DOPPLER BROADENING EFFECT ON LOW-ENERGY PHOTON DOSE CALCULATIONS USING MCNP5 AND PENELOPE

Abstract

Recent releases of the MCNP5 and PENELOPE Monte Carlo codes include the transport algorithm and momentum profiles that are necessary for accounting for Doppler broadening in Compton scattering processes. Such improvements might be particularly important in low-energy photon dose calculations. MCPLIB04 and PENDBASE (PENELOPE photon dataset) are based on the EPDL97 library with Compton momentum profiles, while MCPLIB03 and MCPLIB02 are based on the 1970's old library, with MCPLIB03 including the Compton momentum profiles. To isolate the dosimetric effects of Doppler broadening by the transport algorithm and Compton momentum profiles, we varied the choice of the above photon databases, in the same simulation geometry, using either version of MCNP5 or MCNP4 (no Doppler algorithm). We computed dose rate constants and dose distributions for r = 0.2-10 cm from a point source in a 50-cm-diameter sphere of water. Nine discrete energies for primary photon sources were chosen in the range of 10-150 keV. The results from both versions of MCNP with MCPLIB04 agreed with those of PENELOPE within statistical uncertainties (+/-1%) over the entire ranges of energies and radial distances investigated. MCNP5 with either MCPLIB03 or MCPLIB02 yielded almost identical data within statistical uncertainties (+/-1%) over the entire ranges of energies and radial distances investigated. This implies that in spite of the spectral broadening of scattered photons due to the orbital electron motion, the dosimetric effect of Doppler broadening for Compton interactions in water appears to be insignificant in the energy range investigated. The spectral dose analysis with and without the Doppler broadening supported this conclusion.