Investigation of enhancement in Planar Fast Neutron Detector efficiency with stacked structure using Geant4

Investigation of enhancement in Planar Fast Neutron Detector efficiency with stacked structure using Geant4 1 2 corresp first-author Corresponding author. E-mail address: shivang@igcar.gov.in Corresponding author. E-mail address: shivang@igcar.gov.in shivang@igcar.gov.in 2 2 2 2 2 Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Geant4 based Monte-Carlo study has been carried out to assess the improvement in efficiency of the planar structure of Silicon Carbide (SiC) based semiconductor fast neutron detector with the stacked structure. A proton recoil detector was simulated, which consists of hydrogenous converter i.e. High Density Polyethylene (HDPE) for generating recoil protons by means of neutron elastic scattering ( n , p ) reaction and semiconductor material SiC, for generating a detectable electrical signal upon transport of recoil protons through it. SiC is considered in order to overcome the various factors associated with conventional Si-based devices such as operability in harsh radiation environment, as often encountered in nuclear facilities. Converter layer thickness is optimized by considering 10 9 neutron events of different monoenergetic neutron sources as well as 241 Am-Be neutron spectrum. It is found that the optimized thickness for neutron energy range of 1 - 10 MeV is ~ 400 μmm. However, the efficiency of fast neutron detection is estimated to be only 0.112 %, which is considered very low for meaningful and reliable detection of neutrons. To overcome this problem, a stacked juxtaposition of converter layer between SiC layers has been analyzed in order to achieve high efficiency. It is noted that a tenfold efficiency improvement has been obtained – 1.04 % for 10 layer stacked configuration vis-à-vis 0.112 % of single converter layer detector. Further simulation of stacked detector with respect to variable converter thickness has been performed to achieve the efficiency as high as ~ 3.85 % with up to 50 stacks. Geant4 fast neutron detector silicon carbide recoil proton stacked detector 1 Introduction 1 Nuclear reactors, nuclear recycling and reprocessing facilities often employ neutron detectors as monitoring and control sensor. Operating conditions in such nuclear facilities generally include harsh radiation environment associated with high gamma dose rate and temperature [ 1 1 ] . Thus, the neutron sensors for such facilities should be designed and manufactured so as to make them operable in harsh conditions. Self-Powered Neutron Detectors (SPNDs) [ 2 2 ] and Fission Chambers (FCs) [ 3 3 , 4 4 ] are quite popular for such application requirements. SPNDs have been used widely as in-core flux monitors in thermal reactors owing to the high cross-sections of their emitter materials to the thermal neutrons [ 5 5 , 6 6 ] . However, their sensitivity to both neutrons as well as γ- rays and delayed response for few emitters limits its operation [ 7 7 ] . On the other hand, FCs are good at gamma discrimination by virtue of high charge associated with extremely energetic fission fragments [ 8 8 ] . However, it is a gas-filled type detector that requires comparatively high voltage supply to collect the charge pairs. In addition, these detectors have other limitations viz. (a) high cost of enriching the fissile material in order to attain high efficiency [ 1 1 ] (b) inability to provide energy spectrum information [ 8 8 ] (c) large size. Therefore, there is a compelling need to conceive a detection system which could be employed as an alternative or diverse sensor of neutron detection having features – harsh environment operability, wide neutron spectrum response and compact vis-à-vis gas filled detectors. The wide energy spectrum response is expected to facilitate the study of shielding effectiveness of various materials as well as neutron detector efficiency calibration in a known spectrum. Further, miniaturization of the detector is targeted to ease the handling, installations; finding application where compact nuclear sensors are desirable