Comparison of list mode data acquisition and shift register measurements using the large epithermal neutron multiplicity counter (LEMC)

Neutron multiplicity analysis has been a valuable technique for safeguards measurements of plutonium oxide and mixed oxides. Historically, most of these measurements have been performed using shift register based electronics. The shift register data acquisition lacks certain flexibility because the basic coincidence parameters (e.g., pre-delay, gate width, and long delay) must be fixed prior to the start of the measurement and the values may potentially, therefore, be sub optimal. List mode or time stamped data acquisition records the arrival time of each pulse thereby preserving the history of the pulse stream and allowing analysis and reanalysis using software analogs to the shift register circuit with adjustable parameters. Until recently, the data rates encountered in the assay of modest amounts of plutonium using efficient multiplicity counters were beyond the capacity of readily available personal computers. The calibration of the large epithermal neutron multiplicity counter (LEMC) for assay of plutonium scrap materials is used as a vehicle to compare the performance of the multiplicity shift register and a commercially available list mode acquisition module.