Microdosimetry of negative pions

Abstract

The radiation quality of negative and positive pions of initial momentum 168 MeV/c has been determined at eight different depths in a liquid phantom. The measurements were made with a 2.5 cm diameter spherical proportional counter with Shonka A-150 neutron tissue equivalent plastic walls. The gas pressure in the sensitive volume was chosen to simulate a diameter of 2 $mu$m in unit density material. Dose distributions as a function of lineal energy (y) change slowly in the entrance and plateau regions with a dose mean lineal energy (anti y/sub D/) of 6-8 keV/$mu$m. Less than 3 percent of the dose is delivered in excess of 50 keV/$mu$m in this region. In the Bragg peak region the distributions change rapidly as a function of depth with the dose mean lineal energy increasing to 38 keV/$mu$m at the peak and to 57 keV/$mu$m just beyond the peak. On the basis of these microdosimetric data predictions of RBE and OER have been made with the use of both the theory of dual radiation action and also the delta ray theory of cell survival. The former has been used to predict biological response at low doses and the latter at high doses. A comparisonmore » is made between the two theories at intermediate doses. The results of these calculations are not inconsistent with recent biological data. (6 figures, 1 table, 20 references) (auth)« less