Development of a screened cathode gas flow proportional counter for in situ field determination of alpha contamination in soil

摘要

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 (micro)Ci 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 cm(sup 3) and 87.10 cm(sup 3), respectively, for the small and large detector. An operating voltage of 1,975 volts was determined to be sufficient for both detectors. The average efficiency was 2.59 (+-) 0.12% and 76.71 (+-) 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 17.75 (+-) 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.