Straw-based Portal Monitor 3He replacement detector with expanded capabilities

Boron-coated straw (BCS) detectors can be easily and economically manufactured in large volumes, to replace ³He-based detector modules in Radiation Portal Monitors (RPMs). A neutron detector module (NDM) design based on BCS detectors is presented, maintaining the outer dimensions of the ³He-based NDM in current installations. Multiple (up to 95) BCS detectors are distributed in an array, within a high-density polyethylene moderator block. Each BCS detector is 4.43 mm in diameter, 200 cm long, and individually sealed inside an aluminum tube just large enough to contain the thin walled straw. Straws are organized into 5 rows at different depths within the moderator. The response of five production NDM units to a neutron and gamma source are presented, together with environmental testing aimed to simulate operating conditions encountered in typical RPM applications. The mean neutron absolute efficiency measured at 2 m among 4 units was 3.55±0.06 cps/ng for a moderated ²⁵²Cf source, and 3.13±0.08 cps/ng for an unmoderated source. The mean gamma-ray efficiency measured was 1.2×10^-8 for an exposure rate of 20 mR/hr, and 1.4×10^-8 for 50 mR/hr. The background rate was 2.5 cps at sea level in Houston for all units. Environmental tests showed stable operation over the temperature range from -40°C to +49°C, and during an 18 hour soak at 40°C and 98% relative humidity. Advanced capabilities of the NDM include coincidence/multiplicity counting for characterization of spontaneous fission sources, and encoding of the location of neutron interactions along the depth of the NDM.