Measurement of Ionization Distributions in Tissue-Equivalent Gas

The ionization distributions produced in small, cylindrical, tissue-equivalent sites by beams of protons of cross section much smaller than the sites were measured. Proton energies were 1.7, 3.0, and 4.0 MeV. The heights of the sites equaled their diameters; diameters were 25 and 100 μ g/ cm2. Beams were positioned at distances from 0.3 to 3 times the radius from the center of the site. The properties of compound-Poisson processes are used to explain the trends in the shapes of the distributions and to obtain some information about the ionization distributions due to individual interactions between protons and molecules of the medium. A method of transforming measured distributions into what they would have been had the number of interactions been different is described and applied to comparison of the measured distributions. The ionization distributions for uniform beams of protons were calculated. They show the effects of the straggling in energy loss of the protons and confirm that the increase in the distribution at low energies is due to secondary radiations from protons that do not enter the site.