Measurement of the Average Energy per Ion-Pair with Monoenergetic Electrons of Energy Less than 5keV

The detailed measurement of the integral W-value, that is the mean energy required to form: an ion-pair, has been carried out for monoenergetic electrons in methane and the tissue-equivalent gas of Rossi and Failla (R.1). Measurements have been made over the electron energy range of 16eV
to 5keV.

The principle of the determination of W is the measurement of the ionization produced by the complete absorption of a monoenergetic electron beam of known intensity in the gas under investigation. The apparatus designed and developed to carry out this measurement is fully described as are its experimental characteristics. An analytical estimation of the influence of the ionization chamber collecting field on the energy of the primary electrons is also described.

W has been evaluated from both positive and negative ionization current data. It was seen that below 200eV,; when an ionization chamber gas pressure of around 0.1 Torr. was used, only positive data could be used for the evaluation of W due to the effects of diffusion and the applied collecting field on negative charges.

The results of the measurements show for both gases that above primary electron energies of 2keV the W-value can be considered to be constant. These high energy values of W of 27.2eV and 29.4eV for methane and tissue-equivalent gas respectively agree well with other high energy values found in literature. Below 2keV the variation of W with energy was found to be complex. Apart from a general increase in VY and an even sharper increase within a few electron volts of the ionization potential for each gas there was found a relative maximum of the W-value (40eV) in the region of the K-shell binding energies of the various atoms comprising the gas molecules. Although no theoretical explanation exists at the present time for the position of the relative maxima in W (E), that there is some connection with the K-shell binding energies is further suggested by peaks in the differential W-value,®, which was evaluated from the measurements of W.

Finally, the results show a clear difference between measurements of W using low energy electrons and low energy photons and a qualitative explanation is offered for this.