Development of a Liquid Scintillator Neutron Multiplicity Counter (LSMC)

A new neutron multiplicity counter is being developed that utilizes the fast response of liquid scintillator detectors. The ability to detect fast (vs. moderated) fission neutrons makes possible a coincidence gate on the order of tens of nanoseconds (vs. tens of microseconds). A neutron counter with such a narrow gate will be much less sensitive to accidental coincidences making it possible to measure items with a high single neutron background to greater accuracy in less time. This includes impure Pu items with high (alpha,n) rates as well as items of low mass HEU where a strong active interrogation source is needed. Liquid scintillator detectors also allow for energy discrimination between interrogation source neutrons and fission neutrons, allowing for even greater assay sensitivity. Designing and building a liquid scintillator multiplicity counter (LSMC) requires a symbiotic effort of simulation and experiment to optimize performance and mitigate hardware costs in the final product. We present preliminary Monte Carlo studies using the GEANT toolkit along with analysis of experimental data used to benchmark and tune the simulation.

I. Introduction

Neutron multiplicity counters (NMC) based on ³HE tubes are widely used for non-destructive assay of special nuclear material (SNM). Such detectors use coincidences between neutrons produced in fissions to determine the mass of SNM. Because ³He detectors are sensitive to thermal neutrons, fission neutrons must be moderated in polyethylene prior to detection, and typical coincidence gate lengths are on the order of tens of microseconds. An NMC based on liquid scintillator (LSMC) will detect fast neutrons making possible a coincidence gate on the order tens of nanoseconds. A neutron counter with such a narrow gate will be much less sensitive to accidental coincidences, making it possible to measure items with a high single neutron background to greater accuracy in a shorter time. These items include impure Plutonium items with high (a,n) rates as well as small mass highly enriched Uranium (HEU) items where a strong single neutron source (e.g. AmLi) is needed for active interrogation. Liquid scintillation detectors also allow for energy discrimination between lower energy interrogation source neutrons and higher energy fission neutrons, allowing for greater assay sensitivity of HEU.