Feasibility study of a SiC sandwich neutron spectrometer

Nuclear Instruments and Methods in Physics Research A 708 (2013) 72–77

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Feasibility study of a SiC sandwich neutron spectrometer Jian Wu a, Jiarong Lei a,n, Yong Jiang a, Yu Chen a, Ru Rong a, Dehui Zou a, Xiaoqiang Fan a, Gang Chen b, Li Li b, Song Bai b a b

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, Sichuan Province, PR China Nanjing Electronic Devices Institute, Nanjing 210016, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 June 2012 Received in revised form 14 January 2013 Accepted 15 January 2013 Available online 21 January 2013

Semiconductor sandwich neutron spectrometers are suitable for in-pile measurements of fast reactor spectra thanks to their compact and relatively simple design. We have assembled and tested a sandwich neutron spectrometer based on 4H-silicon carbide (4H-SiC) Schottky diodes. The SiC diodes detect neutrons via neutron-induced charged particles (tritons and alpha particles) produced by 6 Li(n,a)3H reaction. 6LiF neutron converter layers are deposited on the front surface of Schottky diodes by magnetron sputtering. The responses of SiC diodes to charged particles were investigated with an 241 Am alpha source. A sandwich neutron spectrometer was assembled with two SiC Schottky diodes selected based on the charged-particle-response experimental results. The low-energy neutron response of the sandwich spectrometer was measured in the neutron field of the Chinese Fast Burst Reactor-II (CFBR-II). Spectra of alpha particles and tritons from 6Li(n,a)3H reaction were obtained with two well-resolved peaks. The energy resolution of the sum spectrum was 8.8%. The primary experimental results confirmed the 4H-SiC sandwich neutron spectrometer’s feasibility. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.

Keywords: Silicon carbide Sandwich neutron spectrometer Semiconductor detectors

1. Introduction The accurate measurement of neutron spectra is essential for theoretical research and practical application in the field of neutron physics [1]. Usually silicon-based spectrometers with sandwich construction are employed for neutron spectrum measurements [2–5]. It is well known that semiconductor sandwich spectrometers are well suited for in-pile measurements of fast reactor spectra for their compact and relatively simple design [4]. However, their applications are limited by susceptibility to radiation damage due to the low radiation resistance of silicon. Recently, neutron detectors based on 4H-silicon carbide (4H-SiC) are being developed for high-temperature applications in harsh radiation environments [6–11]. The lifetime of 4H-SiC neutron detectors has increased by orders of magnitude compared to commercial silicon based ones [6]. This improvement of lifetime originates from the excellent radiation resistance of silicon carbide. Several researchers have reported that 4H-SiC based detectors are able to effectively detect thermal neutrons [7,12] and fast neutrons [9]. In addition, the wide band gap of 4HSiC (3.25 eV) greatly enhances the temperature resistance of a neutron detector. Consequently 4H-SiC based spectrometers are expected to be usable in elevated temperature and harsh

n

Corresponding author. Tel.: þ86 816 2496754; fax: þ86 816 2495280. E-mail addresses: [email protected] (J. Wu), [email protected] (J. Lei).

radiation circumstances. The conceptual design of a SiC sandwich neutron spectrometer can be found in Ref. [13], which demonstrates that 4H-SiC Schottky diodes are possible candidates of neutron spectrometer. However, the actual performance of the 4H-SiC sandwich neutron spectrometer remains unknown due to the lack of experimental results. In this work, we report the assembling and testing of a 4H-SiC based sandwich neutron spectrometer for the first time. First, we perform the construction and characterization of 4H-SiC Schottky diode. Next, we assemble and then test sandwich neutron spectrometer with low-energy neutrons. Finally, we present and discuss experimental results of the sandwich neutron spectrometer.

