Measurements of $\overline{W}$ for Protons, Helium-4 Ions, and Carbon-12 Ions in Tissue-Equivalent Gas

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Abstract
Absolute measurements of .hivin.W (the mean energy expended in a gas per ion pair formed) have been conducted in methane-based tissue-equivalent gas. A monoenergetic 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 .hivin.W varied from 30.2-31.4 eV for 2.8-0.15 MeV protons, from 30.9-35.3 eV for 7.8-0.36 MeV helium-4 ions, and from 34.7-47 eV for 5.6-0.12 MeV carbon-12 ions. Uncertainties are considered in detail and vary from 0.3-1.2%. The energy dependence is smooth and for energies greater than 1.5 MeV is consistent with a constant value of differential W.

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