Nuclear material identification by photoneutron and X-Ray radiography

Transport of illicit materials, especially nuclear materials, poses a threat to homeland security. Previous studies about Photoneutron and X-Ray Radiography (PNXR) had achieved identification of common materials so that PNXR can be used to detect illicit materials like explosives and drugs [1]. Recent studies have shown that the PNXR can also be used to detect nuclear materials. In our experiment, a 7MeV LINAC working at 160Hz repetition frequency and 11Gy/min@1m dose rate is used to provide X-rays and photoneutrons for active interrogation. Produced by bremsstrahlung and photonuclear reaction, MeV photons and neutrons subsequently penetrate objects or induce fissions in nuclear materials. A V value defined by attenuation information of neutrons versus photons is used for preliminary screening. Beta-delayed fission neutrons are detected within the time window of 2ms to 6ms after each electron pulse by ³He counters to provide further corroboration. When a 4cm×4cm×4cm depleted uranium sample was inspected, 1.74±0.28 cps count rate of delayed neutron was acquired and the sample was confirmed to be nuclear material with probability of approximate unity.