W-value measurements for protons in tissue-equivalent gas and its constituent gases

Absolute measurements of W (the mean energy required to create an ion pair) have been performed for protons with energies between 1.2 MeV and 3.5 MeV for tissue-equivalent gas, and its constituent gases i.e. methane, carbon-dioxide, and nitrogen. The apparatus consisted essentially of two parallel plates enclosed within a vessel containing the gas of interest. Protons entered the measurement chamber through a thin Mylar window and were stopped in the gas region between the plates. A high voltage applied to one plate enabled the ion current to be measured from the opposite plate. The number of protons which produced the measured integrated charge was obtained simultaneously by amplifying signals from the high voltage plate and counting pulses resulting from individual protons. Results for W are somewhat lower than those suggested by ICRU Report 31. For tissue-equivalent gas there have been previous measurements in this energy region by Rohrig and Colvett, and by Larson, and the present measurements are about 2% lower