Intercomparison of low-energy electron transport calculations by different Monte Carlo track-structure simulation codes

To evaluate the uncertainty of physical features of low-energy electron transport in liquid water due to the use of different Monte Carlo track-structure (MCTS) codes. 
Approach. Five MCTS codes developed specifically for liquid water, namely, Geant4-DNA, PHITS-TS, RITRACKS, NASIC, and PARTRAC are compared and used to calculate the electronic stopping power, pathlength and absorption range, dose-point-kernel, and the frequency-mean (y ̅_F) and dose-mean (y ̅_D) lineal energy for primary electron energies from 20 eV to 100 keV. The uncertainty of each calculated quantity is evaluated by the relative standard deviation (RSD) and the maximum relative difference (MRD) among the codes. The medium-to-high-energy (1‒100 keV) performance of the codes is benchmarked against the stopping power and range data for liquid water reported in ICRU Report 90. 
Main Results. For energies above ~1 keV the RSD is moderate and mostly between 5-15%, but increases rapidly at lower energies, reaching 20-100% at sub-100 eV energies. It is noteworthy that the MRD may well exceed 100% below 100 eV, while remaining sizeable (>20%) even at relatively high energies (10-100 keV). Fairly good agreement between the MCTS codes and the ICRU data for the stopping power (1‒100 keV) and range (10‒100 keV) in liquid water is found with average deviations between 1‒16%, depending on the code. 
Significance: The present work reveals significant differences for low-energy electron transport among liquid water MCTS codes, especially below 100 eV. These differences potentially compromise the accuracy of nanoscale simulations where such electrons play a key role. The observed dispersion of results is a consequence of the limitations of the theoretical models used to calculate electron interaction cross sections and the lack of relevant experimental data for their validation and benchmarking. This highlights the need for further development of the physics models used in MCTS codes to reduce the uncertainties associated with low-energy electron transport calculations in liquid water.