A modular 4π counter for β-delayed neutrons

We report on the construction of a detector array for low-energy β-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 γ-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 252Cf 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(Ep) for recoil protons, a careful energy calibration was done by fitting the Compton edges from standard γ-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 4π 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.