The influence of energy indexing algorithm and electron substeps on MCNP4C electron transport: application in Monte Carlo simulation of s-ray spectra in diagnostic radiology and mammography

The random walk for electron transport in the MCNP4C general purpose Monte Carlo code is derived from ITS3.0, which is a well validated code for coupled electron/photon transport. The continuous slowing down approximation energy loss model is used for electron transport where the MCNP4C code breaks the electron's path into many steps. In this study, the influence of substeps and choice of electron energy indexing algorithm for electron transport on the simulation of X-ray spectra in diagnostic radiology and mammography energy range is investigated. For the simulation of X-ray spectra for tungsten (W) and molybdenum (Mo) targets at different tube voltages, the code was run in photon and electron mode with different substeps and energy indexing algorithms using default values for PHYS:P and PHYS:E cards. The simulated x-ray spectra were compared with the spectra calculated by IPEM report number 78. An average relative difference (ARD) of 11.9%, 13.7% and 14.1% were calculated between the simulated W target spectra using MCNP and ITS energy indexing algorithms at tube voltages of 80, 100 and 120 kVp, respectively. The ARD for Mo target spectra at tube voltages of 25 and 30 kVp was 16.6% and 16.7%, respectively. There is no noticeable difference between the spectra simulated using different substeps in both mammography and radiology energy range; however the simulation time increased by 43.8% and 44.2% when the substeps are set to twice the default value for W and Mo targets, respectively. Generally, there is a good agreement between the simulated and IPEM spectra, although the results suggest slight differences between the simulated spectra using different energy indexing algorithms and electron substeps which tend to be reduced by the normalization process. It is concluded that the energy indexing algorithm and electron substeps have limited influence on electron transport in MCNP4C for the purpose of simulating X- ray spectra in diagnostic radiology and mammography.