A Benchmark for Monte Carlo simulation of photoneutron fields from electron accelerators

Sari, A.*; Meleshenkovskii, I.*; Ogawa, Tatsuhiko   ; Tran, K.-T.*; Jinaphanh, A.*; Jouanne, C.*; Zoia, A.*

Electron accelerators cover a wide range of applications, from nuclear waste package assay and security-related tasks to radiation therapy. High-energy photons produced by electron Bremsstrahlung radiation might induce photonuclear reactions emitting secondary neutrons. In this paper, we perform a two-stage benchmark for Monte Carlo simulation of photoneutron fields from electron accelerators operating in the 4-20 MeV range. On the one hand, we benchmark three independently developed Monte Carlo codes, i.e., MCNP, TRIPOLI-4; and PHITS, using ENDF7u photonuclear data from the ENDF/B-VII.1 library. On the other hand, we benchmark two photonuclear data sub-libraries, i.e., ENDF7u and JENDL-5, as well as nuclear reaction models, using the PHITS code. Errors in ENDF7u for W-184 and Ta-181 cause unphysical shapes of the photoneutron energy spectra with the three codes. Questionable behaviors are also found with JENDL-5, which calls for further examinations in this recently released library. For the cases of heavy water and beryllium, the photoneutron energy spectra obtained with MCNP6 is invalidated. We identified that the problem stems from wrong treatment of the reference frame in using the ACE data files by MCNP and previous versions of PHITS. Results obtained for tungsten and tantalum show that photoneutrons are overall emitted in an isotropic manner. However, results obtained for heavy water and beryllium display different tendencies. This investigation should help Monte Carlo code users to refine the current validity domain of the latest releases of MCNP6, PHITS and TRIPOLI-4 for the simulation of photoneutrons.