  To address the problem of the shortage of neutron detectors used in radiation
portal monitors (RPMs), caused by the 3He supply crisis, research on a
cadmium-based capture-gated fast neutron detector is presented in this paper.
The detector is composed of many 1 cm * 1 cm * 20 cm plastic scintillator
cuboids covered by 0.1 mm thick film of cadmium. The detector uses cadmium to
absorb thermal neutrons and produce capture gamma-rays to indicate the
detection of neutrons, and uses plastic scintillator to moderate neutrons and
register gamma-rays. This design removes the volume competing relationship in
traditional 3He counter-based fast neutron detectors, which hinders enhancement
of the neutron detection efficiency. Detection efficiency of 21.66 +- 1.22% has
been achieved with a 40.4 cm * 40.4 cm * 20 cm overall detector volume. This
detector can measure both neutrons and gamma-rays simultaneously. A small
detector (20.2 cm * 20.2 cm * 20 cm) demonstrated a 3.3 % false alarm rate for
a 252Cf source with a neutron yield of 1841 n/s from 50 cm away within 15
seconds measurement time. It also demonstrated a very low (< 0.06%) false alarm
rate for a 3.21 * 105 Bq 137Cs source. This detector offers a potential
single-detector replacement for both neutron and the gamma-ray detectors in RPM
systems.