2. Detector construction and characterization The sandwich neutron spectrometer is composed of two closely-spaced SiC Schottky diodes with a 6LiF converter layer placed in-between (Fig. 1(a)). When neutrons interact with 6Li targets, alpha particles and tritons are produced in the following channel [14]: 6

Liþ 1 n ¼ 4 Heþ 3 H þ 4:780 MeV

ð1Þ

The Q value of the reaction and the kinetic energy of the neutron are shared by the alpha particle and triton. For thermal neutron irradiation, the alpha particle and the triton are emitted

0168-9002/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.01.018

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Fig. 1. (a) Schematic representation of the sandwich neutron spectrometer (dimensions are not to scale). Thermal neutron incident on the spectrometer is represented in the scheme. (b) Schematic drawing of 4H-SiC Schottky diode with 6 LiF neutron converter (dimensions are not to scale).

with an energy of 2.05 MeV and 2.73 MeV, respectively. For higher energy neutron irradiation, an initial neutron energy can be derived by subtracting the Q value of 6Li(n,a)3H reaction from the sum of the kinetic energies of the alpha particle and triton. Fig. 1(b) shows a schematic diagram of SiC Schottky diode. The 4H-SiC epitaxial layers were grown onto 4H-SiC substrate wafers by vapor-phase epitaxy at the Nanjing Electronic Devices Institute, Nanjing (China). A 1 mm n þ buffer layer (  1018 nitrogen atoms per cm3) was epitaxially grown onto a 360 mm conducting 4H-SiC substrate wafer. Then lightly doped n  epilayer with nitrogen doping concentration and thickness of 2.44  1015/cm3 and 13 mm, respectively, was deposited on the n þ buffer layer. The ohmic contact was realized by the deposition of Ni/Au (1/6 mm) multilayer followed by annealing at 1000 1C for 5 min in N2. The Schottky contact was formed on the surface of the epitaxial layer by the deposition of a 100 nm nickel film. The 6LiF (90% enriched in 6Li) converter layer was deposited on the front surface of the Schottky diode by magnetron sputtering. 6 LiF films were subsequently characterized by scanning electron microscopy (SEM). 6LiF layers with a thickness ranging from 10 nm to 2.5 mm were obtained by adjusting the time and power of sputtering. Fig. 2(a) and (b) shows SEM images of surface topography and cross-section of a 2.5 mm 6LiF converter layer, respectively. As shown in these figures, it appears that a 6LiF film successfully forms on the diode with uniform thickness. Finally, a batch of 4H-SiC Schottky diodes with or without 6LiF converter layer were fabricated and mounted on copper base plates individually (Fig. 3). All the diodes were electrically characterized by measuring reverse I–V curve at room temperature. Leakage currents ranging from 0.09 nA to 6.4 nA were measured as the reverse bias varied from 10 V to 600 V. Fig. 4 shows a typical reverse I–V curve of the diode, indicating a successful formation of Schottky contact between Ni and 4H-SiC epitaxial layer. Additionally, no differences in the reverse I–V

Fig. 2. (a) Surface topography of a 2.5 mm 6LiF converter layer and (b) crosssection of a 2.5 mm 6LiF converter layer.

Fig. 3. Photograph of a 4H-SiC diode mounted on a copper base plate. The active area of the diode is 3  3 mm2.

properties were observed between diodes with or without 6LiF converter layer.

3. Alpha particle response of SiC Schottky diode It is necessary to select a pair of diodes with near-identical geometric and electrical properties to assemble the SiC sandwich spectrometer. Hence, an investigation of alpha particle response of SiC Schottky diodes was carried out first by exposing SiC diodes

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to 241Am alpha particles in vacuum at a distance of 6 mm. The experimental equipment used to test the SiC diodes is illustrated in Fig. 5. The diodes were tested by applying the reverse bias at 300 V. A typical 241Am alpha spectrum acquired with the diodes is showed in Fig. 6. The observed energy resolution for 241Am alpha energy peak, defined as full width at half maximum (FWHM) divided by the peak centroid, is 4.3% (5.486 MeV emission energy, 2.86 MeV energy deposited in the 11.5 mm active layer, the remaining energy deposited in the non-active area, calculated by SRIM2011 [15]). The energy resolution is mostly affected by the energy straggling suffered by non-normal incident alpha particles, as the collimator used in this experiment is less efficient than those used in other previous works [16], due to its relatively large diameter (2 mm). However, a higher gross counting rate can be achieved with the large-diameter collimator, so that the process of grouping the diodes into pairs will be faster.

