Measurements of W̄ for Protons, Helium-4 Ions, and Carbon-12 Ions in Tissue-Equivalent Gas

Absolute measurement of anti W (the mean energy expended in a gas per ion pair formed) have been conducted in methane-based tissue-equivalent gas. A monenergetic particle beam enters a 1-m-long tank filled with gas in which the particle stops. The ionization created is collected on a 3-mm-diameter rod. The negative electron pulse at the rod is amplified and the pulse height is analyzed to accumulate a particle energy spectrum. The pulse-height spectrum is integrated to give the number of particles. The charge collected on the rod is simultaneously measured with a vibrating reed electrometer. Measured values of W varied from 30.2 to 31.4 eV for 2.8- to 0.15 MeV protons from 30.9 to 35.3 eV for 7.8- to 0.36-MeV helium-4 ions, and from 34.7 to 47 eV for 5.6- to 0.12-MeV carbon-12 ions. Uncertainties are considered in detail and vary from 0.3 to 1.2%. The energy dependence is smooth and for energies greater than 15 MeV is consistent with a constant value of differential W.