Receipts Assay Monitor: Deadtime Correction Model and Efficiency Profile

摘要

Experiments were performed at Los Alamos National Laboratory to characterize the operating parameters and flatten the axial efficiency profile of a neutron coincidence counter called the Receipts Assay Monitor (RAM). Optimum electronic settings determined by conventional methods included operating voltage (1680 V) and gate width (64 mu s). Also determined were electronic characteristics such as bias and deadtime. Neutronic characteristics determined using a sup 252 Cf neutron source included axial efficiency profiles and axial die-away time profiles. The RAM electronics showed virtually no bias for coincidence count rate; it was measured as -4.6 x 10 sup -5 % with a standard deviation of 3.3 x 10 sup -4 %. Electronic deadtime was measured by two methods. The first method expresses the coincidence-rate deadtime as a linear function of the measured totals rate, and the second method treats deadtime as a constant. Initially, axial coincidence efficiency profiles yielded normalized efficiencies at the bottom and top of a 17-in. mockup UF sub 6 sample of 68.9% and 40.4%, respectively, with an average relative efficiency across the sample of 86.1%. Because the nature of the measurements performed with the RAM favors a much flatter efficiency profile, 3-mil cadmium sheets were wrapped around the sup 3 He tubes in selected locations to flatten the efficiency profile. Use of the cadmium sheets resulted in relative coincidence efficiencies at the bottom and top of the sample of 82.3% and 57.4%, respectively, with an average relative efficiency of 93.5%. (ERA citation 11:055909)