such as space, nuclear reprocessing plants and special nuclear material detection. With above objective, we conceptualized a fast neutron detector which can be exposed to a total neutron flux of the order of 10 10 n/cm 2 s, a gamma field of 10 Gy/h and temperature not exceeding 100°C – a typical ex-vessel location of a fast reactor. A wide band-gap semiconductor, Silicon Carbide (SiC) (3.25 eV for 4H-SiC polytype), has been chosen to study neutronic interaction properties under the above stated conditions. The expected gamma dose at ex-vessel location is quite less than the dose which degrades the SiC detector performance or hinders its detection efficiency. High Density Polyethylene (HDPE) (C 2 H 4 ) n has been employed as a converter material to generate recoil protons upon interaction with fast neutrons [ 9 9 - 15 15 9-15 ] . The details of fast neutron detection through proton recoil method and validation of our model with analytical method could be found in our previous work and references therein [ 10 10 ] . The present work focuses on the study of the improvement in efficiency of planar SiC based semiconductor fast neutron detector with the stacked structure. Geant4 [ 16 16 ] simulations have been performed to optimize the converter layer thickness for different monoenergetic and 241 Am-Be neutron sources. Effect of Low Level Discriminator (LLD) on detection efficiency is analyzed. Also, stacked structure has been simulated to study the improvement in the detection efficiency. 2 A Geant4 Simulation 1 Geant4 is a simulation toolkit for simulating the transport of particles through matter. It offers a complete range of functionality including tracking, geometry, physics models and processes [ 17 17 , 18 18 ] . It is capable of tracking down the secondary particles as well. It provides a vast set of well-defined physics models including hadronic, electromagnetic and optical processes from low energy to very high energy range [ 19 19 ] . It has been developed with software engineering and object-oriented technology and implemented in C++ programming language. 2.1 Model Description 2 The Geant4.10.00.p02 version is used to build the model. The planar detector geometry has been constructed using Geometry Category of Geant4. Geometry, as shown in Fig. 1(a) Fig. 1(a) , consists of a converter layer of HDPE (density 0.94 g/cm 3 ) with 1 cm x 1 cm front face ( XY -plane) and thickness ( Z -axis) is varied from one micrometer to few millimeters in order to optimize the thickness of converter layer. Here optimized thickness represents the thickness at which the detector efficiency is maximum. In other words, the thickness at which, maximum number of recoil protons will punch through the converter layer into the SiC semiconductor region to generate a detectable signal. Adjacent to converter is a detector layer made up of SiC with similar XY -plane and thickness is kept at 600 μmm as shown in Fig. 1(b) Fig. 1(b) . General Particle Source (GPS), class of particle category has been utilized to generate a spectrum of the 241 Am-Be neutron source taken from Table 3 Table 3 of ISO Report 8529-1 [ 20 20 ] . It is positioned at 1 cm distance from the converter face. In practical applications, neutron will not be generated in parallel beams rather fall in random directions on the detector. But in order to achieve maximum efficiency, source is set such that neutrons will impinge perpendicularly and uniformly on the front face of the converter layer (in direction of Z -axis) as illustrated in Fig. 1(b) Fig. 1(b) . Standard physics model QGSP_BERT_HP has been used which employs high precision neutron model used for neutron below 20 MeV [ 21 21 ] . 