A MODULAR 4-PI COUNTER FOR BETA-DELAYED NEUTRONS

We report on the construction of a detector array for low-energy beta-delayed neutrons consisting of sixteen NE 213 scintillation counters. Three pulse-shape discrimination techniques were compared, with special emphasis on the discrimination of neutrons from gamma-rays in the energy region down to 50 keVee (keV electron equivalent). The efficiency as a function of neutron energy was obtained by comparing the precisely known neutron spectrum from the spontaneous fission of Cf-252 and the neutron distribution measured in coincidence with the fission events. The neutron energy was determined with the time-of-flight (TOF) technique. In order to examine the light output function L(E(p)) for recoil protons, a careful energy calibration was done by fitting the Compton edges from standard gamma-ray sources. Neutron efficiencies were also calculated with a Monte Carlo code. Agreement between measured and calculated efficiency curves was observed for two differently shaped detectors and neutron flight paths of 1 and 3 m. This correspondence justifies the efficiency calculation for the 4 pi array with the source in the center. For neutron energies above 800 keV an error of less than 5% in detection efficiency can be reached.