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.