1 1 Nuclear Reactions 1 1 Frequency relative to total neutron interactions (%) 1 1 Q -value 1 1 n + 12 C -- > N γg + 13 C 1 1 0.000427967 1 1 -734.52 keV 1 1 n + 12 C -- > N γg + α + 9 Be 1 1 0.145080736 1 1 -5.7038 MeV 1 1 n + 12 C -- > N γg + n + 12 C 1 1 1.906164005 1 1 -231.52 eV 1 1 n + 12 C -- > α + 9 Be 1 1 0.191729114 1 1 -5.7012 MeV 1 1 n + 12 C -- > n + 12 C 1 1 39.50347295 1 1 139.26 eV 1 1 n + 13 C -- > N γg + α + 10 Be 1 1 0.0012839 1 1 -3.8359 MeV 1 1 n + 13 C -- > N γg + n + 13 C 1 1 0.018402571 1 1 -260.39 eV 1 1 n + 13 C -- > α + 10 Be 1 1 0.001711867 1 1 -3.8358 MeV 1 1 n + 13 C -- > n + 13 C 1 1 0.416839637 1 1 -27.684 keV 1 1 n + 28 Si -- > N γg + 29 Si 1 1 0.008987302 1 1 6.4167 MeV 1 1 n + 28 Si -- > N γg + α + 25 Mg 1 1 0.259775831 1 1 -2.4198 MeV 1 1 n + 28 Si -- > N γg + n + 28 Si 1 1 10.41585531 1 1 -63.193 eV 1 1 n + 28 Si -- > N γg + p + 28 Al 1 1 0.907289558 1 1 -3.6085 MeV 1 1 n + 28 Si -- > α + 25 Mg 1 1 0.411276069 1 1 -2.7349 MeV 1 1 n + 28 Si -- > n + 28 Si 1 1 41.15028909 1 1 31.809 eV 1 1 n + 28 Si -- > p + 28 Al 1 1 0.397581132 1 1 -3.8759 MeV 1 1 n + 29 Si -- > N γg + 2 n + 28 Si 1 1 0.000855934 1 1 -1.149 MeV 1 1 n + 29 Si -- > N γg + α + 26 Mg 1 1 0.024394106 1 1 -31.934 keV 1 1 n + 29 Si -- > N γg + n + 29 Si 1 1 0.738242683 1 1 -48.522 eV 1 1 n + 29 Si -- > N γg + p + 29 Al 1 1 0.013694937 1 1 -2.8964 MeV 1 1 n + 29 Si -- > α + 26 Mg 1 1 0.016690704 1 1 -89.088 keV 1 1 n + 29 Si -- > n + 29 Si 1 1 1.746104432 1 1 31.061 eV 1 1 n + 29 Si -- > p + 29 Al 1 1 0.017118671 1 1 -2.9129 MeV 1 1 n + 30 Si -- > N γg + 31 Si 1 1 0.000427967 1 1 4.3037 MeV 1 1 n + 30 Si -- > N γg + α + 27 Mg 1 1 0.000855934 1 1 -4.2002 MeV 1 1 n + 30 Si -- > N γg + n + 30 Si 1 1 0.411704035 1 1 2.2936 keV 1 1 n + 30 Si -- > N γg + p + 30 Al 1 1 0.000427967 1 1 -6.8197 MeV 1 1 n + 30 Si -- > α + 27 Mg 1 1 0.001711867 1 1 -4.2538 MeV 1 1 n + 30 Si -- > n + 30 Si 1 1 1.29160372 1 1 25.478 eV In order to optimize the converter thickness, five iterations of 10 9 neutrons have been simulated for different monoenergetic as well as 241 Am-Be neutron source. This special case of 241 Am-Be neutron source has been chosen to facilitate the lab-scale experiment of the detector with standard neutron source. Furthermore, five iterations would help in better convergence of results. After determining the optimum thickness, 10 9 neutrons have been fired to illustrate the energy deposition by recoil protons, other charged particles in converter and detector. Furthermore, the number of converters and detectors has been increased to implement the stacked detector configuration as depicted in Fig. 1(c) Fig. 1(c) , and enhancement in efficiency was estimated. Simulations were performed in GNU HPC clusters and all physics processes were kept on so as to reflect the best practical situation. Dead layer and temperature effect was not considered in this work. It should be noted that these effects would further degrade the detection efficiency. 3 Results and Discussion 1 3.1 Nuclear Interactions in HDPE converter 2 To determine the reactions that are taking place in converter, a simulation was performed with 1 cm thick HDPE structure. It was irradiated by 10 6 neutrons of 241 Am-Be neutron source. List of nuclear reactions along with the reaction Q -value is tabulated in Table 1 Table 1 . 1 1 Nuclear reactions   1 1 Frequency relative to total neutron interactions (%)   1 1 Q -value   1 1 n + 12 C -- > N γg + 13 C 1 1 0.0010 1 1 186.34 keV 1 1 n + 12 C -- > N γg + α a + 9 Be 1 1 0.1017 1 1 -5.7038 MeV 1 1 n + 12 C -- > N γg + n + 12 C 1 1 1.3493 1 1 -290.49 eV 1 1 n + 12 C -- > α a + 9 Be 1 1 0.1291 1 1 -5.7012 MeV 1 1 n + 12 C -- > n + 12 C 1 1 26.2475 1 1 144.32 eV 1 1 n + 13 C -- > N γg + α a + 10 Be 1 1 0.0010 1 1 -3.8386 MeV 1 1 n + 13 C -- > N γg + n + 13 C 1 1 0.0131 1 1 -230.71 eV 1 1 n + 13 C -- > α a + 10 Be 1 1 0.0017 1 1 -3.8353 MeV 1 1 n + 13 C -- > n + 13 C 1 1 0.2885 1 1 -28.831 keV 1 1 n + 1 H -- > N γg + 2 H 1 1 0.0003 1 1 578.25 keV 1 1 n + 1 H -- > n + p 1 1 71.8599 1 1 840.86 eV 1 1 n + 2 H -- > n + 2 H 1 1 0.0064 1 1 429.84 eV Table 1 Table 1 represents the reactions which are happening in HDPE material upon interaction with the incident neutrons. It is evident that neutron reaction with hydrogen i.e., (n, p) reaction is dominating (~72 %), which is elastic scattering in nature. This is because, for fast neutrons, elastic scattering cross-section is higher than other reaction cross-sections. In addition to this, neutrons will also undergo elastic scattering reaction with Carbon nuclei, which is the