Development of photon-transport capability in UNIST Monte Carlo code MCS

A photon-transport capability has recently been implemented in the Monte Carlo MCS under development at Ulsan National Institute of Science and Technology (UNIST). The MCS photon fixed-source mode simulates the transport of photons between 1 keV and 20 MeV for all elements from hydrogen to fermium. Motivation for this work is the calculation of photon kerma and/or photon dose in shielding problems for applications in spent nuclear fuel management and reactor shielding. Implemented physics include the simulation of the main four photo-atomic reactions (Rayleigh scattering, Compton scattering, photoelectric effect and pair production) and three secondary processes of photon production (positron-electron annihilation, atomic relaxation and charged-particle Bremsstrahlung). The Doppler broadening of Compton scattered photons due to the pre-collision momentum of the atomic electrons is simulated. In absence of charged-particle transport, a thick-target Bremsstrahlung (TTB) approximation is employed to generate Bremsstrahlung photons, with differentiated physics for electrons and positrons. Validation of the photon-transport capability is currently underway and preliminary verification results against MCNP6.1 are presented for 2 simple geometries (energy distribution of photon flux in an infinite medium and spatial distribution of photon flux in a cylinder) and one application case (spatial distribution of photon body-equivalent dose in a spent nuclear fuel transport cask). Good calculation/calculation agreement is observed in the simple geometries for photon source energies below 1 MeV and for the application case. Discrepancies due to the Bremsstrahlung production are identified for photon source energies over 10 MeV. Ideas for improvement of the photon physics in MCS include the implementation of the energy-loss straggling of charged particles for the emission of Bremsstrahlung photons. (Author)