## 1 Introduction
Metal oxide semiconductor field-effect transistors (MOSFETs) with thin gate oxides have long been accepted as suitable devices to produce dose-rate-independent radiation detectors, confirmed by early investigations over a range of dose rates spanning 11 orders of magnitude [38]. The investigation of MOSFET detectors with thicker gate oxides is still to occur within UHDR fields, however, and is a subject of this paper. The MOS_kin_ detector, developed by the Centre for Medical Radiation Physics (CMRP), is a MOSFET detector constructed with unique, patented technology to allow it to accurately measure the skin dose during radiotherapy [39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49], defined by the International Commission on Radiological Protection as the radio-sensitive basal cell layer situated at an average depth of 70 \(\upmu\)m [50]. Not only is the skin dose itself an important radiotherapy metric but it can also be used, if accurately measured, to assess the efficacy of the delivered treatment plans. Thisis an increasingly important requirement within clinical facilities; many European countries are now mandated to perform in vivo dosimetry according to the Medical Exposure Directive [51]. By adjusting the location of the detector, the MOS_kin_ can also measure dose at any depth within a phantom or in vivo [52; 53; 54; 55; 56]. Including packaging, the entire device is approximately 300 \(\upmu\)m in thickness, available with a gate oxide thickness ranging from 0.55 \(\upmu\)m to 1.0 \(\upmu\)m. The thickness of the sensitive volume is used to control the sensitivity of the device. Due to its small size, the MOS_kin_ can be attached to a patient during treatment, without impacting upon the delivered field [40; 47; 57]. Despite being established as dose rate independent within clinical environments [41; 42] (though as with all solid-state devices, sensitivity varies with beam quality [39]), the MOS_kin_ was later assessed for dose rate independence within an UHDR, VHEE field at the PEER beamline. Pulse dose rates spanning three orders of magnitude were delivered, and the MOS_kin_ response was found to be independent of the dose rate [58]. However, during that period of the PEER beamline development, only low pulse charges were available to ensure the stability of the linac. Therefore, although estimated pulse dose rates were as high as 10\({}^{7}\) Gy/s, the maximum dose delivered in a single pulse was approximately 40 cGy, lower than what is required for eventual UHDR VHEE treatments, and not of sufficient magnitude to assess DPP independence. While the total charge delivered within a pulse is accurately controlled at PEER, to assess the DPP independence of the MOS_kin_, a suitable form of energy- and dose-rate-independent reference dosimetry is required. Gafchromic(tm) film, in particular, EBT-XD [59], has been used for dosimetry in electron FLASH investigations at clinical energies [60]. The manufacturer states a <5% variance in response over an energy range of 100 keV to 18 MeV [61]. Although VHEE lies well outside this range, it is often used during VHEE dosimetric studies [62; 63; 64; 65]. For this investigation, EBT-XD was chosen, as it is suitable for doses up to 40 Gy, covering the range of expected doses for this study. While other dosimeters have been shown to produce accurate results during VHEE studies [30; 35; 66; 67; 68], access to these devices is not readily available outside of their respective research groups. Further, the use of EBT-XD provides a means of consistency with other work in this field. However, during the course of this investigation, EBT-XD exhibited an apparent DPP dependence relative to the charge delivered by the linac. This was in contrast to the previous literature concerning its use within electron irradiations, though a slight dependence has been shown during investigation with UHDR protons [69]. A complimentary experiment was conducted to confirm this result and allow for a correction curve to be applied to all film results.