Simulations and Measurements of Plutonium-Oxide Powder Standards

Measurements on plutonium-oxide samples that are stored at the Performance Laboratory of the Joint Research Center (JRC) in Ispra were performed jointly by the University of Michigan and EURATOM JRC staff. The plutonium- oxide powder is stored in cylindrical steel containers having variable heights and diameters and range in mass from 50 g to 500 g. Measurements were performed on five different samples of varying mass and burnup: 100-, 300-, and 500-g low-burnup samples, and 50- and 100-g high-burnup samples. In these measurements, cross-correlation and pulse height distributions were measured separately for neutron and gamma-ray events. Six cylindrical, EJ-309 liquid scintillation detectors (13-cm high and 13 cm in diameter) were placed symmetrically around the samples at a distance of 30 cm. The liquid scintillation detectors exhibit excellent pulse shape discrimination (PSD) capabilities which enabled, for the first time on fissile samples, the measurement of the partial contributions to the total cross-correlation functions. PSD was performed using a previously-optimized, digital, offline charge integration method. Monte Carlo simulations have been performed using the MCNP-PoliMi code which simulates time-correlated quantities, and includes a correlation between individual neutron interactions and the corresponding gamma-ray production. The experimental results for a range plutonium-oxide sample masses and comparisons to Monte Carlo simulations are shown.