Microdosimetry of a pion beam was carried out at the biomedical pion channel at TRIUMF. Event size spectra were measured at 17 depths in a water phantom along the axis of a pion beam which had a midline momentum of 180 MeV/c. Measurements were performed with a spherical tissue equivalent plastic proportional counter. The pulses were processed with a 4.5 decade logarithmic amplifier which made the recording and analysis quite simple. Measurements were made off the axis and with various effective diameters. A novel time-of-flight microdosimetry technique was employed to differentiate the contributions of the pions and contaminant muons and electrons to the total spectrum. The fractional dose contributions for different lineal energy intervals, the frequency mean lineal energy (anti y/sub F/), and the dose mean lineal energy (anti y/sub D/) were calculated as a function of depth in the phantom. A marked change in the radiation quality was measured between the plateau region, where most of the energy is deposited by passing particles, and the peak region, where a significant amount of energy is deposited by the star products from pion capture. Finally, a map of the varying radiation quality of the pion beam is presented and discussed inmore » relation to other radiation types.« less
