Photoneutron Interrogation of Uranium Samples by a 4 MeV LINAC. A Feasibility Study

It is difficult to reveal smuggled U-bearing material by passive gamma-detection, because the weak radiation can easily be shielded. Neutrons, as penetrate shielding, represent a detection potential, by inducing fission in the nuclear material. A 4 MeV linear accelerator was used as a pulsed neutron source for active interrogation of U-bearing material. Produced in heavy water by bremsstrahlung, neutrons subsequently induced fissions in UO2 samples. Delayed fission neutrons were detected in a neutron collar built up by four He counters in a polyamide container. The counters were gated to be detached from high voltage during the electron pulse. Cyclic irradiation-measurement periods were subsequently used with a 25 Hz pulse repetition rate, as optimum setting, i. e. 500 cycles lasted for 20 s. The time analyser start-up was externally triggered and synchronised by the electron beam pulse. The response of the system was studied as a function of the intensity of the electron current, the amount of heavy water, U enrichment, and total U content. Sensitivity limit was achieved as 0.5 g U and/or 30 g U in a 20 s measurement time (500 cycles) with the amount of heavy water of 100 g and a mean electron current of 2 μA. Because of the long die-away time of neutrons, the duration of the interrogating neutron pulse is about 20 ms, so that a half of the time interval of 40 ms between pulses available for counting delayed neutrons can only be exploited. The response cannot be, however, enhanced by reducing frequency in order to lengthen counting time between pulses, because the saturation level of counting rate would decrease correspondingly. Nor the current is worth increasing, since the pulse tail would accordingly be lengthened.