A Passive Neutron Counter for Determination of Transuranics in High Gamma Activity Samples

The measure of transuranic content in nuclear waste is complicated by the fact that transuranics include a broad spectrum of radioactive decays without any features in common. This paper describes a very sensitive neutron counting instruments which can determine the presence of small quantities of transuranic materials in samples emitting extremely high levels of gamma radiation and which can also determine the isotopic content. The detector is modular to facilitate disassembly for maintenance or decontamination and consists of a neutron counter with internal and external radiation shields. The external shield, designed to minimize the ambient neutron background, is an annular tank containing a saturated aqueous solution of boric acid. The counter module is an annular array of 80 BF/sub 3/ proportional counting tubes arranged in two concentric rows of 40 tubes surrounded by a polyethyene moderator. The internal shield, a lead annulus 10 cm thick, separates the sample chamber from the detector array. The sample chamber accepts a five-gallonbucket sized sample. Gamma radiation levels of greater than 100,000 R/hr can be tolerated without affecting the neutron count rate. Each tube is fitted with a preamplifier/discriminator so that the entire array can operate in the same gamma environment as a singlemore » tube. The electronics permits measurements in a coincidence mode, for isotopic composition measurement, and a spatial mode, for determining the physical distribution of transuranics in the sample.« less