Plastic scintillator response to low-energy photons

Abstract

The plastic scintillator (PS) 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 PS material which maximizes sensitivity and radiological equivalence to water, we have measured the response, epsilon (light output/unit air kerma), of PS 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 PS systems studied were a standard commercial PS, 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, epsilon was 20-57% relative to epsilon for 192Ir photons. Chlorine loading clearly reduced the energy dependence of epsilon, 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 PS, inherent dosimetric sensitivity still varied by 30% over the 20-400 keV energy range. Our work, one of the few measurements of PS 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.

Characterization of a scintillating fibre detector for small animal imaging and irradiation dosimetry

“…Other authors declare an energy dependence of plastic scintillator detectors sensitivity in the radiologic energy range (25-150 kVp). 13,14,[16][17][18][19] As reported by Williamson et al, 16 Lessard et al, 18 and Peralta and Rêgo, 19 it is partly related to energy absorption differences between the reference dose medium and the scintillator medium (air and polystyrene respectively in our case). As shown in Figure 4, the polystyrene-toair mass energy-absorption coefficients ratio shows significant variations ,100 keV.…”

“…This energy dependence of plastic scintillator has first been expressed by Williamson et al 16 who showed a reduction of the scintillation light output with decreasing energy for photons ,133 keV. Other authors declare an energy dependence of plastic scintillator detectors sensitivity in the radiologic energy range (25-150 kVp).…”

“…Nevertheless, these calculations are very approximate and do not take into account accurate spectrum considerations. Indeed, accurate CF should be computed over the all-energy spectrum of each different beam, as carried out by Williamson et al, 16 Lessard et al, 18 and Peralta and Rêgo. 19 Corrections factors do not explain entirely the energy dependence and remaining energy dependence has been observed after medium correction by Williamson et al 16 and Peralta and Rêgo.…”

“…Indeed, accurate CF should be computed over the all-energy spectrum of each different beam, as carried out by Williamson et al, 16 Lessard et al, 18 and Peralta and Rêgo. 19 Corrections factors do not explain entirely the energy dependence and remaining energy dependence has been observed after medium correction by Williamson et al 16 and Peralta and Rêgo. 19 Besides, Frelin-Labalme et al 28 also measured a non-linearity of response ,100 keV for different plastic scintillators.…”

“…[13][14][15] However, other studies also found an energy dependence of the dosemeter response, expressed as a decrease of sensitivity with decreasing energy. [16][17][18][19] In addition, in the kilovoltage energy range, the stem effect is mainly due to the fluorescence of the optical fibre and its contribution has been reported to be small. 8 Its small influence on the final signal allows to simplify the dosemeter readout system.…”