Measurement and simulation of neutron/gamma-ray cross-correlation functions from spontaneous fission

Abstract

For the first time, a technique is presented for the measurement of total and separate neutron and gamma-ray cross-correlation functions from a spontaneous fission source. The cross-correlation functions are unique for given material-geometry configuration, thus represent signatures that can be used for the identification of radioactive materials. The measurement technique allows for the collection of fast coincidences within a time window of the order of a few tens of nanoseconds. A digital pulse shape discrimination technique is used, which allows for the accurate acquisition of the coincidences in all particle combinations. Specifically, separate neutron–neutron, neutron–gamma-ray, gamma-ray–neutron, and gamma-ray–gamma-ray coincidences are acquired with two liquid scintillation detectors. The measurements are compared to results obtained with the MCNP-PoliMi code, which simulates neutron and gamma-ray coincidences from a source on an event-by-event basis. This comparison leads to relatively good qualitative agreement. The measurements and simulations of the separate neutron and gamma-ray contributions to the total cross-correlations provide new signatures that can be obtained using existing experimental systems built to accurately identify nuclear materials. This research has direct applications in the areas of nuclear nonproliferation and homeland security.