Abstract—Fast neutron response measurements are reported

for radiation detectors based on large-volume SiC p-i-n diodes. Multiple reaction peaks are observed for 14-MeV neutron reactions with the silicon and carbon nuclides in the SiC detector. A high degree of linearity is observed for the 28Si(n,αi) reaction set of six energy levels in the product 25Mg nucleus, and pulse height defect differences between the observed 12C(n,α0) and 28Si(n,αi) energy responses are discussed. Energy spectrometry applications in fission and fusion neutron fields are also discussed.

I. INTRODUCTION ilicon carbide (SiC) semiconductor radiation detectors are being developed for a variety of radiation detection

and monitoring applications. The wide band gap of SiC (3.25 eV) compared to more conventionally used semiconductors such as silicon (1.14 eV) and germanium (0.77eV) makes SiC an attractive semiconductor for use in nuclear environments due to increased resistance to radiation effects and the capability to operate stably in elevated and changing temperatures [1].

SiC detectors are under development for alpha particle [1]-[3], beta particle [4], and gamma-ray detection [5]-[7]. Recently, X-ray and low-energy gamma ray spectrometry with SiC detectors has been reported [8],[9]. Measurements of thermal neutron [10]-[13], epithermal neutron [6], and fast neutron [14] fluence rates have also been reported. Fast neutron spectrometry using SiC detectors has also been proposed [15]. In the present paper we will describe the extension of these fast-neutron measurements to SiC detectors with larger active volumes.

Manuscript received October 8, 2004. This research was supported by the Office of Science (BER), U. S. Department of Energy, Grant No. DE-FG02-04ER63734.

Frank H. Ruddy, Abdul R. Dulloo, and John G. Seidel are with the Westinghouse Electric Company, Department of Science & Technology, 1332 Beulah Road, Pittsburgh, PA 15235-5081, USA (Frank H. Ruddy telephone: 412-256-1064, e-mail: ruddyfh@westinghouse.com; Abdul R. Dulloo telephone: (412) 256-2140, e-mail dullooar@westinghouse.com; John G. Seidel telephone: (412) 256-1039, e-mail: seideljg@westinghouse.com).

Mrinal K. Das, Sei-Hyung Ryu, and Anant K. Agarwal are with Cree, Inc., 4600 Silicon Dr., Durham, NC 27703, USA (Mrinal K. Das telephone: (919) 313-5584, e-mail: mrinal das@cree.com; Sei-Hyung Ryu telephone: (919) 313-5541, e-mail: sei-hyung ryu@cree.com; Anant Agarwal telephone: (919) 313-5539, e-mail: anant agarwal@cree.com)

SiC radiation detectors are expected to be useful for detection of fissionable material in nuclear waste management and security screening applications as well as for neutron detection in fission and fusion applications.

II. BACKGROUND Detection of fast neutrons in SiC detectors is based on the

observation of charged-particle products of neutron-induced reactions with silicon and carbon nuclides and other nuclides close to the active volume of the detector as illustrated in Fig. 1. Generally, neutron elastic scattering is possible with Fig. 1. Schematic representation of fast neutron reactions

with silicon and carbon nuclides within the active volume of a SiC Schottky diode.

neutrons of all energies, and other reactions may be possible depending on the neutron energy and reaction energy threshold as indicated by the data in Table I. The pulse height response spectra for a 10-µm x 28 mm2 Schottky diode exposed to 14 MeV neutrons from an electronic deuterium-tritium (D-T) accelerator is shown in Fig. 2 reproduced from [15]. Many of the features observed in this pulse height spectrum can be related to specific nuclear reactions listed in Table I. A comparison of the SiC response to 252Cf fission neutrons (average energy 2.15 MeV) and 241Am-berylium (α,n) neutrons (average energy 4.5 MeV) to the 14-MeV response is shown in Fig. 3 [15]. The responses clearly reflect the energies of the incident neutrons. However, the quality of the neutron response spectrum will depend on the retention of the energy of the reaction product ions in the detector active volume. The 10-µm thick active volume of the detector used

The Fast Neutron Response of Silicon Carbide Semiconductor Radiation Detectors

Frank H. Ruddy, Abdul R. Dulloo, Member, IEEE, and John G. Seidel and

Mrinal K. Das, Sei-Hyung Ryu, and Anant K. Agarwal

S

1-µm gold overSchottky contact

10-100 µm n-

active layer300-µm SiC

substrateBack ohmic

contact

n

n'

