Linear energy transfer spectra of 101 MeV negative pions identified by time-of-flight
Newman, Roger Clarke
Phillips, Gerald C.
Master of Arts
The energy loss spectra in a spherical tissue equivalent proportional counter at various depths in a tissue-equivalent phantom were measured for negative pions. The experiment was performed in the muon cave at SREL with a pion energy of 11 MeV. The Bragg peak for the pions occured at 25 cm depth in the phantom. Measurements were taken at depths of 12.5 cm, 25 cm, and 27.5 cm. Two methods were used to differentiate the pion induced events from those due to muons and electrons. It was assumed that muon events would have LETs of less than 1 keV/micrometer, so all events with LET greater than 1 keV/micrometer were assumed to be due to pions. For events with LET less than 1 keV/micrometer, the particles were identified by their time-of-flight between two scintillators 3 meters apart. From the energy loss spectra, the distributions of track length vs. LET (T(L)), dose vs. LET (D(L)) and LET times dose vs. LET (L*D(L)) at each depth were derived and graphed. The dose average LET, and track average LET were calculated at each depth. The analysis showed that the dose in the plateau region (12.5 cm depth) consisted mainly of low LET events. At the depths of 25 cm and 27.5 cm there was a considerable contribution from events with high LET. This property of pion radiation is one of its main advantages for medical purposes since at high LET, the OER is lower and the RBE is higher.