DEADTIME LOSSES IN MULTIPLICITY COUNTING AT LOW RATES WITH 252Cf

252Cf is used to characterize and in some cases establish calibration parameters for Passive Neutron Multiplicity Counters (PNMCs); for example, determination of counter operating high voltage, dieaway time, coincidence gate width, efficiency, and both coincidence mode and multiplicity deadtime parameters. In contrast to counting a random neutron source (e.g. AmLi), deadtime (DT) losses do not vanish in the low rate limit i.e. deadtime corrected count rates are not equivalent to true count rates. This is because neutrons are emitted from spontaneous fission (SF) events in groups, or time-correlated ‘bursts’, and are detected over a short period commensurate with their lifetime in the detector. Thus, even when SF events themselves are unlikely to overlap, neutron events within the detected ‘burst’ are subject to intrinsic DT losses. This has consequences in scaling the Pu correlation calibration coefficient parameter (e.g. Doubles/ s/fission) from measured 252Cf results. Intrinsic DT losses for Pu will be lower since the multiplicity distribution is softer; hence a correction factor ought to be made for this effect. There are other calibration parameters such as the ratio, ?0, of Doubles-to-Singles and the gate utilization factors for Doubles and Triples counting which may be subject to a small bias. In this work, Monte Carlo neutron pulse train analysis is used to simulate deadtime losses at a range of count rates with 252Cf. Modelling is based on a high efficiency 3He neutron counter with short die-away time, representing the ultimate 3He based detection system. When the ultimate accuracy is needed, or when modeling limits are being assessed, it is important to recognise the subtle effects of DT and quantify them.