Characterization of a Portable Neutron Coincidence Counter

Neutron coincidence counting is a technique widely used for qualitative and quantitative analysis of nuclear material. Because different isotopes possess different coincident neutron characteristics, the coincident neutron signature can be used to identify and quantify a given material. In an effort to identify unknown nuclear samples in field inspections, a new portable neutron coincidence counter has been developed. An indepth analysis has been performed to establish whether the nuclear material in an unknown sample could be quantified with some confidence. The analysis was performed by comparing the true measurements of the system to the calculated output produced using MCNPX and the neutron coincidence point model. Based on the analysis, it is evident that this new portable system can play a useful role in identifying nuclear material for verification purposes. Introduction Neutron coincidence counting is an important passive, nondestructive-assay method that can be used to characterize nuclear material in bulk samples, namely plutonium. Because the number of neutrons emitted simultaneously during the fission process is particular to each isotope, the coincidence neutrons create a useful signature that can be used to describe the isotopes in a sample. Coincidence measurements can be done regardless of background or (α, n) neutrons making them less sensitive to external factors. For plutonium, Pu, Pu, and Pu all have dominant spontaneous fission yields producing coincidence neutrons. The measurement of these spontaneous fission neutrons, however, is complicated by the presence of induced fission neutrons. Induced fission is dependent on the fissile isotopes, primarily Pu, and is caused by the absorption of neutrons in the sample. Both spontaneous fission and induced fission produce coincidence neutrons. Because these neutrons are emitted practically simultaneously, they create a useful signature that can be used to quantify a particular nuclear material. The Portable Neutron Coincidence Counter was developed by Los Alamos National Laboratory to aid inspectors in field measurements of bulk samples. In order to benchmark and characterize the detector, a computer model was created using the Monte Carlo N-Particle eXtended (MCNPX) code and its embedded advanced multiplicity capabilities. The results determined using MCNPX were then compared to the results calculated using the traditional Neutron Coincidence Point Model. This paper discusses both these results and their implications. Background The PNCC consists of four individual high-density polyethylene slabs. The slabs are approximately 7-in. (length) x 10-in. (height) x 3-in. (width) and weigh approximately 8