R5B  Alternative Materials 2

Friday, Nov. 6  10:30-12:30  California

Session Chair:  Robert McLaren, Consultant, United States

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(10:30) R5B-1, invited, Investigation of 12 μm 4H-SiC Epilayers for Radiation Detection and Noise Analysis of Front-end Readout Electronics

K. V. Nguyen1, R. O. Pak1, C. Oner1, F. Zhao2, K. C. Mandal1

1Department of Electrical Engineering, University of South Carolina, Columbia, SC, USA
2Department of Electrical Engineering, Washington State University, Vancouver, WA, USA
Schottky barrier radiation detectors were fabricated on 12 μm n-type 4H-SiC epitaxial layers grown on a 4˚ off-axis highly doped 4H-SiC substrate (0001). Schottky barrier junction properties were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. A diode ideality factor of 1.29 and Schottky barrier height of 1.10 eV were determined from the forward I-V characteristics using a thermionic emission model. A built-in potential of 1.91 V and effective carrier concentration of 1.03 ? 〖10〗^15 〖cm〗^(-3) was calculated from a Mott-Schottky plot of the C-V measurements. Radiation detector performance was evaluated by alpha pulse height spectroscopy (PHS) in terms of energy resolution expressed in full-width at half maxima (FWHM) and charge collection efficiency (CCE). The energy resolution was determined to be 166 keV with a CCE of 22.6% for 5.486 MeV alpha particles. Deep level transient spectroscopy (DLTS) measurements were carried out to investigate the deep levels in the detector active region. An electrically active defect level Z1/2 related to carbon vacancies was identified and characterized. The concentration and capture cross-section of Z1/2 were determined to be 1.58 ? 〖10〗^12 〖cm〗^(-3) and 9.12 ? 〖10〗^(-16) 〖cm〗^2, respectively. Electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system.

(10:50) R5B-2, invited, Spectroscopic Properties of CdMnTe Detector Grown by Seedless THM

K. Kim1, P. Kim1, C. Park1, S. Cho1, J. Lee2, G. Camarda3, A. Hossain3, A. Bolotnikov3, R. B. James3

1Radiologic Science, Korea University, Seoul, Republic of Korea
2AbyzR, Gyeonggi, Republic of Korea
3Brookhaven National Laboratory, Upton, USA
We successfully demonstrated unseeded growth of CMT by using THM and evaluated the spectroscopic properties of 6×6×20 mm3 CMT Frisch-grid detectors by using the57Co, 133Ba, 22Na, and 137Cs isotopes. Mobility-lifetime product calculated from the shift of 662-keV photo-peak vs. the bias, by using Hecht's equation, was 7×10-3 cm2/V. A Frisch-grid CMT detector successfully and simultaneously detected the 122 keV of 57Co and 662 keV of 137Cs. Energy resolution of 6.7 % and 2.1 % were obtained for 122 keV of 57Co and 662 keV of 137Cs, respectively, without using any additional signal processing techniques. In addition, during the data acquisition time of 300 s, 1.211 MeV peak of 22Na with 5µCi activity was successfully detected.

(11:05) R5B-3, Advances in Lead Oxide Technology

O. Semeniuk1,2, G. DeCrescenzo2, A. Reznik1,2

1Department of Chemistry and Material Science, Lakehead University, Thunder Bay, Canada
2Advanced detection devices department, Thunder Bay Regional Research Institute, Thunder Bay, Canada
Polycrystalline PbO is one of the most promising materials for applications in direct conversion X-ray detectors used in radiology and fluoroscopy. It has high X-ray detection quantum efficiency due to its high Z, and it has a theoretically predicted low electron-hole pair creation energy and hence high x-ray to charge conversion gain. In addition, the PbO deposition process is compatible with amorphous silicon (a-Si:H) electronics that allows large detector area coating. In 2005 Simon et al developed a first prototype of a large area flat-panel direct conversion detector where a thick layer of polycrystalline PbO was deposited over a-Si:H TFT (thin film transistors) panel. Despite these advantages, conventional PbO layers are porous, unstable in air, which makes this material challenging for practical utilization in X-ray detectors. In addition, the films are reported to have image lag i.e. residual signal after the end of x-ray exposure. Reported signal lag is the major constraint for application of PbO in real time imaging (i.e. dynamic imaging used in fluoroscopy) and restricts its application to static imaging only (radiology). We propose a new practical approach to the PbO structure for improved dynamic performance – the major drawback of conventional PbO layers. We show that our technological solutions also prevent sample degradation in ambient conditions. Temporal response, dark current and charge yield are investigated and found to compare favorably to published results on conventional polycrystalline film.

