SU-F-T-166: On the Nature of the Background Visible Light Observed in Fiber Optic Dosimetry of Proton Beams.

PURPOSE: The nature of the background visible light observed during fiber optic dosimetry of proton beams, whether it is due to Cherenkov radiation or not, has been debated in the literature recently. In this work, experimentally and by means of Monte Carlo simulations, we shed light on this problem and investigated the nature of the background visible light observed in fiber optics irradiated with proton beams.

METHODS: A bare silica fiber optics was embedded in tissue-mimicking phantoms and irradiated with clinical proton beams with energies of 100-225 MeV at Roberts Proton Therapy Center. Luminescence spectroscopy was performed by a CCD-coupled spectrograph to analyze in detail the emission spectrum of the fiber tip across the visible range of 400-700 nm. Monte Carlo simulation was performed by using FLUKA Monte Carlo code to simulate Cherenkov light and ionizing radiation dose deposition in the fiber.

RESULTS: The experimental spectra of the irradiated silica fiber shows two distinct peaks at 450 and 650 nm, whose spectral shape is different from that of Cherenkov radiation. We believe that the nature of these peaks are connected to the point defects of silica including oxygen-deficiency center (ODC) and non-bridging oxygen hole center (NBOHC). Monte Carlo simulations confirmed the experimental observations that Cherenkov radiation cannot be solely responsible for such a signal.

CONCLUSION: We showed that Cherenkov radiation is not the dominant visible signal observed in bare fiber optics irradiated with proton beams. We observed two distinct peaks at 450 and 650 nm whose nature is connected with the point defects of silica fiber including oxygen-deficiency center and non-bridging oxygen hole center.