Speaker
Description
Photofission reactions represent an important aspect of photonuclear physics, with significant implications for various applications, including the detection of Special Nuclear Materials (SNM) in cargo containers for homeland security, radioactive waste packages characterisation or production of radioisotopes. To obtain the precision required in these high-stakes applications, accurate simulations of industrial or experimental setups are vital. Monte Carlo simulations are widely used for this purpose, using state-of-the-art nuclear data libraries and nuclear reaction models. Benchmark comparisons specifically targeted at the Monte Carlo simulation of photonuclear reactions have revealed significant inconsistencies between simulations and experimental data, but also among the results of different codes. These discrepancies emphasize the need for further research to refine nuclear data, improve modeling techniques, and enhance the reliability of simulation results, thereby ensuring that applications of photofission reactions can be effectively and safely implemented in practice. In this context, the aim of this work is to benchmark several Monte Carlo codes, i.e., TRIPOLI-4, MCNP6, PHITS and DIANE against experimental measurements of delayed neutrons from photofission. The experiments were performed at the SAPHIR facility operated at CEA Paris Saclay, France. The setup is based on a linear electron accelerator, which generates high-energy gamma-rays through the Bremsstrahlung effect on a tungsten target. The accelerator, designed by Varex Imaging Corp., is a Linatron® M9A. In our experiments, a photon beam from 6 or 9 MeV electrons pulsed at 100 Hz is produced. Reference photofission samples (235U, 238U, 239Pu) are investigated. The detection part of the experiment is composed of ten 3He filled-gas proportional counters embedded in a block made of high-density polyethylene, which is surrounded by a cadmium layer, in order to thermalize and detect delayed neutrons from photofission, without contribution from spurious neutrons thermalized in the irradiation hall. The simulation is conducted in two key steps for comparison with experimental results. In the first step, the photofission rate within the sample is calculated using the electron source from the accelerator. In the second step, the simulation accounts for the delayed neutron source, tallying the reaction rate for the neutroninduced (n,p) reactions within the 3He gas. Subsequently, a time dependent correction factor is taken into account to address the six families of delayed neutrons associated with each sample. The findings of this study could help Monte Carlo practitioners and codes developers, as well as nuclear data evaluators, to improve the accuracy of the photofission reaction simulation.