(11:20) R5B-4, Study of Spectral Distortion in Pixel CZT Detector

J. Fu1,2, Y. Li1,2, Y. Liu1,2, L. Zhang3, Y. Li1,2

1Department of Engineering Physics, Tsinghua University, Beijing, China
2Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education, Beijing, China
3Nuctech Company Limited, Beijing, China
Pixel CZT detectors are widely applied in gamma ray spectroscopy, medical imaging and so on. However, observed 662keV gamma spectrum of a single pixel has a low tail of full energy peak and presents high counts below 150keV, which will cause efficiency variations and spectral degradation for the identification of gamma ray sources.

In this study, a Monte Carlo simulation was developed to gain an insight of the reason for spectral distortion. The physics model taken into account includes electron trapping, backscatter and charge sharing. The model produced spectrum in good agreement with the measurement of a 4 × 4 pixelated detector.

This study confirmed that the reason for high counts below 150keV is not backscatter, but charge sharing. Another remarkable conclusion was that the low tail of full energy peak is mainly due to charge sharing rather than electron trapping. Veto spectrum was investigated to mitigate the effect of spectral distortion and demonstrated effective.

(11:35) R5B-5, Neutron Detection Using the Semiconductor 6LiInSe2

J. Tower1, H. Hong1, A. Kargar1, A. C. Stowe2,3, B. Wiggins2,3, Z. Bell4, P. Bhattacharya5, E. Tupitsyn5, L. Matei5, M. Groza5, A. Burger5

1Radiation Monitoring Devices, Inc., Watertown, MA, USA
2Vanderbilt University, Nashville, TN, USA
3Y-12 National Security Complex, Oak Ridge, TN, USA
4Oak Ridge National Laboratory, Oak Ridge, TN, USA
5Fisk University, Nashville, TN, USA
Our team is developing the wide-bandgap semiconductor LiInSe2 for room temperature neutron detection. The advantageous properties of LiInSe2 include high thermal neutron detection efficiency due to the presence of the lithium-6 isotope, a large bandgap and high resistivity for low-noise operation, and the ability to grow high quality single-crystals using melt based crystal growth techniques. Considerable progress has been achieved in the growth of high quality crystals and subsequent fabrication of semiconductor devices that can be used for neutron detection. We report on the crucial recent achievements leading to breakthroughs in detector performance. These include: • Controlled synthesis for reproducible composition. • Single-crystal ingots grown at 16-18 mm diameter. • Large-batch purification of enriched 6Li metal to 5 nines purity. • Crystal growth without 2nd phase precipitates. • Reduced point defects for better mobility-lifetime. • High resistivity for room temperature detector operation (without doping). • Radiation detection with good separation between neutrons and gamma rays. Detector performance has been correlated with measurable material properties to identify and control the crucial parameters. Photoluminescence, X-ray excited optical luminescence (XEOL), IR microscopy, optical transmission, and Laser Induced Breakdown Spectroscopy (LIBS) have been conducted to understand how the chemical purity and stoichiometry affect the detector performance. Electrical and radiation testing were performed using two-terminal planar devices, which were characterized for the current-voltage (I-V) characteristic and mobility-lifetime product (mu-tau), as well as radiation spectral responses. Pulse height spectra were collected from 241AmBe (thermal neutron source), 137Cs and 60Co (gamma rays), and 241Am (alpha particles). The resistivity of the LiInSe2 crystals is in the range 1011 - 1013 Ohm-cm, which is sufficiently high for low-noise operation at room temp.

