A LARGE MULTIPLICITY COUNTER FOR THE MEASUREMENT OF BULK PLUTONIUM*

We have considered the problem of designing a thermal neutron counter for the neutron multiplicity measurement of large, high-mass plutonium items. Three neutron multiplicity counters have been built at Los Alamos to date; two are used for in-plant applications. The third counter is an experimental prototype and is used for research. The sample cavities of these counters can accommodate only relatively small samples. The largest item that can be measured optimally by the largest of these counters is 20 cm wide by 36 cm high. Now that the multiplicity technique has proven to be accurate and timely for the measurement of plutonium-bearing items, several facilities in the DOE complex have identified the need for larger counters. Several sources have identified a counter that could measure items contained in 30-gal. drums as the most desirable. For a multiplicity measurement to be successful, the neutron counter must have a large detection efficiency independent of changes in the neutron energy of the emitted neutrons, a short die-away time, and a uniform response over the sample cavity. These requirements have been achieved in the smaller counters by using large numbers of ^He tubes placed in concentric rings. This method can be extrapolated to a larger counter but at great expense. We have conducted a design study for a 30-gal. drum counter. The goal of this study is to reduce the number of -^He tubes required for such a large counter while still maintaining good measurement performance. This paper will report on the results of this study.