Technical Note: Implications of using EGSnrc instead of EGS4 for extracting electron stopping powers from measured energy spectra.

Purpose: NRC Report PIRS-0626 (https://doi.org/10.4224/40000364) describes how measured electron energy deposition spectra can be used to determine the electronic stopping power. The stopping power is obtained by comparing measured spectra with spectra calculated using Monte Carlo techniques. The stopping powers reported in PIRS-0626 were obtained using the EGS4 Monte Carlo code. Since then, the EGSnrc code has been released which has more accurate electron transport algorithms. We calculate the effect on the measured stopping powers of using EGSnrc instead of EGS4.

Method: The EGS4 spectra calculated in PIRS-0626 were based on primary electron histories. We first show that those spectra, calculated in 1997, are consistent with current EGS4 spectra calculated using histories. EGSnrc spectra are also calculated using histories and these high-precision spectra are compared to extract any energy difference. The energy differences between the spectra are used to estimate the effect on the measured electronic stopping powers.

Results: The energy differences depend on the absorber material, the absorber thickness and the beam energy. The improved electron elastic scattering cross section of EGSnrc accounts for only part of the difference between the two codes. The effect on the extracted stopping power is largest for the lowest electron energies and can be as large as 0.9%. The calculated spectra show differences for lower energies, with the EGSnrc spectra having a larger proportion of low-energy electrons.

Conclusion: The differences introduced by using EGSnrc instead of EGS4 can affect the estimated stopping power by almost 1% in the worst case but generally the effect is much smaller. We report corrections that can be applied to all the stopping power data in PIRS-0626. An experiment to measure the average energy to create an ion pair in air, , using aluminum detectors will provide an interesting test of the aluminum stopping power data as reported in PIRS-0626 and revised by this work.