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%.

Some experimental results on W values for heavy particles.

The total ionization produced by ions stopped in argon and tissue-equivalent (TE) gas has been measured in the energy range 25-500 keV. A large ionization chamber was used for this study. The chamber was alternately operated as a proportional counter and as a ionization chamber to measure particle rate and the total ionization produced by them, respectively. The average energy loss per ion pair (W value) was found to be dependent on both the energy and mass of the incident ions. For argon gas the accelerated ions were H+, He+, Ar+; the W value ranges from 23.72eV for 25 keV H+ to 63.12 eV for 50 keV Ar+; Irregularities in the W value were found for He+ in the region 70-130 keV. For TE gas the accelerated ions were H+, He+, C+, N+, O+; the W value ranges from 29.13 eV for 25 keV H+ to 51.45 eV for 50 keV O+. Comparisions with existing data show a good agreement in the absolute values for TE gas and in the relative variations in argon gas. Differences between absolute values in argon might be due to impurities in composition.