Radiation Scanning

Radiation portal monitors (RPMs), consisting of large-area gamma-ray detectors (usually plastic scintillation detectors) and neutron detectors (He-3 detectors in polyethylene moderator material), allow the passive detection of nuclear materials or other radioactive materials to be detected in cargo containers or trucks entering or leaving a port. The high detection sensitivity of RPMs allows 100% scanning of cargo with minimal impact on throughput. However, false positive alarms resulting from cargo which is naturally radioactive (e.g., certain ceramic materials, kitty litter) can slow-down the flow of commerce unless efficient means are provided for resolving these false positives.
Figure 4 shows RPMs installed at the Port of Felixstowe in the UK under a pilot program. RORO (roll-on, roll-off) cargo entering this port is scanned by a gamma-neutron passive portal monitor. Those trucks which alarm at the primary detection station are diverted to a secondary inspection station where the truck is radiographed (either gamma-ray or x-ray) and a handheld isotope identifier unit isused to determine the specific radioisotope responsible for triggering the portal monitor alarm.