next reaction in terms of dominance (~26 %). Other than these reactions, neutron will also undergo several inelastic scattering reactions with Carbon and hydrogen nuclei [ 12 C(n , n ' ) 12 C; 12 C(n, n') 13 C; 1 H(n, n') 2 H ] and a very few ( n , α ) reactions [ 12 C( n , α ) 9 Be; 13 C( n , α ) 10 Be] due to very low cross-section. Some reactions have negative Q -value which shows these reactions will occur only when the incident neutrons will have energy equal to or greater than the threshold energy. Table 2 Table 2 shows the list of particles generated due to neutron interaction with HDPE along with their mean kinetic energy and range of the kinetic energies at the time of their production. 1 1 Generated Particles   1 1 Number   1 1 Mean Kinetic Energy   1 1 Range of Kinetic Energy   1 1 10 Be 1 1 8 1 1 1.4649 MeV 1 1 303.12 keV --> 2.7232 MeV 1 1 9 Be 1 1 649 1 1 1.1741 MeV 1 1 9.0611 keV --> 3.5147 MeV 1 1 12 C 1 1 77577 1 1 409.14 keV 1 1 1.3303 eV --> 2.9667 MeV 1 1 13 C 1 1 851 1 1 360.36 keV 1 1 897.91 eV --> 2.5871 MeV 1 1 α 1 1 657 1 1 1.8154 MeV 1 1 206.39 keV --> 4.8089 MeV 1 1 2 H 1 1 19 1 1 1.5774 MeV 1 1 151.93 keV --> 5.3214 MeV 1 1 γg 1 1 10607 1 1 1.6236 MeV 1 1 1.0013 keV --> 9.142 MeV 1 1 n 1 1 3830 1 1 2.2004 MeV 1 1 31.444 keV --> 6.1871 MeV 1 1 p 1 1 202004 1 1 1.5502 MeV 1 1 1.0023 eV --> 10.726 MeV 3.2 Nuclear Interactions in SiC detector 2 Similar approach was adopted for simulating the nuclear interactions in SiC material. List of nuclear reactions along with the reaction Q -value is tabulated in Table 3 Table 3 . From Table 3 Table 3 , it is evident that the elastic scattering reaction with Carbon and Silicon nuclei is dominant with ~ 39 % and ~ 41 % reactions respectively in SiC, followed by inelastic scattering reaction with Si nuclei and other threshold reactions resulting in α a -particles, Mg, Al, Be... as a reaction products. The dominant reaction products produced in SiC have lower energy range ( Table 4 Table 4 ) than the products generated in HDPE ( Table 2 Table 2 ). Therefore, an energetic reaction product generated in HDPE converter and transported to SiC, would deposit higher energy in SiC in comparison to the products generated in SiC itself, provided, the thickness of converter is optimized. Thereby, a HDPE converter layer over SiC detector will result in a better detection efficiency and detectable signal. Franceschini et al. [ 22 22 ] , have also reported a better detection efficiency in the presence of converter layer over SiC detector. 1 1 Generated Particles   1 1 Number   1 1 Mean Kinetic Energy   1 1 Range of Kinetic Energy   1 1 28 Al 1 1 3049 1 1 367.77 keV 1 1 16.171 keV --> 1.1204 MeV 1 1 29 Al 1 1 72 1 1 349.65 keV 1 1 28.833 keV --> 1.0893 MeV 1 1 30 Al 1 1 1 1 1 581.19 keV 1 1 581.19 keV --> 581.19 keV 1 1 10 Be 1 1 7 1 1 1.2123 MeV 1 1 160.58 keV --> 2.1968 MeV 1 1 9 Be 1 1 787 1 1 1.1595 MeV 1 1 14.152 keV --> 3.137 MeV 1 1 12 C 1 1 96759 1 1 395.14 keV 1 1 0.34078 eV --> 2.9473 MeV 1 1 13 C 1 1 1018 1 1 347.16 keV 1 1 180.32 eV --> 2.4177 MeV 1 1 25 Mg 1 1 1568 1 1 876.6 keV 1 1 43.409 keV --> 2.3849 MeV 1 1 26 Mg 1 1 96 1 1 928.07 keV 1 1 110.96 keV --> 2.0859 MeV 1 1 27 Mg 1 1 6 1 1 788.25 keV 1 1 194.37 keV --> 1.5206 MeV 1 1 28 Si 1 1 120493 1 1 145.37 keV 1 1 0.26237 eV --> 1.3631 MeV 1 1 29 Si 1 1 5826 1 1 154.35 keV 1 1 15.233 eV --> 1.1235 MeV 1 1 30 Si 1 1 3980 1 1 139.39 keV 1 1 2.4636 eV --> 1.1573 MeV 1 1 31 Si 1 1 1 1 1 70.041 keV 1 1 70.041 keV --> 70.041 keV 1 1 α   1 1 2464 1 1 3.428 MeV 1 1 197.08 keV --> 8.2182 MeV 1 1 γg 1 1 49939 1 1 1.5854 MeV 1 1 406.57 eV --> 8.9998 MeV 1 1 n 1 1 31526 1 1 2.9368 MeV 1 1 6.335 keV --> 9.4629 MeV 1 1 p 1 1 3122 1 1 3.153 MeV 1 1 238.19 keV --> 7.5591 MeV In Table 4 Table 4 , one can see that significant number of γ g -rays is also generating due to various inelastic scatterings and other reactions. Also, neutron fields are generally accompanied with γ g -background. These, γ g -rays can undergo photoelectric, Compton interactions with atomic electrons and it can also be converted into an electron-positron pair, but it would hardly affect the operation of the SiC detector as it has been reported that change in SiC-based detector response is negligible even after 22.7 MGy exposure to γ g -rays [ 23 23 ] . 