Study of field assessment methods and worker risks for processing alternatives to support principles for FUSRAP waste materials. Part 2: Preliminary development of a wall-less gas-flow proportional counter for in-situ field analysis of nuclear contamination in soil Page: 4 of 40

                                
Preliminary Development of a Wall-Less Gas-Flow Proportional
Counter for In-Situ Field Analysis of Nuclear Contamination in Soil
S.P. Bush, D.M. Hamby and J.E. Martin
ABSTRACT
This study resulted in the design, construction and testing of a gas flow proportional counter for
in-situ determination of soil contamination. The uniqueness of this detector is the screened
material used for the cathode. A Pu-239 source of 0.006 gCi was mounted to the outside of the
cathode to simulate radioactive soil. The detector probe was placed into a laboratory mock-up
and tested to determine operating voltage, efficiency and energy resolution.
Two gas flow proportional counters were built and tested. The detectors are cylindrical, each
with a radius of 1.905 cm, having an anode wire with a radius of 0.0038 cm. The length of the
smaller detector s anode was 2.54 cm, and the length of the larger detector's anode was 7.64 cm.
Therefore, the active volumes were 28.96 cm3 and 87.10 cm3, respectively, for the small and large
detector.
An operating voltage of 1975 volts was determined to be sufficient for both detectors. The
average efficiency was 2.59'0.12% and 76.71 t. 10.81% for the small volume and large volume
detectors, respectively. The average energy resolution for the low-energy peak of the small
detector was 4.24' 1.28% and for the large-energy peak was 1.37   0.66%. The large detectors'
energy resolution was 17.75tL 3.74%. The smaller detector, with better energy resolution,
exhibited a bi-modal spectrum, whereas the larger detector s spectrum centered around a single
broad peak.
Acknowledgment. The research was funded under grant No. DE-FG05-96EW00001. During the
course of the majority of this work, Ms. Bush was funded under appointment to the Applied
Health Physics Fellowship program administered by Oak Ridge Institute for Science and
Education for the U.S. Department of Energy.

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