Quantification of Thorium and Uranium by β-delayed neutron from photofission

Advanced instrumentation for non-destructive assay (NDA) of fissile and fertile materials is required in the reprocessing plants for safety, safeguards, accountability and process control. Active interrogation, a measurement technique which uses a radiation source to probe materials and generate unique signatures is a powerful tool for assaying highly shielded fissile and fertile materials. Photon source with energies above 6 MeV can be used as active interrogation device for quantification of special nuclear materials. The method involves usage of intense beam of high-energy Bremsstrahlung photons produced from electron LINAC in order to induce photo fission in actinides. The measurement of delayed neutrons and photons emitted by fission products bring specific information on localization, quantification and finding relative proportions of different types of the fissile materials. A set of photo fission experiments have been conducted at the 10 MeV LINAC of EBC, Kharghar with Thoria and Natural Uranium samples fabricated by AFD, BARC. The radiological safety consideration favours the use of electron beam of upto 10 MeV energy. The photofission experimental data available on 10 MeV end point energy is very scarce. Literature describes about quantification of pure samples by photofission, however not much information is available on quantification of relative mass of fissile materials mixed together. In the present paper the mass of Thorium and natural Uranium have been determined in pure as well as in mixed state by measuring time dependent delayed neutrons and photons after photofission. In experiments Electron beam of 10 MeV energy, 50 mA peak current, 50 Hz PRF, 1.1 Hz scan frequency and 1 meter scan length was used. The Bremsstrahlung beam generated by the Tantalum converter was impinged on the samples. The samples were placed below the converter at the geometric centre of the scan horn. The beam ON time was of 60 seconds. Set of irradiations were performed with the pure samples as well as mixed samples. After the electron beam was OFF, photofission delayed neutrons were measured with the help of 'C' shape neutron detection system consisting of six nos. of high sensitivity (250 cps/nv) He-3 detectors. Decayed gammas produced after photofission were also acquired using BGO scintillator. The data were analysed in ROOT minuit software taking considerations of corrections for the pulsed and scanning of electron beam. Mass v/s delayed neutron/gamma count rate were established. The experiments conclude that with the present set-up unshielded 100 mg of Thorium and natural uranium can be detected with reasonable accuracy. The relative proportions of the Thorium and Uranium was determined within 10 % accuracy by analyzing the delayed neutron decay signature. (author)