Characterization of a mixed multiplicity counter based on liquid organic scintillators

A measurement system capable of multiplicity measurements for both neutrons and gamma rays has been developed. The benefit of such an approach is in the increased number of available observables. A pure neutron assay results in three observables for third-order multiples, while a combined neutron/gamma-ray assay results in nine observables for the same order of multiples. The idea is to use the additional observables 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-309s) 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 characterizing the measurement system. Comparison of the measured and simulated multiples is discussed in detail and a relatively good agreement is found.