Technical Note: Improvements in geant4 energy-loss model and the effect on low-energy electron transport in liquid water.

PURPOSE: The geant4-DNA physics models are upgraded by a more accurate set of electron cross sections for ionization and excitation in liquid water. The impact of the new developments on low-energy electron transport simulations by the geant4 Monte Carlo toolkit is examined for improving its performance in dosimetry applications at the subcellular and nanometer level. METHODS: The authors provide an algorithm for an improved implementation of the Emfietzoglou model dielectric response function of liquid water used in the geant4-DNA existing model. The algorithm redistributes the imaginary part of the dielectric function to ensure a physically motivated behavior at the binding energies, while retaining all the advantages of the original formulation, e.g., the analytic properties and the fulfillment of the f-sum-rule. In addition, refinements in the exchange and perturbation corrections to the Born approximation used in the geant4-DNA existing model are also made. RESULTS: The new ionization and excitation cross sections are significantly different from those of the geant4-DNA existing model. In particular, excitations are strongly enhanced relative to ionizations, resulting in higher W-values and less diffusive dose-point-kernels at sub-keV electron energies. CONCLUSIONS: An improved energy-loss model for the excitation and ionization of liquid water by low-energy electrons has been implemented in geant4-DNA. The suspiciously low W-values and the unphysical long tail in the dose-point-kernel have been corrected owing to a different partitioning of the dielectric function.