Neutron-Photon Pile-Up and its Effects on Multiplicity Correlations of Fission

In nonproliferation applications, an accurate knowledge of fission signatures is of paramount importance and can lead to the refinement of current assaying techniques, as well as the development of new techniques based on yet unknown signatures. One of the distinguishing signatures of a fission reaction is the virtually simultaneous emission of neutrons and photons, with both particle types emitted in multiplicity. A correlation between the multiplicities of neutrons and photons from fission has been investigated, but not yet conclusively determined. Indeed, values of this correlation determined from experimental data and theoretical models, respectively, differ significantly. One possible source of this disagreement is that experimental setups sensitive to this correlation are often affected by significant sources of bias. Specifically, organic scintillators are sensitive to both neutrons and photons, and thus represent an ideal detector type in assaying neutron-photon multiplicities; however, they are susceptible, among other things, to neutron-photon pile-up. We present the results of our investigation on the effects of neutron-photon pile-up on the covariance between the detected</i> multiplicities of neutrons and photons from fission. We show that pile-up disproportionately affects events which are important in the determination of multiplicity covariance: even at a low pile-up rate, the multiplicity covariance in the observed data is significantly distorted. We develop a closed form expression which models the effects of pile-up on the detected data. The expression is validated in simulations in the Monte Carlo radiation transport code MCNPX-PoliMi, with sources drawn from several models of fission emission, with a range of positive and negative correlations. We develop an unfolding technique based on these results, with associated error and sensitivity analysis.