OPTICAL FILTERING AND SPECTRAL MEASUREMENTS OF RADIATION-INDUCED LIGHT IN PLASTIC SCINTILLATION DOSIMETRY

A small, water-equivalent plastic scintillation detector system has previously been developed for radiation therapy dosimetry. A light signal, proportional to dose, is generated in the scintillator and is transmitted to a remote photomultiplier tube (PMT) via optical fibres. Ionizing radiation also produces light in the fibres, which, if not properly accounted for, could limit the accuracy of the scintillator system. The fibre light is shown to have both a Cerenkov radiation and fluorescent component. The differences in the measured optical spectra of the fibre light and plastic scintillator light lead to the possibility of reducing the fibre light component by optical filtering. Optical spectral measurements of a commercially available orange-emitting plastic scintillator revealed that incomplete light-wavelength-shifting to the orange region of the visible spectrum occurs due to the size of the small scintillators that were used. Spectral measurements of orange and green scintillators with higher concentrations of wavelength-shifting fluor have been performed. Quantitative results indicate that using the highest doped orange scintillator and appropriate optical filter can decrease the fibre light contribution by about 50% when compared to the conventional blue scintillator, non-filtered case in typical radiotherapy dosimetry situations.