A new algorithm for EGS4 low-energy electron transport to account for the change in discrete interaction cross-section with energy

This paper presents a new algorithm for the EGS4 Monte Carlo computer code system to account for the change in electron discrete interaction cross-section as a function of energy for low-energy electron transport. The current EGS4 algorithm uses a crude method to deal with this change which is only valid for high-energy cutoffs (AE > 560 keV) and therefore results in a systematic error in low-energy electron transport. In the new algorithm a variable electron mean free path correction factor is used to account for the effect of the overestimated electron mean free path at the beginning of an electron path. The electron path length is then more accurately sampled from the averaged electron discrete interaction cross-section along the path using a linear variation model. Calculations of the absorbed dose and particle fluence in different geometries have been carried out. The results show that the number of discrete interactions for low-energy electron transport (AE = 512 keV) increases by more than 20% with the new algorithm compared with the current EGS4 algorithm. The depth-dose calculations show a 1-2% difference around the depth of maximum dose for electron energies of 10 keV to 1 MeV. The difference in the electron fluence obtained with the new algorithm and that with the current algorithm is about 4% at energies of 100-300 keV. It is recommended that the new algorithm should be implemented in the EGS4 system for the calculations of the absolute dose or fluence with electron energies below 1 MeV and energy cutoff AE smaller than 531 keV.