, et al., Nucl. Instrum. Methods A 618 (2010) 275. [12] D. Testov, et al., Nucl. Instrum. Methods A 815 (2016) 96. [13] N. Colonna, et al., Eur. Phys. Journal Plus 130 (3) (2015) 53. [14] S. Lo Meo, L. Cosentino, A. Mazzone, P. Bartolomei, P. Finocchiaro, Nucl. Instrum. Methods A 866 (2017) 48. [15] A. Pappalardo, et al., Nucl. Instrum. Methods A 810 (2016) 6. [16] A. Pappalardo, C. Vasi, P. Finocchiaro, Results Phys. 6 (2016) 12. [17] J. Allison, et al., Nucl. Instrum. Methods A 835 (2016) 186. [18] E. Mendoza, et al., IAEA technical report INDC(NDS)-0612, Vienna, 2012. Data available online at http://www-nds.iaea.org/geant4. [19] E. Mendoza, et al., IEEE Trans. Nucl. Sci. 61 (2014) 2357. [20] A. Kaplanov, et al., J. Instr. 8 (2013) P10025.
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