Average energy required to produce an ion pair, revisited

TL;DR

This study uses Monte Carlo simulations to examine how the average energy needed to produce an ion pair in air varies with electron energy, confirming its constancy at megavoltage levels and increase at lower energies.

Contribution

It extends electron degradation spectra calculations down to 200 eV and confirms the constancy of the w value in the megavoltage range using a novel simulation approach.

Findings

w value remains constant at megavoltage energies

w value increases at energies below several keV

Simulation method extends to lower electron energies

Abstract

The present work is a theoretical/computer simulation study aimed at investigating the dependence of the w value in air on the initial energy of the ionizing electrons. The current assumption of a constant w value underpins the accurate determination of absorbed dose in megavoltage radiotherapy. The transport of electrons in the energy range between 1 keV and 10 MeV has been studied using the EGSnrc Monte Carlo code system. We have extended the electron degradation spectra calculations down to 200 eV by solving an integral equation derived from the cavity integral formulation for the absorbed dose. The present study confirms the constancy of the w value in the megavoltage energy range though our calculation method is not capable of accurately predicting its absolute value. When the average electron energy falls below several keV, it is well known that w exhibits an increase in its absolute value which our calculations also indicate.