12C

n

α

9Be

nα

αα

n'

n

n'

28Si

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TABLE 1 FAST NEUTRON-INDUCED REACTIONS IN SILICON CARBIDE

REACTION Q Value (MeV)

Energy Threshold (MeV)

12C(n,n’)12C ground state 0 0 12C(n,n’)12C 2+ -4.4389 4.8088

12C(n,α)9Be -5.7012 6.4196 12C(n,n’)3a -7.3666 8.4286 12C(n,p)12B -12.5865 13.7401

28Si(n,n’)28Si ground state 0 0 28Si(n,n’)28Si 2+ -1.7790 1.8425

28Si(n,α)25Mg -2.6537 2.7653 28Si(n,p)28Al -3.8599 4.0042

1

10

100

1000

10000

0 2000 4000 6000 8000 10000 12000

Energy (keV)

Cou

nts

per C

hann

el

12C(n,α)9Be8.428 MeV

12C(n,n')3αEmax=6.756 MeV

28Si(n,n')gs Emax=1.881 MeV

28Si(n,n')2+ Emax=1.644 MeV

12C(n,n')gsEmax=4.011 MeV

12C(n,n')2+ Emax=2.751 MeV

28Si(n,p),(n,α)

Fig. 2. Response of a 28-mm2 x 10-µm SiC Schottky Diode

to 14-MeV neutrons.

1

10

100

1000

10000

0 2000 4000 6000 8000 10000Energy (keV)

Cou

nts

per C

hann

el

Cf-252 Neutrons

Am-Be Neutrons

14-MeV Neutrons

Fig. 3. Comparison of the response spectra to 252Cf (2.15

MeV), 241Am-Be (4.5 MeV) and D-T (14 MeV) for a 28-mm2 x 10-µm Schottky Diode.

in [15] resulted in the loss of some of the reaction product energy due to finite-volume effects. This effect is most apparent in the data of Fig. 2, where the silicon reaction peaks are not well defined.

Recently, larger-volume SiC detectors have become available due to the improved capability to manufacture lightly doped epitaxial layers with thickness of 100 µm and higher. Furthermore, the dopant concentration can be kept small enough to allow the entire epitaxial region to be depleted at a modest voltage so that the active volume of the SiC detector will be thick compared to the recoil ranges of the fast neutron-induced reaction products. The retention of the energies of

these reaction products within the active volumes of the detectors will greatly improve the quality of the fast-neutron response spectra.

III. EXPERIMENTAL P-i-n diodes with 4.4-mm x 4.4-mm dimensions were

fabricated by chemical vapor-phase deposition (CVD) epitaxy onto 350-µm 4H-SiC n-type substrate wafers with 20 milliohm-cm resistivity. A SiC n- layer with a nitrogen dopant concentration of (1.5-2.5) x 1014 cm-3 and 90-µm thickness was grown on the Si face of the substrate. Next, a 2-µm SiC p+ layer with an aluminum concentration of (5-8) x 1018 cm-3 was grown over the n- layer followed by a 0.5-µm p++ layer containing an aluminum concentration of 5 x 1019 cm-3. The p++ contact layer was covered with an 8- µm layer of gold and the back ohmic contact consisted of 4 µm of gold. Devices were terminated with a double-zone, 400 µm Junction Termination Extension designed for over 5 kV of blocking. Reverse biases in the 0-1000 V range were applied to create the depleted regions, which act as the active region in the diode for detection of the ionization produced by charged particles. Reverse biases up to -900 V were applied in most measurements to create depletion depths up to 90 µm while avoiding excessive diode leakage currents. A schematic representation of the p-i-n diode structure is shown in Fig. 4.

Fig. 4. Schematic representation of the layered structure for a SiC p-i-n diode. Not drawn to scale.

IV. MEASUREMENTS The p-i-n diodes were exposed to 14-MeV neutrons from a

D-T electronic source with the SiC detector plane positioned 5 cm from the source target and the plane of the detector 900 to a line drawn to the source. The p+ layer was closest to the target. Under these conditions, the neutron energy impinging on the SiC detector is 14.0 MeV.

