The plastic scintillator is a promising dosimeter for brachytherapy and other low-energy photon applications because of its high sensitivity and approximate tissue equivalence. As part of our project to develop a new plastic scintillator material which maximizes sensitivity and radiological equivalence to water, we have measured the response, ε (light output/unit air kerma), of plastic scintillator to low-energy bremsstrahlung (20 to 57 keV average energies) x-rays as well as photons emitted by 99mTc, 192Ir, and 137Cs sources, all of which were calibrated in terms of air kerma.
The plastic scintillator systems studied were a standard commercial plastic scintillator, BC400 (Bicron Corporation, Newbury, OH), and our new sensitive and quench-resistant scintillator (polyvinyltoluene base and binary dye system) with and without 4% Cl loading intended to match the effective atomic number of water. For low-energy x-rays; ε was 20-57% relative to ε for 192Ir photons. Chlorine loading clearly reduced the energy dependence of ε, which ranged from 46% to 85% relative to 192Ir. However, even after using Monte Carlo photon-transport simulation to correct for the non-air equivalence of the plastic scintillator, inherent dosimetric sensitivity still varied by 30% over the 20-400 keV energy range.
Our work, one of the few measurements of plastic scintillator response to low-energy photons, appears to confirm Birks’ 1955 finding that ionization quenching reduces sensitivity to electrons below 125 keV. However, our results cannot be explained by Birks’ widely used unimolecular quenching model.