Induced luminescence in water and PMMA - spectral analysis for irradiations with proton beams within the clinical energy range

It was recently discovered that water and PMMA emit a weak luminescence signal when irradiated with protons within the clinically used energy range. This could offer a fast approach for range measurements in water. However, a complete explanation or investigation on the origin of the signal has not been published. In this work, a setup for the high-resolution spectral measurement of the weak luminescence signal in water and PMMA was designed. The measurement environment in the vicinity of a proton accelerator represented a major challenge for the sensitive optical measurements due to the presence of ionizing scattered radiation. A high-sensitive spectrometer in combination with a custom-made fiber was used to build a foundation for further analysis of the luminescence signal by providing accurate spectral information. For water, a broad distribution in the range from 240 to 900 nm with a maximum at 480 nm was obtained. A comparison of the spectra with previously published work indicates that the signal originates from excited states produced during the radiolysis of water. In comparison, differences between the water and the PMMA spectrum were observed. When examining the signal in PMMA, spectral differences were found compared to the measurements in water. The signal in PMMA was approximately 10 times stronger, had a narrower distribution and was shifted to lower wavelengths. Nevertheless, for the investigated proton energies, no spectral energy dependence was detected. In addition to the results for water and PMMA, a further luminescence signal was measured when the silica fiber used was directly irradiated with primary protons. All spectra, obtained in this work, describe the signal of proton-induced luminescence in water and PMMA with a high resolution of 3.4 nm and thus form a basis for further research, which could be a powerful tool in proton range verification.