SMALL SAMPLE ASSAY SYSTEM

A new high-precision assay system for fissile nuclear material has been built. The new system, called Small Sample Assay System (SSAS), is based on the principle of thermal (or epithermal) neutron activation analysis, where the observed response is the delayed fission gamma rays emitted from thefissionproductafewsecondstoafewminutesafterthefission. The system takes advantage of the high initial yields of the delayed gamma rays by starting the counting very shortly after the end of the irradiation. The SSAS operates with a basic timing cycle which begins with a background meas- urement, continues by irradiating the sample in the neutron flux of a properly moderated 252(jf source, and concludes with a measurement of the signal plus background. Thebasicphysicsofthesystem,whichmakesefficientuseof the delayed gamma spectrum and time dependence, yield very high count rate per gram of fissile material, even when moderately small Cf sources (10 to 50 pg) are employed. The SSAS can be operated with either or both of two types of elec- tronic processing systems. The basic system employs a dead-time-less tech- nique which allows handling of high count rates and intrinsically gives increased weight to the higher energy portions of the gamma spectrum, thus reducing the gamma self shielding. The other system involves a fast linear system which retains pulse-height information, found to be a very useful feature, especially for mixed oxide pellets. The prototype SSAS was subjected to an intensive series of tests aimed at determining the precision of the device and its reliability. Extensive tests to determine the precision of the SSAS were per- formed with sintered LWR pellets and UC>2 powder spanning the enrichment range from natural to 3.3%. The results show that the typically obtained precision is about 0.5 and 1% at the 1-a confidence level for pellets and oxide powder, respectively.