System Design Of Photofission Reaction Ratio With High-energy Bremsstrahlung Photons For Nuclear Security

The applicability of the photofission reaction ratio (PFRR) method to identify high-enriched uranium is studied by switching the photon source from the Gaussian spectrum in extant studies to a bremsstrahlung spectrum. The principle of PFRR method is based on the difference of photofission cross section between 235U and 238U nuclides and it was validated using Gaussian photons in extant studies where PFRR increases linearly with uranium enrichment. A mock experiment was conducted using Californium-252 (Cf) neutron source as the reference target to estimate the minimum requirement of number of neutrons emission for detection by natural uranium target. The Cf-252 target used had the activity of 1.04x107 Bq and 1.21x106 neutrons/s. A 5” diameter x 5” thick organic liquid scintillator, BC-501A was used as the detector to measure neutron/gamma from Cf-252 source. To measure neutron in the mixed neutron/gamma field, electronic circuit with a PSD function was used. High voltage of 800V was supplied to the circuit and the signal is delayed using delay amplifier ORTEC 460 with 20x20 coarse gain and displayed on MCA coupled with rise time to height converter. A ROI of 21x85 on the contour plot of neutrons and gamma-rays distribution (512x512 arbitrary units) was defined as the region containing mostly neutrons. With 0.3% confidence, the minimum detectable activity for both natural background and background with beam operating at 18 MeV were 3.95 kBq and 10 kBq respectively. An accelerator experiment was conducted with natural uranium target with 18.0 MeV incident electron energy. The natural uranium target had a size of 4 x 4 x 0.6 cm, and the cross section of 238U is estimated to be approximately 300 mb at peak around 14.0 MeV. As such, with the same ROI region defined, the natural uranium showed a fission rate larger than the minimum required reaction rate based on Cf-252 source for natural background.