Energy dependence of W-values for heavy charged particles in gases

For determination of W, an average energy to produce an ion pair, in gases at atmospheric pressures for heavy ions, an apparatus consisting of a particle-energy degrader, an ionization chamber and a time-of-flight energy spectrometer was constructed, where two types of pulse operation modes in an ionization chamber were employed. Using heavy-ion beams from an accelerator, the values of W were measured for He²⁺, C⁶⁺, and N⁷⁺ ions in air and tissue-equivalent gas of the pressures of 500 to 760 Torr as a function of the energy of ions. For C⁶⁺ and N⁷ ions, the energy dependence of W was clearly observed in both gases, while W for He²⁺ ions was approximately constant over a wide rage of the energy. The differential W-values were also measured in two gases. In air, the differential W-values for C⁶⁺ and N⁷ ions are almost equal to the value of W for high energy electrons.