Photofission method for quantification of fissile material in large drums

Abstract Photofission is one of the active non-destructive analysis (ANDA) techniques used for quantification of fissile materials in large containers. Sets of photofission experiments were conducted at an industrial 10 MeV electron LINAC facility with the aim to develop an active monitoring system. A mock-up drum of 50 cm diameter and 100 cm length was placed horizontally and irradiated with high-energy photons axially. Two banks of high sensitivity Helium-3 proportional counters, with six detectors in each bank were used to measure delayed neutrons after photofission. Average absolute efficiency of the neutron detection system with Cf-252 source was measured as 9.2 %. Photofission experiments were conducted using a 15.6 gram thorium dioxide sample. Minimum detection limit was estimated at eight angular positions along periphery of the drum. The minimum detection limit from the measurement of delayed neutrons for the farthest position of the sample was estimated as 976 ± 57 mg for 1 kW electron beam in scan mode which actually impinged around 40 W beam power on the thorium dioxide sample. The average photofission delayed neutron yield/100 fissions for Th-232 is estimated from the experiments as 3.6 ± 1.1.