Calibration of Active Well Coincidence Counter (AWCC) with Highly Enriched Uranium (HEU) and AmBe Active Neutron Interrogation Source at the Savannah River National Laboratory

摘要

The Active Well Coincidence Counter (AWCC) is a neutron multiplicity counter developed at Los Alamos National Laboratory (LANL) and used primarily to verify total fissile mass (~(235)U + ~(239)Pu) present in a sample of special nuclear material (SNM). The AWCC is comprised of a sample well surrounded by the two rings of ~3He detector tubes aligned in a vertical axis to measure neutrons emitted from SNM samples. Each of the ring consists of 21 detector tubes housed inside high-density polyethylene (HDPE) moderator wells. The AWCC was designed to use two AmLi fixed neutron sources in active interrogation mode. Since these sources can be difficult to acquire, experimental and MCNP studies were performed at the Nuclear Measurements group at the Savannah River National Laboratory (SRNL) to calibrate the AWCC with AmLi and AmBe active neutron sources. The objective of this work was to obtain an alternative active neutron source for this nondestructive assay (NDA) instrument and possibly for other NDA instruments that rely on traditional AmLi neutron sources. AmLi and AmBe active background rates, induced fission (IF) singles and doubles rates, and IF doubles to singles ratio were compared. The instrument was calibrated in MCNP for 93% enriched HEU samples of different density. Finally, effects of sample geometry to the IF count-rates was also investigated. Results showed that the AmBe source had a higher active singles background than the AmLi source (15% versus 5%). AmBe also had a higher active doubles background count rate (8.647 cps versus 0 cps). In addition, AmBe had a lower net IF singles and doubles rate than AmLi (by approximately 52% and 31% respectively). Even though AmBe provided higher active singles and doubles background rates and lower net IF singles and doubles rates, AmLi resulted in lower net doubles to net singles ratio (-40%) compared to AmBe. This shows AmBe can be used in lieu of AmLi in the AWCC instrument.