Average energy to produce an ion pair for heavy charged particles in gases measured as a function of particle energy

In order to determine 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.