Frank H. Ruddy

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.

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.

Annealing phenomena in solid state track recorders

Abstract Accepted descriptions of the annealing process in Solid State Track Recorders (SSTR) are reviewed with emphasis on current misconceptions. In particular, it is shown that the Arrhenius equation should not be used to describe the decrease in observable track density induced by annealing. Results of annealing experiments in different SSTR media are examined. On this basis, a general reaction rate theory of the annealing process in SSTR is advanced. This formalism is used to introduce the new concept of the energy per etchable detect for SSTR. An important qualitative outgrowth of this work is the establishment of a general sensitivity-annealing correlation for SSTR. Observed annealing induced correlations between track size and track density for fission fragments are readily understood in terms of this general theory. Qualitative explanations of current enigmas in SSTR cosmic ray work are also advanced.

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.

Silicon Carbide Neutron Detectors

The potential of Silicon Carbide (SiC) for use in semiconductor nuclear radiation detectors has been long recognized. In fact, the first SiC neutron detector was demonstrated more than fifty years ago (Babcock, et al., 1957; Babcock & Chang, 1963). This detector was shown to be operational in limited testing at temperatures up to 700 oC. Unfortunately, further development was limited by the poor material properties of SiC available at the time.

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.

The status of automated nuclear scanning systems

Abstract Present day minicomputers and microprocessors enable a range of automation, from partial to total, of tasks once thought beyond approach. The status of three computer controlled systems for quantitative track measurements is reviewed. Two systems, the Hanford optical track scanner (HOTS) and an automated scanning electron microscope (ASEM) are used for scanning solid state track recorders (SSTR). The third system, the emulsion scanning processor (ESP), is an interactive system used to measure the length of proton tracks in nuclear research emulsions (NRE). Current limitations of these systems for quantitative track scanning are presented. Experimental uncertainties attained with these computer controlled systems are described using results obtained from reactor neutron dosimetry.