Experimental study of fission counter design optimization with a simplified analytical model of performance

Abstract

Trends inferred from this study of fission counter design confirm previously known relationships between design variables and counter performance. Most important is the proved existence of an optimum electrode spacing for a specific design that increases with electrode area and decreases with gamma dose rate. The PD product also has an optimum value, with low values being limited by the presence of electronic noise and high values by enhanced gamma pileup. Conventional 50-..cap omega.. signal cables do not always provide the best performance, particularly for large-area counters that have high interelectrode capacitances and operate in high gamma background. Values as low as 16.7 ..cap omega.. yield an improvement of as much as 20%. The performance of a counter closely follows the E/P versus drift velocity characteristic of its gas mixture, and is superior for fill gases having higher electron drift velocities. The optimum filter time constant of the counting channel depends on the gamma background dose rate. At low gamma levels, when electronic noise dominates, longer filter time constants give better signal-to-noise ratios. Counters with shorter collection times more effectively suppress the gamma pileup at high dose rates, and filters with higher break-frequencies process the faster pulses more effectively. Anothermore » conclusion from this study is that the usable neutron sensitivity per gram of neutron-sensitive material is a monotonically decreasing function of electrode area. These sensitivity losses can be minimized for high-sensitivity fission counters by use of neutron-sensitive material with low intrinsic alpha activity (such as electromagnetically enriched uranium), as well as by careful specification of the major design variables.« less