A reverse bias of -900 volts was applied to the SiC detector, which was connected to a Canberra 2003BT preamplifier. Bias to the SiC diode was provided by an Ortec 428 Bias Supply through the 2003BT preamplifier. A Canberra 2024 amplifier was used. Output of the amplifier went to four Ortec 921E Buffers connected in parallel. Gating of these buffers was provided by an EG&G 9650A Digital Delay Generator. The 921E Buffers are hardware components of the Ortec ADCAM architecture and were interfaced to a personal computer, for display of the data, using Ortec’s GammaVision software.

SiC Conducting Substrate

n- Layerp+ Layer 2-µm (5-8)x1018 Al/cm3

90-µm (1.5-2.5)x1014 N /cm3

350-µm conductingsubstrate (20 milliohm-cm)

Au (8 µm)

Au (4 µm)

0.5-µm p++ (5x1019 Al/cm3)4.4-mm x 4.4-mm detector area

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V. RESULTS

A representative response spectrum is shown in Fig. 5. In contrast to the spectrum of Fig. 3, which was taken with a detector with a much smaller active volume thickness, detailed peak structure is now observed for multiple branches of the 28Si(n,α)25Mg reaction as well as for the previously observed 12C(n,α0)9Be reaction. A summary of the expected reaction branches and corresponding energies is contained in Table II.

TABLE 1I

FAST NEUTRON-INDUCED REACTION BRANCHES AND ENERGIES REACTION BRANCH ENERGY (keV)

12C(n,α)9Be Q=-5701.2 keV Ground State - α0 8298.8 1st Excited State - α1 6614.1 2nd Excited State - α1 5869.4

12C(n,p)12B Q=-12586.5 Ground State - p0 1413.5 1st Excited State - p1 460.36

28Si(n,α)25Mg Q=-2653.7 keV Ground State - α0 11346.3 1st Excited State - α1 10761.26 2nd Excited State - α2 10371.56 3rd Excited State - α3 9734.533 4th Excited State - α4 9381.69 5th Excited State - α5 8544.83 6th Excited State - α6 7941.1 7th Excited State - α7 7932.33 8th Excited State - α8 7438.6 9th Excited State - α9 7375.6 10th Excited State - α10 7286.7 11th Excited State - α11 7069.5 12th Excited State - α12 6986.9

28Si(n,p)28Al Q=-3859.9 keV Ground State - p0 10140.1 1st Excited State - p1 10109.46 2nd Excited State - p2 9167.72 3rd Excited State - p3 9126.474 4th Excited State - p4 8767.15

0

50

100

150

200

250

300

4000 5000 6000 7000 8000 9000 10000 11000 12000Energy (keV)

[Based on 28Si(n,α) Reactions]

Cou

nts

per C

hann

el

Gaussian Fits

28Si(n,alpha) Peak Positions

14-MeV Neutron Response Data

α0α1

α2α3α4

α5

α6α7

α8

α9

α10

α11α12

12C(n,α0)9Be

Fig. 5. Response spectrum for a SiC p-i-n diode exposed to

14.0-MeV neutrons.

The 28Si(n,α)25Mg ground state and first four excited state branches are resolved in energy and are well represented by Gaussian shapes as shown in Fig. 5. The peak for the branch

to the fifth excited state of 25Mg can not be resolved from the 12C(n,α0) peak. The branches to the sixth and seventh excited states are only separated by 8.77 keV and the peak can be fit to a single Gaussian. A plot of the calculated peak energy as a function of observed detector pulse height (channel number) for the α0, α1, α2, α3, α4, and α6/α7 peaks is shown in Fig. 6.

y = 3.8753x + 107.3022R2 = 0.9998

7500

8500

9500

10500

11500

2000 2200 2400 2600 2800 3000

Channel Number

Ener

gy (k

eV)

25Mg Peaks12C(n,alpha)9Be Peak9Be CalibrationLinear (25Mg Peaks)

Fig. 6. Energy calibration for the 28Si(n,α) peaks in SiC. A

different pulse height defect is observed for the 12C(n,α0) peak due to differences in energy deposition between 9Be and 25Mg ions.

The fit of the six 28Si(n,α)25Mg data points to a straight line indicates a high degree of linearity of the SiC detector energy response.

