The Evaluation of CdTe for Nucleonic Fuel Gauging

A nucleonic system using an array of source-detector pairs to gauge propellant tanks of up to approximately 20,000 lb under zero g conditions was analyzed. The sources of error in determining the propellant mass were outlined and ways of reducing the sampling error were considered. Detector characteristics, the source strength and photon energy, and the consequential radiation dose and source collimator mass were specified. To test the feasibility of a multi source-detector pair nucleonic gauging system six 200 mm CdTe detectors were positioned opposite six collimated 137Cs sources on an eight foot diameter sphericaltank containing up to 19,000 lb of salt water as a fuel simulant. An accuracy of approximately 0.6% of full tank loading was obtained under 1 g conditions. Propellant loading was determined by counting photoevents above a fixed energy threshold. A separate analysis of the detectors showed that counting efficiencies ranged from 66% to 83% of the theoretical values due to tailing of the photopeak. Due to trapping and detrapping, count rates increased nearly linearly with increasing temperature from -40°C to 65°C with the value 0.35%/°C. Count rates also increased with increasing bias, with values as low as 0.025%/volt. These studies indicate the feasibility of using a highly collimated system of source detector pairs to gauge propellants under zero g conditions.