Subsequently, the peak centroid and FWHM of the 241Am alpha spectrum measured by each SiC diode were utilized to group the diodes into pairs. Two SiC Schottky diodes, designated as A1 and A2, were selected as a pair based on the experimental results. The summary testing results of diodes A1 and A2 are reported in Table 1. Then, a SiC sandwich neutron spectrometer was assembled with A1 and A2 (Fig. 7).

Fig. 6. Typical response of a SiC Schottky diode to 241Am alpha particles at applied bias voltage of 300 V. Counting time ¼300 s.

Table 1 The summary testing results of diodes A1 and A2 at Vreverse ¼300 V. Sample no.

Peak centroid

FWHM

Leak current (nA)

Description

A1 A2

2032.3 2032.2

100.0 102.5

3.40 3.38

2.5 mm 6LiF layer

Fig. 4. Reverse I–V characteristic for the 4H-SiC Schottky diode.

Fig. 5. Experimental setup for SiC diode testing. Preamplifier—ORTEC 142A, Amplifier—ORTEC 671, Multichannel Analyzer—ORTEC 926, and PC—Personal Computer with a MAESTRO-32 software installed.

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4. Neutron response of the SiC sandwich neutron spectrometer Only the 6Li(n,a)3H reaction is considered as the measurements were carried out in low-energy neutron fields. The experimental equipment used to test the SiC sandwich neutron spectrometer is illustrated in Fig. 8. Fission neutrons generated

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by the Chinese Fast Burst Reactor-II (CFBR-II) were slowed down by paraffin with a thickness of 9 cm. The SiC sandwich neutron spectrometer was placed close to the core of the CFBR-II at a distance of 97 cm. A vacuum pump was then used to maintain low vacuum environment (4.5  103 Pa) when data acquisition was carried out. An oscilloscope was attached to the pulse-processing system occasionally to observe the signal pulses from both diodes of SiC sandwich neutron spectrometer. By connecting only one diode to a Dual Sum and Invert Amplifier, spectra with two well-resolved peaks were acquired utilizing the A1 and A2 diodes (Fig. 9). The two peaks in each spectrum refer to alpha particles and tritons induced by low-energy neutrons. The reverse bias voltage (100 V) determines a 6.68 mm depletion layer width of 4H-SiC Schottky diode, which is sufficient to completely stop all the alpha particles with kinetic energy of approximately 2 MeV. The value of 2.73 MeV tritons in SiC, calculated by SRIM2011 as 27.93 mm, is larger than the 13 mm thickness of 4H-SiC epilayer (i.e. the maximum active layer of the diode); thus most of the tritons will pass through the active layer without depositing all their energies. Using the SRIM2011 code, we find that the kinetic energies deposited in the 6.68 mm depletion layer for 2.73 MeV triton and 2.05 MeV alpha particle are estimated at 0.47 MeV and 2.05 MeV, respectively. The kinetic energy deposited in the neutron converter layer and dead layer (100 nm nickel) is ignored in the calculation. According to the calculated results, peaks at lower channels refer to tritons, while peaks at higher channels refer to alpha particles. The tails of the triton peak in each spectrum is attributed to non-normally incident tritons. The sum spectrum was acquired by connecting both diodes to the Dual Sum and Invert Amplifier (Fig. 10). A Gaussian curve with 204.9-channel FWHM provides a good fit to the measured spectrum. The energy resolution is 8.8% for the fitted peak. However, this does not represent the energy resolution of 4H-SiC sandwich neutron spectrometer, since triton kinetic energy deposition in the depletion layer was not complete. In fact, the resolution of the system as a fast neutron spectrometer can be estimated by using the following expressions:  2 ðFWHMneutron Þ2 ¼ FWHMalpha þ ðFWHMtriton Þ2 ð2Þ and

Fig. 7. Unassembled SiC sandwich neutron spectrometer. A pair of closely-spaced 4H-SiC diodes is mounted on the top of a steel wire.