Only the ground state branch for the 12C(n,α)9Be reaction is observed, because the branches to the excited states are obscured by the 12C(n,n’), 12C(n,n’)3α and 28Si(n,n’) pulse height continua. The data point for this peak does not fall on the line established by the 25Mg peaks, because the pulse height defect is different for the 12C and 28Si (n,α) reactions. Generally, more of the energy of the 9Be ions will go into ionization (electronic stopping) than for the 25Mg ions resulting in a higher pulse height for the 12C(n,α0)9Be reaction.

Pulse height defect effects are further evident in the shapes of the observed reaction peaks. The peaks for the 12C(n,α0)9Be and 28Si(n,α0)25Mg reactions are shown in Fig. 7. Note that the full width at half maximum (FWHM) for the 28Si(n,α0)25Mg reaction peak is 1.4 times the value for the 12C(n,α0)9Be peak. Also, the widths of both peaks are much broader than would be observed for alpha particle peaks of corresponding energies. Both the 9Be and 25Mg reaction product ions are formed with a range of energies depending on the reaction angle, because the 14-MeV neutrons impart momentum to the reaction system. In the case of 25Mg, the minimum and maximum ion energies are 336.8 keV and 3542.0 keV respectively. The corresponding energies for the 9Be ions are 397.6 keV and 5625.1 keV. In both cases, the fraction of the energy corresponding to electronic stopping will decrease with energy and cause peak broadening. More energy will go into non-electronic stopping

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processes for the higher atomic number 25Mg ions, resulting in a broader peak for the 28Si(n,α0) reaction.

0

50

100

150

200

250

300

8300 8400 8500 8600 8700 8800 8900Energy (keV)

Cou

nts

per C

hann

el

DataGaussian Fit

FWHM = 192 keV

12C(n,α0)9Be

E=8298.8 keV

(a)

0

20

40

60

80

100

120

140

160

180

200

11100 11200 11300 11400 11500 11600 11700

Energy (keV)[Based on 28Si(n,α) Reactions]

Cou

nts

per C

hann

el

DataGaussian Fit

28Si(n,α0)25Mg

E=11346.3 keV

FWHM = 271 keV

(b)

Fig. 7. Peaks observed for the 12C(n,α0)9Be (a) and 28Si(n,α0)25Mg (b) reactions

Noticeably absent in Fig. 5 are any peaks due to 12C(n,p)

and 28Si(n,p) reactions. If the values of Gaussian fits to the peaks in Fig. 5 are subtracted from the data, the plot shown in Fig. 8 results. Although peaks corresponding to (n,p) reactions

0

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30

40

2200 2300 2400 2500 2600 2700 2800 2900 3000

Channel Number

Cou

nts

per C

hann

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Data minus Si(n,alpha)Gaussian FitsGaussian Fits to (n,p)Peaks

28Si(n,p0)28Al

+28Si(n,p1)

28Al

28Si(n,p2)28Al

+28Si(n,p3)

28Al

Fig. 8. Spectrum obtained by subtracting the Gaussian fits

to the (n,α) peaks in Figure 5 from the data. The peaks in the residual data correspond to (n,p) reactions.

can be identified in the data, these peaks are weak and are not well defined, because the reaction-product proton ranges are larger that the thickness of the active volume of the detector. Therefore, much of the reaction energy is deposited outside of the detector active volume, and the observed peaks are not

fully representative of the reaction energy. Much thicker active volumes are necessary to observe the (n,p) reaction peaks quantitatively.

VI. DISCUSSION AND CONCLUSIONS Previous work has been reported for fast neutron reactions

with the components of diamond (carbon) [16]-[18] and silicon [19]-[23] radiation detectors.

Kaneko, et al., [16] observed the peak from the 12C(n,α0)9Be reaction produced by 14-MeV neutrons in a synthetic type IIa single diamond crystal. The FWHM for the reaction peak was 2%. The corresponding energy value, ~170 keV, is comparable to the observed SiC FWHM, 192 keV. Although Kaneko, et al. attributed the 2% energy resolution mainly to electron trapping effects in the diamond detector, the detector pulse height defect may be a more likely source for the energy broadening of the peak in the diamond detector.

