## 4 Discussion  
The film sensitivity was unchanged between IMBL and ARPANSA, confirming its energy independence and allowing for the future use of on-site facilities for film calibration requirements at PEER. However, the results presented in this paper demonstrate that the DPP and dose-rate dependence of EBT-XD at UHDR must be checked and accounted for if they are to be used for reference dosimetry. With a stable beam and accurate charge diagnostics, this dependence can be corrected for, allowing for reference dose measurements at depth within a phantom, something that a charge measurement cannot truly provide in isolation. The normalised response of the film to a 20 pulse delivery corresponds to the clinical average dose rates that it was designed for, and the range of responses agree well with the Geant4 simulations, confirming that the corrected film provides a true representation of the absorbed dose. While other forms of reference dosimetry exist, such as calorimetry, they also require corrections and may not be immediately suitable for a small, diverging Gaussian beam. Hence, in the absence of suitable reference dosimetry, EBT-XD provides a reference measurement that may not be easily attainable through other available means. The use of EBT-XD with correction factors will, much like dose calibration curves, require evaluation of their ongoing suitability, due to time-based darkening, as well as intra-batch variation. It is not expected that the correction curve will hold over time and across batches. Though this effect was not known prior to the experiment, further reviews of the available literature uncovered a similar observation with the use of UHDR protons and EBT-XD at 7500 Gy/s [69], and recently, a thorough investigation within UHDR electron fields [70]. This work extended the range of DPP and dose rates within which EBT-XD was evaluated. Together, these works confirm that the observed dependence may not be confined to a single batch. It should be restated for clarity, however, that the use of EBT-XD within these novel fields is orders of magnitude outside of the manufacturer's intended use. In hindsight, the existence of a dependence should be rather unsurprising. Comparing the MOS_kin_ response to the total absorbed dose revealed a linear response as expected; the Geminate model allows for this expectation. The MOS_kin_ threshold voltage is measured before and after exposure to radiation, and the change in threshold voltage is essentially a measure of radiation damage. A simple explanation, therefore, is that in the absence of e-h pair overlap, the level of radiation damage is not proportional to the duration of radiation exposure, only to the total dose. The linearity of the device over its lifetime was also demonstrated, with consecutive irradiations and build up of dose, as well as single-pulse irradiations adhering to the same linear trend line with a slope of 3.386 \(\pm\) 0.022 mV/cGy, representing the sensitivity of the device to the 100 MeV electron field. When the sensitivity was determined in a clinical 20 MeV field, an increase of 11.1% was found relative to the 100 MeV measurements. As the pair spacing along the track is larger for 20 MeV electrons, recombination is expected to be lower, and therefore an increase in sensitivity is expected. The ratio of stopping powers (and e-h pair spacing along a track) predicted an approximately 8% increase in sensitivity would occur. To model the exact value of such an increase requires significant further investigation, including the effect of perturbations of the MOS_kin_ electric field due to the moving charges of different energies. Such work is beyond the scope of this paper. However, a difference of only 3% between the observed and predicted change in sensitivity is considered very minor, and the results are certainly within the expectations, considering a difference of 80 MeV between the two electron energies investigated, and the non-uniform fluence distribution of the PEER electron beam.