High sensitivity portable neutron detector for fissile materials detection

Efforts to contain weapons of mass destruction have recently intensified, in response to increasing terrorist activities worldwide. The ultimate scenario for nuclear terrorism involves the smuggling of nuclear weapons, or the material to make them, and subsequent detonation in densely populated cities. Widespread monitoring in ports of entry is part of the solution proposed by experts. We propose a portable, economical monitor with high sensitivity for both neutron and gamma detection. It consists of a close-packed array of many thin-walled copper straws, lined with a 1 μm thick coating of enriched boron carbide (¹⁰B₄C). Neutrons are converted in ¹⁰B, while gammas are converted in Cu. We present performance tests for a prototype monitor that has dimensions of 40×5×5 cm³ and weighs 3.6 kg. Results show a neutron background of 0.03 cps, a thermal neutron sensitivity of 36 cps/nv (18% efficiency), and an angular resolution of 40 degrees (FWHM). We estimate that a monitor of the proposed design (with enriched boron carbide), traveling at 3 mph, can positively detect 230 g of ²⁴⁰Pu, from a distance of 2.5 m, with 90% sensitivity and with fewer than 1 false alarms every two hours.