Estimation of dose equivalent and average quality factors for pi- and pi+ beams.

Abstract

A Monte Carlo computer program, used previously to compute absorbed dose as a function of position in a water phantom exposed to pion beams, is used to study dose equivalent and average quality factors. Detailed calculations for a typical beam (∼6 cm diameter, 175 MeV/c mean momentum) of negative pions show that the average quality factor is 1.6 to a depth of about 16 cm, after which it rises to a maximum of 6.4 at a depth of 22 cm, some 2 cm beyond the depth at which the absorbed dose is maximum. Similar calculations for positive pions show that the quality factor decreases with depth, because of the decreasing cross section for nuclear interactions. Adding muons and electrons to the pi− beam changes only slightly the relative contributions made by heavy fragments (atomic mass number > 1) and neutrons to the dose equivalent in the region where it is greatest. Average quality factors are also computed as a function of depth for pi− beams with mean momenta of 150 and 190 MeV/c. The maximum value of the average quality factor changes only slightly (from 5.5 to 7.0) over this momentum range.