Abdul R. Dulloo

Development of a silicon carbide radiation detector

The radiation detection properties of semiconductor detectors made of 4H silicon carbide were evaluated. Both Schottky and p-n junction devices were tested. Exposure to alpha particles from a /sup 238/Pu source led to robust signals from the detectors. The resolution of the Schottky SiC detector was 5.8% (FWHM) at an energy of 294 keV, while that of the p-n junction was 6.6% (FWHM) at 260 keV. No effect of temperature in the range of 22 to 89/spl deg/C was observed on the characteristics of the /sup 238/Pu alpha-induced signal from the SiC detector. In addition, testing in a gamma field of 10,000 rad-Si h/sup -1/ showed that the alpha-induced signal was separable from the gamma signal.

Demonstration of an SiC neutron detector for high-radiation environments

Neutron response studies have been performed on Schottky diodes fabricated using 4H-SiC material. These studies indicate that neutron detection using SiC diodes is possible without significant degradation in the energy resolution, noise characteristics or, most importantly, the neutron counting rate even after exposure to neutron fluences of 3.4/spl times/10/sup 17/ n/sub th//cm/sup 2/ (1/spl times/10/sup 17/ n/sub fast//cm/sup 2/; E/sub n.fast/>1 MeV), the highest yet examined. The results represent orders of magnitude increased device lifetime in neutron fields compared to commercial silicon based detectors. Additionally, detector response was found to be linear up to thermal neutron fluxes of 2000 n/sub th//cm/sup 2//s. However, degradation in the charge collection efficiency due to neutron damage-induced defects prevented self-biased operation after exposures above /spl sim/5.7/spl times/10/sup 16/ n/sub th//cm/sup 2/. A carrier removal rate of 9.7/spl plusmn/0.7 cm/sup -1/ was calculated from C-V doping profile measurements on neutron irradiated samples. These results demonstrate the viability of SiC-based detectors for a variety of radiation monitoring applications.

Simultaneous measurement of neutron and gamma-ray radiation levels from a TRIGA reactor core using silicon carbide semiconductor detectors

The ability of a silicon carbide radiation detector to measure neutron and gamma radiation levels in a TRIGA reactors mixed neutron/gamma field was demonstrated. Linear responses to epicadmium neutron fluence rate (up to 3/spl times/10/sup 7/ cm/sup -2/ s/sup -1/) and to gamma dose rate (0.6-234 krad-Si h/sup -1/) were obtained with the detector. Axial profiles of the reactor cores neutron and gamma-ray radiation levels were successfully generated through sequential measurements along the length of the core. The SiC detector shows a high level of precision for both neutrons and gamma rays in high-intensity radiation environments-1.9% for neutrons and better than 0.6% for gamma rays. These results indicate that SiC detectors are well suited for applications such as spent fuel monitoring where measurements in mixed neutron/gamma fields are desired.

The fast neutron response of 4H silicon carbide semiconductor radiation detectors

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 /sup 28/Si(n,/spl alpha//sub i/) reaction set of six energy levels in the product /sup 25/Mg nucleus, and pulse height defect differences between the observed /sup 12/C(n,/spl alpha//sub 0/)/sup 28/ Si(n,/spl alpha//sub i/) energy responses are discussed. Energy spectrometry applications in fission and fusion neutron fields are also discussed.

High-resolution alpha-particle spectrometry using 4H silicon carbide semiconductor detectors

SiC detectors with active volume dimensions sufficient to stop alpha particles have been manufactured and tested. A linear energy response and excellent energy resolution have been obtained for various alpha emitters in the 3.18-MeV to 8.38-MeV energy range. Evaluation of the contributing factors to the SiC detector energy resolution indicates that the measured values for the full width at half maximum (FWHM) are limited by energy straggling of the alpha particles as they pass through the metallic contact layers that comprise the entrance window to the detector. Even with this component included in the measured FWHM, the measured values are comparable to those achievable with silicon alpha spectrometers. The possibility that the energy resolution that can be achieved with SiC may surpass that of silicon can not be excluded. SiC alpha spectrometers are expected to be useful in many nuclear applications where the ability to operate in high-temperature and high-radiation environments is required. Such applications include monitoring of alpha particles, neutrons, and low-energy gamma rays and X-rays in actinide waste-tank environments as well as neutron and gamma-ray monitoring of spent nuclear fuel assemblies.