3.3 Optimization of HDPE converter thickness for wide energy range of neutrons 2 The recoil proton generated in converter will tend to lose its energy continuously along the path, which limits its range in the material. Figure 2(a) Figure 2(a) shows the range of proton in HDPE and SiC computed using SRIM [ 24 24 ] . It is evident that 1 MeV proton can traverse up to 20 μmm while 10 MeV proton can travel up to 1200 μmm. In spite of the fact that having a HDPE converter of around 9 cm will result in approximately 99 % neutron interactions [ 10 10 ] , we could not employ a converter of such thickness because most of the recoil protons will be stopped in converter itself, before reaching the SiC detector volume. Thus, Geant4 simulation has been performed to optimize the HDPE converter thickness for different neutron energies ranging from 0.5 MeV to 10 MeV and a special case of 241 Am-Be neutron source spectrum as shown in Fig. 2(b) Fig. 2(b) . The optimized thickness is the thickness at which the maximum number of recoil protons is detected. It is assumed that each recoil proton, reaching the detector with sufficient energy, will give a detectable signal i.e. proton detection efficiency is 100 %. 10 9 neutron events have been simulated 5 times to determine the detector's absolute efficiency i.e. number of recoil protons reaching SiC detector volume divided by total incident neutrons [ 25 25 ] . Figure 3(a) Figure 3(a) depicts the detector efficiency for different energy neutrons and different converter thicknesses. It could be observed that after a certain thickness of converter, efficiency is decreasing. This is due to the self-absorption of proton energy in converter itself. This self-absorption limits the efficiency of planar detector. Since, neutrons of different energies, provide maximum efficiency for different thickness, there is a need to select a thickness which could be efficient for a specific energy range that is 0.5-11 MeV in the context of this work. Therefore, in order to accommodate a wide energy range of neutrons, we have considered 400 μmm thickness as an optimized one in this work. From Fig. 3(a) Fig. 3(a) , the optimized HDPE converter thickness for different monoenergetic neutron source can also be chosen. For e.g. a thickness of 100 μmm can be selected as an optimized thickness for a 2.5 MeV D-T neutron source. Similarly, for an application which requires 10 MeV neutrons to be detected, a converter layer of ~ 1.1 mm should be considered as an optimum one. Further, a special case has been taken to optimize the HDPE thickness for 241 Am-Be neutron source and illustrated in Fig. 3(b) Fig. 3(b) . It is apparent that for thicknesses of HDPE converter ranging from 400 μmm to 1000 μmm, the detector will provide maximum efficiency of 0.112% to 0.117%. Thus, we can say that for the thickness of 400 μmm to 1000 μmm, detector efficiency is approximately constant. It should be noted that there are relatively few neutrons significantly higher than 6 MeV in the 241 Am-Be spectrum. Also, larger thickness would restrict the number of recoil protons reaching the detector. Thus, to cover the wide range of specified source spectrum a 400 μmm thickness could be regarded as the optimum thickness, which is well in agreement with the optimum result for the 6 MeV monoenergetic neutron source obtained in Fig. 3(a) Fig. 3(a) . The efficiency of neutron detector with 400 μmm thick HDPE converter with respect to different monoenergetic neutron source is illustrated in Fig. 3(c) Fig. 3(c) . It can be observed that with 400 μmm thick HDPE converter, maximum efficiency of ~ 0.18 % can be achieved for up to 7 MeV monoenergetic neutron source. For neutron energies higher than 7 MeV, efficiency is decreasing since the optimized value of HDPE thickness is higher than 400 μmm, for high energy neutron sources as shown in Fig. 3(a) Fig. 3(a) . 