A neutron capture gamma-ray facility

A neutron capture gamma-ray facility was constructed for prompt gamma-ray spectrometry, and its characteristics were measured. In the facility, a neutron beam is extracted from the H-6 horizontal experimental hole of Japan Research Reactor No.3, JAERI, and a target outside the reactor is irradiated by the beam. Thermal neutron flux at the target position was 8.0 x 107 n/cm2.s and the cadmium ratio (Au) was 21. Characteristics of a Ge gamma-ray spectrometer using a 7 in. diameter x 8 in. length guard NaI (Tl) detector were measured for 3 modes, single, Compton (escape) suppressed and pair, with neutron capture gamma-rays from nitrogen, chromium etc. Characteristics were: (1) in single spectrometer, full-energy peak efficiency 2.3 x 10-5 (1 MeV), 4.6 x 10-6 (5 MeV) and 1.25 x 10-6 (10 MeV); (2) in Compton (escape) suppressed spectrometer, Compton suppression factor 3.5 (1-2 MeV), 5.1 (3-5 MeV) and 5.5 (6-8 MeV), and escape suppression factor 2.57 +- 0.2 (single) and 8.34 +- 1.2 (double); (3) in pair spectrometer, detection efficiency 4.5 x 10-7 (3 MeV), 6.5 x 10-7 (4 MeV), 7.1 x 10-7 (5 MeV) and 7.3 x 10-7 (5.5-10 MeV). Lower detection limits in determination of boron, cadmium and gadolinium by Compton suppressed spectrometry of prompt gamma-rays were evaluated on the basis of the experimental data. The limits under the conditions of 2 g aqueous solution, standard error 10% and counting time 1000 s were 9.0 μg for B, 12.3 μg for Cd and 2.9 μg for Gd. (author)