## 2 Theory and Methods
### MOSkin Characterisation  
To begin the experiment, first, a Faraday cup (FC) [28] was used to confirm the required linac settings to produce a wide range of stable charge delivery. Charge is delivered at PEER in 100 ps bunches, with an inter-bunch spacing of 2 ns. For the following work, all doses were delivered within a single pulse consisting of 100 bunches, resulting in a pulse length of 200 ns. The total charge and, therefore, dose were varied by controlling the charge within the bunches. Transverse beam profiles were measured with a scintillating screen, and these measurements were combined with previous knowledge to estimate the parameters required for a dose of 1 Gy. FC readings at PEER are used to calibrate a fast current transformer (FCT) for diagnostic purposes during a beamtime, and can be used afterwards during data analysis for a confirmation of the relative charge delivered during the experiment. To mount film, as well as three MOS_kins_ within the linac tunnel, a phantom was designed and manufactured from PMMA, with dimensions sufficiently large to ensure no overlap of beam spots will occur on the film. The location of the MOS_kin_ sensitive volumes were marked on the surface of the phantom to allow for precise alignment within the beam. As VHEE beams are generally small (relative to clinical beams) and Gaussian, with steep dose gradients, both the EBT-XD and MOS_kins_ were positioned at 20 mm depth to ensure some flattening of the beam in the form of decreased amplitude and increased \(\sigma\). Positioning at this depth is also more clinically relevant than the surface, or in-air as sometimes seen in other studies. The phantom was positioned on a set of linear stages with the scintillating screen fixed to the top right corner, shown in Figure 2.  
The scintillator was imaged with a fixed camera, and the resulting images were used to find the centre of the beam, shown in Figure 2. Once known, the coordinates were marked within the camera software and the marks on the phantom representing the sensitive volume of the MOS_kins_ could be aligned to these coordinates. The beam alignment was checked repeatedly throughout the experiment to confirm its transverse position had not changed. The linear stages at PEER have a position accuracy of 65 \(\upmu\)m and repeatability better than 5 \(\upmu\)m. After exposure of the MOS_kins_, the positions of the film exposures were easily found due to the geometry of the phantom, allowing a grid of nine positions to be accurately reproduced during each iteration of the experiment. The use of a grid also provided secondary confirmation of the beam's transverse positioning stability throughout the experiments, as any substantial deviations from the grid would be visible on the scanned films. The readout system used to apply the threshold voltage to the MOS_kin_ is limited to 28 V, allowing for approximately 50 Gy of the absorbed dose to be measured in clinical settings. This value is, of course, different for varying beam qualities. To ensure the MOS_kin_ was used within this range, an estimated dose of 1 Gy was delivered first, followed by increasing doses until the limit was reached. This was repeated for three MOS_kin_s. A further three MOS_kin_s received the same doses in reverse order to rule out the presence of any total absorbed dose effects that may appear as DPP effects. Due to the reader limit, higher doses had to be delivered singularly, to new MOS_kin_s, so for higher levels of pulse charge, three unirradiated MOS_kin_s were mounted in the phantom for each set of exposures. Each time, a new piece of EBT-XD film was also mounted. The individual doses were delivered to film three times. Figure 2: **Top left** and **top right**: The PMMA phantom holding unexposed EBT-XD film with a scintillator attached for beam alignment, as well as three MOS_kin_s. Also displayed is the post-irradiation grid of nine film exposures, which allows the assessment of transverse beam stability. Clear separation of the exposures is evident.