Initial recombination of ions in electron tracks: an evaluation of the predictions of Lea's model and a modified track structure model

Abstract

The formulas estimating initial recombination of ions created by electrons in high pressure parallel plate ionization chambers are modified and developed to comply with a modified model for recombination. Appropriate average values for restricted ionization density related parameters of recombination are derived to describe the situation of irradiation with electron or photon spectra. It is shown that these average values are related to the dose mean of the restricted ionization density (cut-off energy 500 eV) in methane based tissue equivalent gas. An experimental comparison between the predictions of the modified model and Lea's model for electron recombination is presented and the modified model shows a closer agreement with experiment in all situations studied. The modified model predicts the experimentally observed currents to within 0.1% over the photon energies studied (60–1225 keV) and over all high field strengths applied to the chamber, at a constant pressure of 8 atm.