Plastic scintillator response to low-energy photons

The plastic scintillator (PS) is a promising dosimeter for brachytherapy an d other low-energy photon applications because of its high sensitivity and approximate tissue equivalence. As part of our project to develop a new PS material which maximizes sensitivity and radiological equivalence to water, we have measured the response, epsilon (light output/unit air kerma), of P S to low-energy bremsstrahlung (20 to 57 keV average energies) x-rays as we ll as photons emitted by Tc-99m, Ir-192, and Cs-137 sources, all of which w ere calibrated in terms of air kerma. The PS systems studied were a standar d commercial PS, BC400 (Bicron Corporation, Newbury, OH), and our new sensi tive and quench-resistant scintillator (polyvinyltoluene base and binary dy e system) with and without 4% Cl loading intended to match the effective at omic number of water. For low-energy x-rays; epsilon was 20-57% relative to epsilon for Ir-192 photons. Chlorine loading clearly reduced the energy de pendence of epsilon, which ranged from 46% to 85% relative to Ir-192. Howev er, even after using Monte Carlo photon-transport simulation to correct for the non-air equivalence of the PS, inherent dosimetric sensitivity still v aried by 30% over the 20-400 keV energy range. Our work, one of the few mea surements of PS response to low-energy photons. appears to confirm Birks' 1 955 finding that ionization quenching reduces sensitivity to electrons belo w 125 keV. However, our results cannot be explained by Birks' widely used u nimolecular quenching model.