The measurement of passive neutron multiplicity counter gate utilisation factors and comparisons with theory

In a recent paper, a new method of calculating the doublets and triplets gate utilisation factors f₂ and f₃ for passive neutron multiplicity counters has been developed resulting in the MCF computer code. In this paper we arrange the basic multiplicity equations and generate nuclear data parameter combinations to enable f₂, f₃, f₂/ f₃ and f₂²/ f₃ to be derived from measurements taken using a calibrated ²⁵²Cf source and multiplicity shift-register electronics. The two approaches have been compared with reference to Harwell Instruments Ltd. N95 neutron coincidence counting measurement chamber operated both with and without an internal cadmium liner. The Monte Carlo code MCNP™ was used to calculate neutron capture time distributions and these were fed into the MCF code to produce theoretical gate utilisation factors to compare with the experimental data. Pre-delays of 2 and 4 μs and coincidence gate widths between 16 and 240 μs were investigated. The predicted values agreed with those measured within 1.5% and 5% for the f₂ and f₃ results, respectively. These results confirm the power of the new theoretical treatment in accurately predicting gate utilisation factors in an absolute sense and with confidence at the design stage. Relative predictions can be made with greater accuracy.