Recent experimental results on W-values for heavy particles.

The total ionisation produced by ions stopped in nitrogen, methane and carbon dioxide has been measured in the energy range 25-375 keV using a chamber described previously. The chamber was operated alternately as a proportional counter and as an ionisation chamber to measure respectively the particle rate and the total ionisation produced. This procedure was repeated 20 to 40 times and a statistical treatment of the data was carried out. The average energy loss per ion pair (W-value), the associated standard deviation and the systematic error (+/- 2.5%) were determined for H+, He+, C+, N+, O+ and Ar+ ions stopping in the three gases. The W-value was found to be dependent on the velocity of the incident ions. For nitrogen gas, values range from 35 eV for 25 keV H+ to 84 eV for 25 keV O+. For methane, the values are 30.3 and 66.4 eV. For carbon dioxide, W-values range from 33.4 eV for 50 keV H+ to 135.9 eV for 25 keV Ar+. Finally, the Bragg additivity formula was tested for tissue-equivalent gas. The agreement between W-values calculated by this formula and those measured was found to be within 2% except for 25 keV O+ where the discrepancy is 10%.