FWHMalpha 4FWHMtriton

ð3Þ

Fig. 8. Experimental setup for SiC sandwich neutron spectrometer testing. Preamplifier—ORTEC 142A, Amplifier—ORTEC 671, Timing Single-Channel Analyzer—ORTEC 551, Universal Coincidence—ORTEC 418A, Linear Gate—ORTEC 426, Delay Amplifier—ORTEC 427A, Dual Sum and Invert Amplifier—ORTEC 533, Multichannel Analyzer—ORTEC 926, and PC—Personal Computer with a MAESTRO-32 software installed.

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Fig. 9. Spectra obtained by diodes A1 and A2 at 100 V. Counting rates for diodes A1 and A2 are 69.33 cps, and 69.44 cps, respectively.

5. Conclusions and future works

Fig. 10. Sum spectrum obtained by 4H-SiC sandwich neutron spectrometer and fit of a Gaussian shape to the spectrum. Counting rate is 69.28 cps.

A 4H-SiC sandwich neutron spectrometer prototype was fabricated and tested in this work. The manufacture of 4H-SiC Schottky diodes, the deposition of 6LiF converter on the diodes, and their characterization were performed. Low-energy neutron detection was achieved at the CFBR-II and spectra of alpha particles and tritons from 6Li(n,a)3H reaction were obtained with two wellresolved peaks. The energy resolution of the sum spectrum is 8.8%. The preliminary experimental results confirmed the feasibility of SiC sandwich neutron spectrometer. As triton kinetic energy deposition in the depletion layer was not complete, the initial neutron spectrum cannot be obtained by deconvolution of the sum spectrum. In order to fabricate a sandwich neutron spectrometer that is suitable for the measurement of neutrons with kinetic energy ranging from a few hundred KeV to several MeV, manufacture of 4H-SiC Schottky diodes with thicker epilayer is under way. Further experiments of neutron spectrum measurement at CFBR-II and other reactors are also planned to be carried out in the future.

Acknowledgment where FWHMneutron, FWHMalpha and FWHMtriton are FWHM values of neutron peaks, alpha peaks, and triton peaks, respectively (Fig. 9 and Fig. 10). It is assumed that the empirical relationship between FWHMalpha and FWHMtriton (see Eq. (3)), which is common for Si sandwich neutron spectrometers [3], is applicable in the case of 4H-SiC based ones. By using Eqs. (2) and (3), one obtains:

The authors would like to thank Bo Liu of the Institute of Nuclear Science and Technology Sichuan University, Chun Zheng, Wenqing Sun, Youli Yang, Hongchao Zhao and other workers of China Academy of Engineering Physics for their invaluable contributions in fabrication and testing of these detectors, and in obtaining the neutron detections performed in this study.

pffiffiffi FWHMalpha o FWHMneutron o 2FWHMalpha

References

ð4Þ

In order to include the possible impact of the 2 mm air gap between the LiF layer and one of the SiC diodes (Fig. 1a), FWHMalpha of A1 (Fig. 9) is used. According to Eq. (4), FWHMneutron is in the interval of (163 and 230 KeV), and this is even lower than FWHMs of Si based ones [3]. Consequently, the estimated energy resolution for thermal neutrons is in the interval of 3.4% and 4.8%. The neutron-induced signal can be further sharpened by inserting a pinhole collimator in the sandwich neutron spectrometer [3]. Additionally, no g related peaks were observed in either spectrum though the measurements were carried out in a mixed neutron/g field, indicating the g pulses were successfully eliminated through coincidence measurement and threshold adjustment.

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