## 3 Results  
### EBT-XD Film Calibration and Correction  
The EBT-XD film calibration curves, shown in Figure 3, show no difference whether exposed to low energy X-rays at IMBL, or 15 MeV electrons at ARPANSA. As increasing pulse charge was delivered throughout the experiments, the beam size was found to be very stable, with an average \(\sigma_{x}\) of 2.58 \(\pm\) 0.08 mm and \(\sigma_{y}\) of 2.38 \(\pm\) 0.03 mm. With a relatively unchanged spot size, the dose should scale proportional to the charge, as the distribution is unchanged. When analysing the response of EBT-XD films, this was not found to occur. Instead, a super-linear response to charge was found. These results are presented in Figure 4, and it can be seen that this effect becomes pronounced as pulse charge increases above 1000 pC, corresponding to doses of approximately 4 Gy. This effect was investigated further by delivering three levels of charge over three different timescales, achieved by varying the charge within a bunch, the number of bunches in a pulse, and the number of pulses within the total irradiation. This allowed a range of DPP and dose rates from ultra-high, to conventional. Figure 5 provides the response normalised to charge. A clear trend is established, showing a lower response for low pulse charge, and most importantly, the responses for all three levels of charge converge for the 20 pulse deliveries. To complement this finding, Geant4 simulations were used to predict the response to ensure the range of film responses were not significantly over- or underrepresenting the true absorbed dose. A normalised response of 0.004 Gy/pC was predicted by the simulation, about 7.7% higher than the point of convergence in Figure 5. This result validates the range of the EBT-XD normalised responses, confirming the dependence upon the DPP and dose rate. Figure 3: The EBT-XD film calibration curves performed at the Imaging and Medical Beamline of the ANSTO Australian Synchrotron, adjacent to PEER, as well as at the Australian Radiation Protection and Nuclear Safety Agency. The presence of energy dependence impacting upon calibrations, as reported in the literature for other types of dosimetry film, is not evident for EBT-XD used in this investigation. Uncertainty is within markers. Figure 4: A super-linear response of the EBT-XD relative to charge measurements was observed, prompting the investigation of potential DPP or dose-rate dependence within the UHDR field at PEER. The data plotted in red is a linear series scaled to the first data point ratio of Gy/pC. The maximum deviation observed is for the highest charge measurement, where the reported dose from EBT-XD film is 20% higher than expected. Figure 5: To investigate the presence of DPP or dose-rate effects, EBT-XD film was exposed to three levels of charge, each delivered over three sets of temporal structures; a single pulse as used during the rest of this paper, 2 pulses, to halve the DPP and halve the average dose rate, and 20 pulses, to reduce the DPP by a factor of 20 and create dose rates of less than 1 Gy/s. Due to the non-standard operation of the linac, the charge delivery was not as stable as during the normal, single-pulse operation that PEER is optimised for (though this can be improved for future work). The baseline of the film exhibited some uneven darkening, giving rise to larger than usual uncertainties, shown as one standard error of the mean. However, the dependence upon DPP is evident, especially for single and two pulse deliveries. The trend against average dose rate is much clearer, and the normalised response for all levels of charge converges for the twenty-pulse delivery, confirming the presence of a large dose-rate dependence within the UHDR used in this study. Although the EBT-XD film was confirmed to exhibit a DPP and dose-rate dependence, reference dosimetry at the same depth as the MOS_kin_ was still required to validate the detector response to increasing DPP.