Some Observations on the Use of 252Cf in the Determination of the Deadtime Parameters of Neutron Multiplicity Counters

A popular way to determine the deadtime of passive neutron coincidence counters is to count a set of 252Cf sources ranging from weak to strong. Because the source characteristics (same fissioning system) remain fixed the deadtime and background corrected Singles, Doubles and Triples count rates should be in fixed proportion independent of source strength. This expectation may therefore be used as a constraint in the analysis of the observed 252Cf data permitting the parameters of the selected deadtime correction model to be extracted. The 252Cf system has some special properties even for real physical sources. The self multiplication is essentially unity and the (a,n) production rate is, for all practical purposes, zero. The sources are physically small and point like so that the concept of a unique efficiency holds. These properties mean that the point-model equations linking the basic nuclear properties to chamber parameters can be expected to hold rather well. For a measurement chamber with an exponential dieaway profile (which governs the gate utilization factors) these special conditions lead to a degeneracy in the point-model. In this paper we examine the consequences of this redundancy in the context of forming empirical Triples deadtime correction relationships.