Optimization of a mixed multiplicity counter using Monte Carlo simulations and measurements

A measurement system capable of multiplicity measurements for both neutrons and gamma rays has been developed at the University of Michigan. The benefit of such an approach is in the increased number of available measurables. A pure neutron assay results in three measurables for third-order multiples, while a combined neutron/gamma-ray assay results in 9 measurables for the same order of multiples. The idea is to use the additional measurables to achieve greater accuracy when determining unknown parameters of the sample such as the fissile mass. The measurement system is based on liquid scintillation detectors (EJ-309) which feed detected pulses to a digital data-acquisition system. The excellent pulse shape discrimination capabilities of the EJ-309s allow for accurate differentiation between gamma-ray pulses and neutron pulses. The PSD is vital to correctly identify the different multiples up to the third order: n, γ, nn, nγ, γγ, nnn, nnγ, nγγ and γγγ. Previous investigation of the measurement system showed that good counting statistics can be achieved within minutes for spontaneous-fission sources such as 252Cf. In this paper, we present new measurement results and corresponding Monte Carlo simulations aimed at charaterizing the measurement system. Comparison of the measured and simulated multiples count rates is discussed in detailed and a relatively good agreement on trends and count rates is found.