THE CHARACTERISTICS OF THE PROMPT GAMMA-RAY ANALYZING SYSTEM AT THE NEUTRON BEAM GUIDES OF JRR-3M

A reactor-neutron-induced prompt gamma-ray analyzing system was constr ucted at JRR-3M. The system can be set at cold and thermal neutron bea m guides with a neutron flux at the sample position of 1.1 x 10(8) and 2.4 x 10(7) n cm-2 s-1, respectively. The system was designed to achi eve the lowest gamma-ray background. This is done by using lithium flu oride tiles as neutron shielding, by placing the samples in a He atmos phere, and by using a Ge-BGO detector system for Compton suppression. The gamma-ray spectrometer is equipped to acquire three modes of spect ra simultaneously: single mode, Compton suppression mode, and pair mod e in an energy range of up to 12 MeV. Due to the cold neutron guide be am and the low background system, sensitivities and detection limits b etter than those in other PGA systems have been achieved. For typical elements such as H, B, Cd, and Gd, analytical sensitivities with the c old neutrons are 10 to 18 times higher, and detection limits are lower ed by factor of 6 to 8, compared to the case with thermal neutrons und er the same geometrical condition. Detection limits down to a range of 2 to 7 ng were recorded for B, Cd, Sm, and Gd by the cold neutrons. H ydrogen, one of the most important elements in material studies, could be detected down to 1.0 and 2.8 mug by cold and thermal neutrons, res pectively.