Passive (NMIS) Measurements on Plutonium Oxide Samples

Abstract

This paper describes the measurements and Monte Carlo simulations that were performed in March and April, 2005 on plutonium oxide standards stored at the EURATOM Joint Research Center in Ispra, Italy. The simulations were performed with the MCNP-PoliMi code, which correctly simulates the statistics of the neutron and photon field in a multiplying medium. The plutonium samples were oxide powder having mass in the 50 to 2500 g range, with three compositions corresponding to three different burnups. The measurements were performed with 8 large (25 by 25 by 8 cm) liquid scintillators, arranged in several different configurations. The measurements rely on the Pu-240 spontaneous fission, which emits multiple neutrons and gamma rays. These particles are detected with the scintillators and correlations among detectors are studied, on the time scale of a few tens of ns. The correlations include gamma-gamma, gamma-neutron, and neutron-neutron coincidences. The paper presents results from the measurements, together with Monte Carlo simulations and analysis. Contamination of the signature by (alpha, n) reactions and detector cross-talk are analyzed and discussed. Finally, a method to determine Pu-240 mass is described.