Background and purpose: The investigation of the FLASH effect requires experimental accelerators capable of delivering ultra-high dose rate (UHDR) beams. Rapid widespread use of this technology could be achieved by modifying clinical electron linacs, originally designed to deliver megavoltage photon radiation up to a few Gy per minute to the isocenter, to deliver electron beams at 40 Gy/s and beyond. Only limited experience has been reported on the radiation safety of UHDR electron beams. This work aims to evaluate the performance and applicability of radiation detectors to quantitatively assess the radiation exposure in this context.
Methods: A Varian TrueBeam linac has been modified to deliver 16-MeV electron UHDR with dose rates up to 3⋅10⁵ Gy/s (instantaneous) and 256 Gy/s (average) at the isocenter and used to investigate the detectors performances. A short-term survey was performed at the first UHDR beam-on with passive and active detectors. Then, a long-term survey was conducted with passive detectors during the first three months of operation of the UHDR linac. Moreover, linearity of detector response, activation of the linac components and secondary radiation inside the bunker were evaluated.
Results: Selected active survey metres were shown to have a linear response for the detection of the ambient radiation outside the bunker when performing pulsed UHDR irradiations. The most critical locations outside the bunker were identified at the bunker door and at the control room. The results showed that the operation of the linac with a workload limit of 1000 Gy/week at the isocenter would allow respecting a limit of 0.02 mSv/week to the personnel. The activation of the linac head with 16-MeV electron beam was more than ten times greater with conventional beams compared to UHDR. The secondary radiation inside the bunker was also reduced by −27% when employing UHDR beams.
Conclusions: This work provides a comprehensive evaluation of the suitability of active and passive detectors to perform a radiation safety assessment for a 16-MeV electron UHDR linac. The conditions under which commonly available survey metres for photons (FLUKE 451P) and neutrons (Ludlum Model 3007) can safely be employed in controlled areas outside the bunker were investigated. Moreover, we showed that if a radiation vault is safe for 16-MeV electron beams at conventional dose rates, this applies also to UHDR when fixing the linac weekly workload to a given amount of dose at the isocenter.