Monitoring of Ultra-High Dose Rate Pulsed X-ray Facilities with Radioluminescent Nitrogen-Doped Optical Fiber

Abstract

We exploited the potential of radiation-induced emissions (RIEs) in the visible domain of a nitrogen-doped, silica-based, multimode optical fiber to monitor the very high dose rates associated with experiments at different pulsed X-ray facilities. We also tested this sensor at lower dose rates associated with steady-state X-ray irradiation machines (up to 100 keV photon energy, mean energy of 40 keV). For transient exposures, dedicated experimental campaigns were performed at ELSA (Electron et Laser, Source X et Applications) and ASTERIX facilities from CEA (Commissariat à l’Energie Atomique—France) to characterize the RIE of this fiber when exposed to X-ray pulses with durations of a few µs or ns. These facilities provide very large dose rates: in the order of MGy(SiO2)/s for the ELSA facility (up to 19 MeV photon energy) and GGy(SiO2)/s for the ASTERIX facility (up to 1 MeV). In both cases, the RIE intensities, mostly explained by the fiber radioluminescence (RIL) around 550 nm, with a contribution from Cerenkov at higher fluxes, linearly depend on the dose rates normalized to the pulse duration delivered by the facilities. By comparing these high dose rate results and those acquired under low-dose rate steady-state X-rays (only RIL was present), we showed that the RIE of this multimode optical fiber linearly depends on the dose rate over an ultra-wide dose rate range from 10−2 Gy(SiO2)/s to a few 109 Gy(SiO2)/s and photons with energy in the range from 40 keV to 19 MeV. These results demonstrate the high potential of this class of radiation monitors for beam monitoring at very high dose rates in a very large variety of facilities as future FLASH therapy facilities.

Preirradiation Treatment Influence on the Radioluminescence of a Nitrogen‐Doped Optical Fiber Dosimeter

“…[ 15–17 ] This RIL was observed, for different radiation types (photons, neutrons, protons), to linearly depend on the dose rate over more than 10 decades. [ 18 ] For this work, we chose a fiber length of 2.5 cm, to avoid any RIA effect accordingly with another study [ 16 ] for doses below 90 kGy(SiO 2 ) and to guarantee, at the same time, an intense RIL signal over the whole range of investigated dose rates. In another study, [ 19 ] we estimated the X‐Ray RIL measurement reproducibility for both SM (as the samples discussed here) and multimode (MM) versions of this fiber type.…”

“…This latter was a PMT (Hamamatsu H7421), optimized for photon detection in the 380–720 nm spectral range and then adapted to the RIL of our samples centered around 550 nm. [ 18 ] The photon counting integration time was set to 2 s for all the acquisitions runs. To perform our study about the influence of the dose effects, we adopted the following methodology.…”

Silica-based scintillators: basic properties of radioluminescence kinetics

“…RL is produced by recombination of free carriers at recombination centers (RCs) formed by charges of opposite polarity captured in a thermally stable state by the luminescent ions introduced as dopants. The latter can be REs [12,13,15], copper [13,15], germanium [14,16,17], phosphorus [18] or nitrogen [19,20]. In silica, luminescent centers are most often reduced by electron capture under irradiation [23][24][25][26].…”

“…Silica-based all-fibered systems, featuring a short piece of scintillating silica spliced to a radiation-hard transport silica fiber, provide an interesting alternative to the preceding options. Silica can be activated through doping by a variety of dopants [12][13][14][15][16][17][18][19][20] to produce very sensitive radioluminescent fiber detectors. The smaller size of silica fibers (down to 125 µm in total diameter) and of their sensitive volume (core diameter down to 50 µm) allow an even better spatial resolution while avoiding affecting dose delivery to the patient during in-vivo measurements.…”

“…The assessment of various silica-based RL dosimeters essentially consisted in checking the linear growth of this equilibrium level against dose rate, under x-or gamma-ray irradiations. Proportionality has been found over up to six decades of dose rate for germanium or rare earth (RE)-doped silica (the explored range) [14] and over 10 decades for nitrogen-doped silica (10 −1 -10 9 Gy s −1 ) [20]. A straightforward dosimetric principle with a dose-rate independent sensitivity is therefore clearly demonstrated up to very high dose rates, so the question of 'linearity' is no longer the central concern.…”