PERFORMANCE REVIEW OF THE N94 PNCC CHAMBER

The Harwell Instruments standard N94 Passive Neutron Coincidence Counter is capable of assaying plutonium at levels ~ 10 mg 240Pu (effective)in containers up to approximately 200 l in capacity for a 600 second count. We review the experience gained in plant using several N94 units, and compare the operating parameters including detection efficiency, die-away times and matrix effects. The result is a database that may be used to access the performance of various waste types. We also discuss some recent work on the implementation of a Matrix Interrogation Source (MIS) technique for the N94, in which a small 252Cf source was introduced to the underside of 200 l drums containing various simulated waste matrices. The variation between the MIS count rates and the specific response from Pu within the drum, allow a matrix compensation to be derived through a suitable calibration. Wheras conventional MIS counting relies on totals neutron counting, we use both the Totals and Reals responses. The Reals response allows for variations in the coincidence gate utilisation factor, as well as the detection efficiency, and therefore potentially provides for accurate matrix compensation. We comment on the general applicability of the MIS technique in coincidence mode, for general waste forms.