Schmid, et al. [18] reported 2.9% resolution for the 12C(n,α0)9Be reaction peak in a single-crystal CVD diamond detector and noted that the measurement was limited by the energy spread of the incident neutrons. Based on an extrapolation of high-energy alpha particle data, these authors predict 0.4% resolution for the carbon reaction peak, but this value is unlikely to be achieved due to pulse height defect effects in the diamond detector.

Mingay, et al., [22] reported that their best energy resolution for the (n,α) groups observed in a silicon detector was 120 keV for a detector with a resolution of 28 keV for 5.477-MeV 241Am alpha particles. The broad (n,α) resolution was attributed to the energy loss defect caused by the 25Mg ions as discussed above. The reported silicon FWHM value is considerably smaller than the 271 keV FWHM observed in SiC. Part of this discrepancy is due to differences in ion stopping in SiC and silicon and to finite-size effects in the SiC detector, but further investigation is needed.

Elevant, et al., [19] have reported that the temperatures of D-T plasmas can be measured through reactions produced by the fusion neutrons in silicon detectors. Based on a measured FWHM of 150 keV, D-T plasma ion temperatures can be measured at temperatures from 2 to 6 keV with estimated errors of 25% to 2%, respectively [19]. Although the observed energy resolution for silicon (n,α) groups for SiC detectors appears to be less than for silicon detectors in measurements to date, the superior radiation resistance and ability to operate at elevated temperatures [24],[1] will favor SiC detectors in some fusion applications.

For lower-energy, fission-neutron applications, recoil ion spectrometry appears to be possible. The recoil-ion energy spectrum can be calculated based on the observed pulse height spectrum using appropriate range-energy calculations such as SRIM [25], and the recoil-ion spectrum can be unfolded to determine the corresponding incident neutron energy spectrum.

SiC neutron detectors are also expected to provide an effective means for detecting concealed special nuclear

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materials in security screening applications as well as for quantifying actinide isotopes in nuclear waste management.

VII. ACKNOWLEDGMENT We gratefully acknowledge the contributions of Haoquian

Chen with computational aspects of this work. We also acknowledge Bojan Petrović for helpful discussions.

VIII. REFERENCES [1] F. H. Ruddy, A. R. Dulloo, J. G. Seidel, S. Seshadri, and L. B. Rowland,

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[2] F. Nava, P. Vanni, C. Lanzieri, and C. Canali, “Epitaxial silicon carbide charged particle detectors”, Nucl. Instr.& Meth., A437 (1999) pp. 354-358.

[3] F. H. Ruddy, A. R. Dulloo, J. G. Seidel, J. W. Palmour, and R. Singh, “The charged particle response of silicon carbide semiconductor radiation detectors”, Nucl. Instr. & Meth. A505 (2003) pp. 159-162.

[4] H.-C. I. Chen, A. R. Dulloo, J. G. Seidel, and F. H. Ruddy, “Detection of minimum ionizing beta-particle radiation with a silicon carbide radiation detector”, Westinghouse Science & Technology Report 98-1TK1-SICBX-R1, October, 1998.

[5] F. H. Ruddy, A. R. Dulloo, and J. G. Seidel, “The gamma-ray response of silicon carbide radiation detectors”, Trans. Am. Nucl. Soc. 79 (1998), pp.113-114.

[6] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, C. Davison, T. Flinchbaugh, and T. Daubenspeck, “Simultaneous measurement of neutron and gamma-ray radiation levels from a TRIGA reactor core using silicon carbide semiconductor detectors”, IEEE Transactions on Nuclear Science vol. 46,no. 3, June 1999, pp. 275-279.

[7] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, T. Flinchbaugh, C. Davison, and T. Daubenspeck, “Neutron and gamma ray dosimetry in spent-fuel radiation environments Using Silicon Carbide Semiconductor Radiation Detectors”, in Reactor Dosimetry: Radiation Metrology and Assessment, ASTM STP 1398, John G. Williams, David W. Vehar, Frank H. Ruddy, and David M. Gilliam, Eds., American Society for Testing and Materials, West Conshohoken, PA, 2001, pp 683-690.

[8] G. Bertuccio, R. Casiraghi, A. Cetronio, C. Lanzieri, and F. Nava, “Silicon carbide for high resolution X-ray detectors operating up to 100 0C”, Nucl. Instr.& Meth., A522 (2004), pp.413-419.

