Analysis of Neutron Reflection in Correlation Measurements

The Nuclear Materials Identification System (NMIS) procedures that rely on the fast correlation measurement of neutrons and gamma rays from fission are in use at the Y-12 National Security Complex and elsew h e re. In active measurements, an external s o u rce of neutrons is used to induce fission in the sample to be analyzed. Typically, a Cf-252 source inside an ionization chamber is used. Previous studies and measurements showed that the environment, primarily the proximity of the floor or a wall to the i n s t ruments, affects the measured signatures. In this paper, we present an analysis of neutron reflection based on a large number of simulations performed with the MCNP- PoliMi code. The simulations were performed for the time-of-flight configuration. The Monte Carlo program and its post- processor allow us to partition the total signature into the direct and scattered components. The direct component consists of uncollided neutrons and gamma rays traveling from the source to the detector. The scattered component is composed of part i c l e s that were reflected from the floor. The aim of this paper is to identify and quantify the latter component. The analysis of the data consisted of a search for an empirical fitting curve for the scattered component of the signature. The fitting curve depends on a number of parameters that are mainly related to the geometry of the setup. The results show that the fitting pro c e d u re was able to model floor reflection with good a p p roximation for the range of cases considered. Equations have been developed that approximate the neutron floor reflection and can be used in applications to calculate the floor reflection component so that it may be removed from the measured signatures.