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 specia l emphasis on the discrimination of neutrons from gamma-rays in the en ergy region down to 50 keVee (keV electron equivalent). The efficiency as a function of neutron energy was obtained by comparing the precise ly known neutron spectrum from the spontaneous fission of Cf-252 and t he neutron distribution measured in coincidence with the fission event s. The neutron energy was determined with the time-of-flight (TOF) tec hnique. In order to examine the light output function L(E(p)) for reco il protons, a careful energy calibration was done by fitting the Compt on edges from standard gamma-ray sources. Neutron efficiencies were al so calculated with a Monte Carlo code. Agreement between measured and calculated efficiency curves was observed for two differently shaped d etectors and neutron flight paths of 1 and 3 m. This correspondence ju stifies the efficiency calculation for the 4 pi array with the source in the center. For neutron energies above 800 keV an error of less tha n 5% in detection efficiency can be reached.