Abstract
A mathematical model has been developed for predicting the ability of a fission counter to operate at a high gamma dose rate. There are several potential applications in reactor programs for detectors that can accurately measure neutron fluxes in the range 0.1 to 1.0 nv(th) while subjected to gamma dose rates of 10/sup 4/ to more than 10/sup 6/ R/hr. Fission counters for such service have up to now been designed on the basis of laborious testing of prototypes combined with extrapolation from existing designs. The objective of this work is to make it possible to arrive at an optimum design without expensive testing. In this model, the complete system of counter, cables, preamplifier, main amplifier, filters, and discriminator is considered, including the effects of interaction of counter capacitance with cable and preamplifier impedance. Electrical noise theory is used to calculate the frequency spectrum of the interfering signal generated by pileup of gamma pulses, to add to it a contribution from electronic noise, and to obtain the rate at which the total noise signal triggers the discriminator. The shapes and widths of neutron and gamma pulses are calculated from the electron drift velocity and electrode spacing.
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