Dependence of the anti W-value on the atomic number of heavy ions

Anti W-values (the average energy required to form an ion pair) were determined for 35Cl ions in nitrogen and tissue-equivalent gas. These values were compared with previously reported anti W-values for oxygen ions and alpha particles in the same media. This comparison was made at two specific values of energy per atomic mass unit (u) of the incident ions. At an energy of 2.57 MeV/u, the comparison shows that anti W is 12% and 10% higher for oxygen ions in tissue-equivalent and nitrogen gas, respectively, relative to the alpha particle anti W. At an energy of 0.77 MeV/u, a similar comparison shows that anti W is 20% higher for 35Cl ions and 10% higher for 16O ions in tissue-equivalent gas; and 12% and 10% higher, respectively, in nitrogen gas, relative to the alpha particle anti W. These results indicate that anti W-values depend not only on the energy per atomic mass unit of heavy ions but also on their charge. (author)