A new neutron multiplicity counter is being developed which utilizes the fast response of liquid scintillator (NE-213) detectors. Current uranium coincidence counting methods rely on the assay samples to conform to the calibration standards with respect to the sample uniformity, geometry, material type, etc. There exists a wide range of material throughout the DOE complex where these attributes are non-standard or unknown. A neutron counter with short die-away time makes possible the measurement of higher order coincidences. This information can be used to more accurately assay many of the problem items in the inventory. In addition, such a counter would allow for rapid inventory measurements of all forms of uranium. Liquid scintillator detectors also allow for energy discrimination between interrogation source neutrons and fission neutrons, allowing for even greater assay sensitivity. Liquid scintillator detectors are sensitive to? and neutron radiation. Differences in the timing of scintillation light produced in?-ray and neutron interactions allows for separation of these events using pulse shape discrimination (PSD). PMT pulses resulting from neutron and? interactions in the Liquid scintillator are read in using a fast waveform digitizer with a 1 GS/s sampling rate. The pulse shapes are then compared to? and neutron pulse templates and a?2 comparison determines the species. Integrated rise time can also be used to discriminate between they and neutron pulses. The results of these studies are presented.

A new technique is presented for the measurement of neutron and/or gamma-ray coincidences. Separate neutron neutron, neutron gamma-ray, gamma-ray neutron, and gamma-ray gamma-ray coincidences are acquired with liquid scintillation detectors and a digital pulse shape discrimination (PSD) technique based on standard charge integration method. The measurement technique allows for the collection of fast coincidences in a time window of the order of a few tens of nanoseconds between the coincident particles. The PSD allows for the acquisition of the coincidences in all particle combinations. The measurements are compared to results obtained with the MCNP-PoliMi code, which simulates neutron and gamma-ray coincidences from from a source on an event-by-event basis. This comparison leads to good qualitative agreement.