Abstract
The frequency distributions of event size in the deposition of energy produced by the passage of protons in a pathlength of 6.99*10-5 g cm-2 tissue-equivalent gas have been measured after penetration of a 44 MeV monoenergetic proton beam through various amounts of bone and muscle equivalent plastic. Corresponding thicknesses of the absorbers of the two types were chosen such that the mean energy of the residual beam of protons entering the gaseous test volumes were identical. Each pair of absorbers thus produced the same relative ionization density as measured by the Bragg curve. The resultant energy deposition distributions obtained for each pair of the two absorbing materials were identical. This indicates that both primary and forward scattered secondary particle spectra in a degraded beam are not significantly different after the primary beam passes through equivalent amounts of bone or soft tissue. The results obtained on a microscopic scale are in agreement with both calculations of the absorbed dose and dose equivalent under similar conditions and with experimental measurements of absorbed dose.