[9] G. Bertuccio, R. Casiraghi, A. Cetronio, C. Lanzieri, and F. Nava, “A new generation of X-ray detectors based on silicon carbide”, Nucl. Instr.& Meth., A518 (2004), pp. 433-435.

[10] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, J. M. Adams, J. S. Nico, and D. M. Gilliam, “The neutron response of miniature silicon carbide semiconductor detectors”, Nucl. Instr. & Meth ,A422 (1999) , pp. 47-48

[11] F. H. Ruddy, A. R. Dulloo, J. G. Seidel, F. W. Hantz, and L. R. Grobmyer, “Nuclear reactor power monitoring using silicon carbide semiconductor radiation detectors”, Nucl. Technology 140, (2002). Pp. 198-208.

[12] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, J. M. Adams, J. S. Nico, and D. M. Gilliam, “The thermal neutron response of miniature silicon carbide semiconductor detectors”, Nucl. Instr. & Meth, A498, (2003), pp. 415-423.

[13] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, S. Lee, B. Petrović and M. E. McIlwain, “Neutron fluence rate measurements in a PGNAA 208-liter drum assay system using silicon carbide detectors”, Nucl. Instr. & Meth., B213 (2004) pp. 400-405.

[14] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, and B. Petrović, “Monitoring of D-T accelerator neutron output in a PGNAA system using silicon carbide detectors”, Applications of Accelerators in Research and Industry – Sixteenth International Conference, AIP CP576 (2001), pp. 499-503.

[15] F. H. Ruddy, A. R. Dulloo, B. Petrović, and J. G. Seidel, “Fast neutron spectrometry using silicon carbide detectors”, in Reactor Dosimetry in the 21st Century, J. Wagemans, H. A. Abderrahim, P. D’hondt, and C. De Raedt (Eds.), World Scientific, London (2003) pp. 347-355.

[16] J. Kaneko, Y. Ikeda, T. Nishitani, and M. Katagiri, “Response function measurement of a synthetic diamond radiation detector for 14-MeV

neutrons”, 12th Topical Conference on High-Temperature Plasma Diagnostics, Princeton, NJ, June 7-11, 1998

[17] A. V. Krasilnikiv, J. Kaneko, M. Isobe, F. Maekawa, and T. Nishitani, “Fusion neutronic source Deuterium-Tritium neutron spectrum measurements using natural diamond detectors”, Rev. Sci. Instrum. 68 (1997), pp. 1720-1724.

[18] G. J. Schmid, J. A. Koch, R. A. Lerche, and M. J. Moran, “A neutron sensor based on a single crystal CVD diamond”, ”, Nucl. Instr.& Meth., A527 (2004), pp. 554-561.

[19] T. Elevant, H. W. Hendel, E. B. Nieschmidt, and L. E. Samuelson, “Silicon surface barrier detector for fusion neutron spectroscopy”, Rev. Scientific Instruments 57, (1986) 1763.

[20] T. B. Ryves and K. J. Zieba, “A measurement of the energy of neutrons from the T(d,n)4He reaction using a silicon detector”, Nucl. Instr.& Meth., 167 (1979), pp. 449-453.

[21] S. Dési, “Fast neutron flux measurement and spectroscopy using silicon semiconductor detectors”, Nucl. Instr.& Meth., 140 (1977), pp. 467-471.

[22] D. W. Mingay, J. P. F. Sellschop, and P. M. Johnson, “Neutron induced reactions in silicon semiconductor detectors”, Nucl. Instr.& Meth.., 94 (1971), pp. 497-507.

[23] J. A. Shannon and J. B. Trice, “Using the 28Si(n,q) interaction in fast neutron spectroscopy”, Nucl. Instr.& Meth., 41 (1966), pp. 255-260.

[24] S. Seshadri, A. R. Dulloo, F. H. Ruddy, J. G. Seidel, and L. B. Rowland, “Demonstration of a SiC neutron detector for high radiation environments”, IEEE Transactions on Electronic Devices vol. 46, No.3, pp. 567-571, March 1999.

[25] J. F. Ziegler and J. P. Biersack, “SRIM-2003.26: The stopping and range of ions in matter”, SRIM.com, Annapolis, MD (2003).

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