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

As recently discovered, water and PMMA emit a weak luminescence signal when irradiated with protons within the clinically used energy range. The potential use of this signal has been shown for example on range measurements in water. An explanation or investigation on the origin of the signal has not been clearly published. Furthermore, there is a lack of high-resolution spectral investigations in the literature that could form a basis for further work on this topic.
High-resolution spectral analysis of the signal is challenging due to the low signal intensity of the effect. In this work, a setup for high-resolution spectral measurement of the weak luminescence signal was designed. 
The measurement environment in the vicinity of a clinical accelerator presents a particular challenge for low light optical measurement systems due to the presence of scattered radiation which interfere with the sensitive system. Therefore, a high-sensitive spectrometer in combination with a custom designed fiber was used. 
For water, a broad distribution in the range from 240 to 900 nm with a maximum at 480 nm could be determined.
The comparison of the measurements for water and PMMA showed clear differences.
The signal in PMMA is approximately 10 times stronger, has a narrower distribution and is shifted to lower wavelengths, indicating a different origin.
For the investigated proton energies, no spectral energy dependance was detected for both materials. 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. 
These measurements confirm previously published works on this fiber effect.
Finally, spectra with a resolution of 3.4 nm were obtained in this work which accurately describe the signal of proton-induced luminescence in water and PMMA and thus form a basis for further research on this topic.&#xD.