3.4 Calculation of optimum thickness of SiC for 241 AmBe neutron source 2 For 241 Am-Be neutron source, maximum recoil energy of proton that can be generated in HDPE is 10.726 MeV ( Table 2 Table 2 ). The range of proton in SiC is computed using SRIM code and it is found to be approximately 571 μmm for proton of energy 11 MeV ( Fig. 2a Fig. 2a ). Thus, to allow the maximum recoil proton to deposit all of its energy in detector volume, the thickness of the SiC detector active volume is chosen to be 600 μmm. 3.5 Simulation of SiC detector in mixed neutron-gamma field 2 The primary concern of neutron detector in a mixed neutron-gamma field, is the inability of the detector to accurately discriminate between the two. Also, a significant number of γ g γ -rays are being generated along with the other charged particles in SiC as well as in HDPE converter. Therefore, to analyze the effect of gamma background over neutron detection, a simulation was performed with gamma and neutron source of energies ranging from 100 keV to 10 MeV with 10 9 particles. Figure 4(a) Figure 4(a) represents the comparison of their energy deposition in SiC detector. Gammas would generally undergo photoelectric effect at low energy region while Compton scattering and pair production at high energy region. Pair production reaction generally occurs when the incident gamma energy is greater than 1.02 MeV. On the other hand, neutron induced events are mainly due to energy deposited by recoil protons generated in converter layer. It is evident that such a heavy gamma background signals would be comparable to neutron signals below 1 MeV range. Between 1 MeV to 2 MeV, there would be gamma effect on neutron signals which could be easily distinguishable and can be discriminated with the pulse height spectrum or by applying Low Level Discriminator (LLD). Beyond 3 MeV there is a very minimal effect of gamma events which could be neglected in case of fast neutrons. Figure 4(b) Figure 4(b) shows the effect of LLD on detector efficiency. As can be seen, simply increasing the LLD to higher values can limit the background radiation, as from γ g γ -rays and other charged particles, but at the same time some proton recoil events will inevitably be discriminated, because the proton recoil energy distribution extends all the way to zero. Therefore, the detector with a finite discrimination level will always have a slightly lower efficiency. The efficiency achieved with this proton recoil detector is very small. However, for a reliable and consistent monitoring, this kind of detector efficiency is often regarded as unacceptably low. Therefore, to enhance the efficiency of the detector, with major characteristics unaffected, a special configuration consisting several layers of converters and active SiC layers (stacked configuration), as shown in Fig. 1(c) Fig. 1(c) , has been considered and assessed. 3.6 Stacked Detector simulation 2 Converter and detector volume is regarded as a single layer and by using the G4Replica Class of Geant4 it is replicated in order to generate stacked structure as shown in Fig. 1(c) Fig. 1(c) . The optimum value of thickness is used for converter and detector i.e. 400 μmm for HDPE and 600 μmm for SiC. Simulations have been performed for 10 9 neutron events and up to 10 layers. Efficiency, as defined previously, is estimated by increasing the number of layers in the stack. From Fig. 5(a) Fig. 5(a) , it is apparent that as we increase the number of stacks, efficiency will also increase but not linearly. As we increase the number of layers from 1 to 2, the increment in efficiency is almost double i.e. 99.33 % increment. Further, from 2 nd layer to 3 rd layer increment is ~ 99 % of the first increment. From 3 rd to 4 th layer increment is ~ 98.22 % of the first increment. Similarly, from 9 th layer to 10 th layer increment is ~ 90.49 % of the first increment. Thus, it appears that adding the 10 th layer is approximately the same increase in total detection as adding the 2 nd layer. Further addition of 50 th layer offers only ~ 48 % increment in efficiency to what we achieved by adding 2 nd layer. It shows that after a certain number of layers efficiency will saturate. This is due