Year
1994
Abstract
Harwell Instruments model N94 Passive Neutron Coincidence Counter is a moderating chamber capable of assaying plutonium in containers of up to approximately 208/ in capacity. It has been systematically characterised by a combination of experiment and calculation as described in this paper. A program of Monte-Carlo modelling, benchmarked against experiment, has been carried out to explore the potential to improve the performance of the system by making some simple design modifications. The result is an assay chamber with a volume weighted detection efficiency of 30%, approximately double the starting value. diameter and 2 atmosphere fill pressure, sandwiched between two layers of low density polyethylene (LDPE) moderator 70mm apart. The inner LDPE layer is25.4mmthick and the outer layer is 38.1mm thick. The summed signals from the 36 detectors is fed into a NIM format Neutron Coincidence Unit (953021-1). The coincidence unit analyses the signal and reports the Totals (T), Reals plus Accidentals coincidences (R+A), and Accidentals (A) count rates according to the traditional shiftregister time correlation analysis approach [2]. The Reals count rate is then obtained by subtraction. A waste container is input into the chamber via a hinged door which comprises one of the four vertical walls.