The fast neutron response of silicon carbide semiconductor radiation detectors

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 /sup 28/Si(n,/spl alpha//sub 1/) reaction set of six energy levels in the product /sup 25/Mg nucleus, and pulse height defect differences between the observed /sup 12/C(n,/spl alpha//sub 0/) and /sup 28/Si(n,/spl alpha//sub 1/) energy responses are discussed. Energy spectrometry applications in fission and fusion neutron fields are also discussed.

Nuclear reactor power monitoring using silicon carbide semiconductor radiation detectors

Abstract Silicon carbide semiconductor neutron detectors are being developed for use as ex-vessel power monitors for pressurized water reactors. Key features such as neutron response, radiation resistance, and high-temperature operation have been explored for silicon carbide detectors, and the results are consistent with their use in the ex-vessel environment. Prototype silicon carbide ex-core neutron detectors have been assembled and tested under research reactor conditions simulating ex-core neutron monitoring requirements. Linear, pulse-mode operation without the need for gamma compensation has been demonstrated with these prototype detectors. The silicon carbide detectors are compared to presently deployed gas-filled ex-vessel detectors, and several advantages of the silicon carbide technology can be seen. It is anticipated that a wide-range silicon carbide neutron detector can be designed to replace the combined functions of the multiple power range detectors in use. Furthermore, the need for gamma-ray compensation will be eliminated, and more efficient reactor operation and simplified reactor operating procedures will result.

Detection Limits of a Laboratory Pulsed Gamma Neutron Activation Analysis System for the Nondestructive Assay of Mercury, Cadmium, and Lead

Detection of Hg, Cd, and Pb within concrete matrices located in 8-gal drums was successfully demonstrated using a pulsed gamma neutron activation analysis system. Real-time assays of 600 s led to the detection of these metals at concentration levels ranging, in parts per million (ppm) by weight, from 487 to 19,820 for Hg, 485 to 8181 for Cd, and 9927 to 19,950 for Pb. The measurements of Hg and Cd relied on the observation of thermal neutron-induced prompt gamma rays, whereas the Pb measurements relied on the observation of decay gamma rays from {sup 207m}Pb, a product of fast neutron-induced reactions in Pb. The projected lower limits of detection of the current system for a 600-s run are 15, 170, and 8600 ppm for Cd, Hg, and Pb, respectively. Up to a one-order magnitude of improvement in sensitivity is anticipated through the enhancement of the system`s detector and neutron source. The results obtained confirm the potential of prompt and decay gamma neutron activation analysis as an effective method of the nondestructive analysis of hazardous metals in mixed-waste drums.

Spent Fuel Monitoring with Silicon Carbide Semiconductor Neutron/Gamma Detectors

Silicon carbide semiconductor radiation detectors have been demonstrated for neutron and gamma-ray monitoring of spent nuclear fuel. Neutrons and gamma rays were monitored simultaneously over a 2050-h period, resulting in a gamma dose of over 6000 Gy to the SiC detector. No changes in the neutron and gamma-ray sensitivity were observed as a result of the gamma-ray exposure. After the spent-fuel measurements, the absolute neutron sensitivity was determined in a standard neutron field, and the degree of gamma-ray and neutron spectrum overlap were determined through exposures in intense gamma ray fields. No overlap was observed for gamma-ray dose rates up to 100 G/h.

Pulsed neutron interrogation for detection of concealed special nuclear materials

A new neutron interrogation technique for detection of concealed Special Nuclear Material (SNM) is described. This technique is a combination of timing techniques from pulsed prompt gamma neutron activation analysis with silicon carbide (SiC) semiconductor fast neutron detector technology. SiC detectors are a new class of radiation detectors that are ultra-fast and capable of processing high count rates. SiC detectors can operate during and within nanoseconds of the end of an intense neutron pulse, providing the ability to detect the prompt neutron emissions from fission events produced by the neutrons in concealed SNM on a much faster pulsing time scale than has been achieved by other techniques. Neutron-induced fission neutrons in 235U have been observed in the time intervals between pulses of 14-MeV neutrons from a deuterium-tritium electronic neutron generator. Initial measurements have emphasized the detection of SNM using thermal-neutron induced fission. Neutron pulsing and time-sequenced neutron counts were carried out on a hundreds of microseconds time scale, enabling the observation of prompt fission neutrons induced by the die-away of thermal neutrons following the 14-MeV pulse. A discussion of pulsed prompt-neutron measurements and of SiC detectors as well as initial measurement results will be presented.