to the fact that probability of interaction for neutron with HDPE will saturate at certain thickness [ 10 10 ] . Also, each HDPE layer will attenuate some fraction of incident neutron flux thereby decreasing the incident flux for subsequent converter layers. D.S. McGregor et al. have shown a similar effect in the case of thermal neutron detector with 10 B and LiF converters [ 26 26 ] . Thus, we can achieve efficiency of up to 1.04 % with 10 layers of stacked detector. Furthermore, the thickness of converter was varied for 5, 10, 15, 20 and 50 stacks and the maximum efficiency of ~ 3.85 % could be achieved at the converter thickness of around 400 μmm. Efficiency for different stacked numbers with respect to converter thickness is represented in Fig. 5(b) Fig. 5(b) . Altering the thickness of converter of different layers may help in further enhancing the efficiency, but it is the subject of further detailed study by changing the thicknesses of different layers. 3.7 Calculation of Energy Deposition 2 A data analysis framework has been integrated with Geant4 in order to illustrate the contribution of recoil protons, alpha particles and other charged particles in SiC detector. Figure 6{(a), (b), (c) & (d)} Figure 6{(a), (b), (c) & (d)} represents the histograms of the total energy deposited, energy deposited by the recoil protons generated in HDPE converter, energy deposited by alpha particles and energy deposited by other charged reaction products in SiC detector volume respectively. A LDD of 2 MeV would significantly discriminate the signals due to other charged particles while signals due to α a -particles above 2 MeV might provide spurious counts. Furthermore, a discrimination strategy may have to be adopted based on rise time or amplitude to avoid spurious signals. One must find a suitable balance between LLD setting and detector efficiency as per one’s radiation environment. 4 Conclusion 1 Neutron transport and interaction in SiC detector along with HDPE converter have been investigated using Monte Carlo based Geant4 simulation toolkit. Reactions occurring in HDPE and SiC are tabulated along with their mean energy and frequency. Optimization of the converter layer thickness has been done for different energies of neutrons and a special case of 241 Am-Be neutron source and it is found to be ~ 400 μmm. Effect of LLD on SiC detector efficiency was presented which shows reduction in efficiency from 0.112 % (at LLD of 0 keV) to 0.07 % (at LLD of 2 MeV). A stacked detector configuration has been demonstrated to significantly improve the efficiency to 1.04 % with 10 layers of stacked detectors and 3.85 % with 50 stacked layers. Energy deposition by protons, α a , and other charged particles along with total deposition has been demonstrated using ROOT tool. Further investigation is planned to estimate the effect of dead layer and temperature on the efficiency of single and stacked structure. Also, fabrication of single and stacked detectors will be taken up for demonstration of improvement in efficiency by comparing the simulated results with experimental results. References M. Sivaramakrishna , C.P. Nagaraj , K. 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A , 500 , 272 - 308 ( 2003 ). doi: 10.1016/S0168-9002(02)02078-8 http://doi.org/10.1016/S0168-9002(02)02078-8 Design considerations for thin film coated semiconductor thermal neutron detectors—I: basics regarding alpha particle emitting neutron reactive films The authors would like to acknowledge Head CD, IGCAR for providing HPC cluster facility to carry out the simulation work. Also, the authors would like to thank Director, IGCAR for providing the opportunity to carry out this study. 10.1007/s41365-017-0315-7 This work was supported by the grant of research fellowship from Indira Gandhi Centre for Atomic Research, Department of Atomic Energy, India. 2017-01-26 2017-05-25 2017-06-02 2017-10-25 2017-11-01 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2017 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-017-0315-7 http://dx.doi.org/10.1007/s41365-017-0315-7 . 2017 Nuclear Science and Techniques 11 28