(11:50) R5B-6, Advancements on Dual-Sided Microstructured Semiconductor Neutron Detectors

R. G. Fronk1, S. L. Bellinger2, L. C. Henson2, D. E. Huddleston3, T. R. Ochs1, C. J. Rietcheck1, J. K. Shultis1, C. T. Smith1, T. J. Sobering3, D. S. McGregor1

1Mechanical & Nuclear Engineering Dept., Kansas State University - S.M.A.R.T. Laboratory, Manhattan, KS, USA
2Radiation Detection Technologies, Inc., Manhattan, KS, USA
3Kansas State University - Electronics Design Laboratory, Manhattan, KS, USA
Microstructured semiconductor neutron detectors (MSNDs) represent a low-cost, high-efficiency means of solid-state thermal neutron detection. Straight trenches are etched into a pn-junction diode and backfilled with nano-sized 6LiF neutron converting material. Blocking contacts are formed on the top and bottom of the device, which is then biased to -2.6 V. Neutrons absorbed within the conversion material produce charged reaction products that interact within the semiconductor substrate and generate electron-hole pairs. The electron-hole pairs are swept out under the applied bias and are collected, generating an electronic pulse indicating a neutron event. Presently, single-sided MSNDs are approaching their theoretical maximum detection efficiency with devices nearing 35% intrinsic thermal neutron detection efficiency, and represent an order-of-magnitude improvement over common thin-film-coated thermal neutron detectors. MSNDs are inherently limited in their detection efficiency due to the presence of streaming paths between the trenches; incident neutrons rarely interact with the semiconductor substrate and will pass through the detector without detection. Dual-side microstructured semiconductor neutron detectors (DSMSNDs) alleviate this issue with an additional set of trenches etched into the backside of the diode. Neutrons streaming through the first set of trenches can be absorbed in the second set of trenches. Careful design of DSMSNDs can allow for detection efficiencies exceeding 70% for a single 1-mm thick detector. Previously reported DSMSNDs yield 10-16% intrinsic thermal neutron detection efficiency for a 0.5-mm thick device, limited by poor conversion material backfilling and poor charge collection efficiency. Device redesign, controlled junction forming, and better neutron conversion backfilling has led to substantial improvement to the prototype DSMSND neutron response rate.

(12:05) R5B-7, Evaluation of AlSb Crystal for Radiation Detector Applications

T. Wang, Y. Li, C. Wang, F. Yang, B. Zhou, Z. Yin, J. Li, W. Jie

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi, China
AlSb crystal is considered as one of the most promising candidate of radiation detectors to approach the high purity germanium, while can be used at room temperature. However, the development of AlSb has been suffered from the high reactivity of Al to the crucible materials and the high volatility of Sb, as well as the low purity of Al. Here we present our recent effort on the synthesis and crystal growth of AlSb. Several crucible materials were used for AlSb synthesis, including carbon coated silica, ZrO2, Al2O3 and p-BN. The reaction products of AlSb with crucible were determined by EDS and was found to be often Al. Crystal ingots with diameter from 20 mm to 50 mm were successful fabricated. Single crystal was easier to obtain compared to the ternary compound CdZnTe. By adjusting the starting ratio of Al and Sb, nearly inclusion free crystal was obtained. Hall measurements show that the carrier concentration has great relationship to the purity of Al. The resistivity of AlSb crystal was adjusted by doping of Te and change of the stoichiometry. The evaluation of AlSb as radiation detector is on the progress by improving the fabrication process.

(12:20) R5B-8, Concluding Comments

R. B. James, M. Fiederle

Brookhaven National Laboratory, Upton, New York, USA
Brief comments pertaining to the 2015 RTSD will be provided at the end of the workshop.