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.

Keywords: X-rays; beam monitoring; optical fibers; optical materials; radiation effects; radioluminescence.

Figure 1

MCNP6 (Monte-Carlo N-Particle Transport Code version 6) [26] simulation of the energy spectrum at 1 cm from the multilayer electron X-ray converter.

Figure 2

ELSA experimental irradiation area and experimental setup developed to measure the RIE induced in the N-doped fiber under test. The inset describes the temporal evolution of the RIE signal for 8 successive ELSA X-ray pulses.

Figure 3

ASTERIX experimental irradiation area and experimental setup developed to measure the RIE induced in the N-doped fiber under test.

Figure 4

RIL versus dose rate calibration setup under steady-state X-ray irradiation.

Figure 5

RIL dose rate dependence of the nitrogen-doped optical fiber performed with LabHX 40 keV X-ray machine.

Figure 6

RIE peaks related to the interaction of the N-doped fiber with 8 ELSA X-ray pulses.

Figure 7

RIE evolution following the dose variations, as a function of the X-ray pulses. The first pulse set in blue was performed with a dose rate average of ~1.3 MGy(SiO₂)/s, while the second set in pink was performed at ~1.7 MGy(SiO₂)/s.

Figure 8

Illustration of the detected RIE for the N-doped sample before, during and after an X-ray shot at ASTERIX. In the inset are the reported normalized RIE corresponding to the dose rates associated with the 5 different shots (black triangles).

Figure 9

Comparison between the nitrogen-doped optical fiber RIE dose rate dependencies as measured with steady-state 40 keV X-ray source and µs (up to 19 MeV) and ns (~1 MeV) scale pulsed X-ray sources.