NMIS PASSIVE TIME-DEPENDENT COINCIDENCE MEASUREMENTS FOR PLUTONIUM MASS AND MULTIPLICATION

With the support of the National Nuclear Security Administration’s Office of Nonproliferation Policy (NNSA NA- 241), the Oak Ridge National Laboratory (ORNL) and the Russian Federal Nuclear Center, All-Russia Scientific Research Institute of Experimental Physics (RFNC-VNIIEF) have taken first steps to jointly develop and implement a radiation measurement technique to inspect plutonium. In June and July 2000, personnel from ORNL and VNIIEF performed joint experiments on unclassified plutonium metal (d-phase, 1.77%-240Pu) spherical shells at VNIIEF facilities in Sarov, Russia. Measurements of eight plutonium spherical shells were performed using the Nuclear Materials Identification System (NMIS). The shells’ inner radii varied from 10 to 53.5 mm; their outer radii varied from 31.5 to 60 mm. Consequently, plutonium mass spanned 1829 to 4468g. The subsequent analysis demonstrates that NMIS can be applied to passively measure the mass and multiplication of plutonium spherical shells. In its passive mode, NMIS employs an array of fast plastic scintillators to accumulate the distribution of coincident detector counts. This coincidence-distribution is the real coincidence (i.e., total coincidence less accidental coincidence) as a function of the time-delay between detector signals (at zero time-delay, the detectors’ signals are synchronized). The features of the time-dependent coincidence-distribution used to estimate the multiplication parameter and mass attribute are derived from the distribution’s width and integral. For this study, the distribution width was taken as its full-width at tenth-maximum, or FWTM. The distribution’s integral was taken as the area beneath the FWTM, a.k.a, its FWTM area. The model applied to the width and integral features measured for all the VNIIEF shells yields fairly accurate estimates of mass and multiplication for each shell. In a root-mean-square (RMS) sense, the inverse model estimates multiplication to within 5% of its actual value and mass to within 7% of its actual value. Consequently, this jointly developed method shows promise as a means to inspect plutonium for domestic and international safeguards and disarmament agreement verification. Further joint experiments will expand its applicability.