## 3 Results
### Irradiation Response and Linearity  
Generally, X-ray photon interactions lead to the creation of Compton electrons, photoelectrons, or electron-positron pair production, which in turn create electron-hole pairs and phonons. Under the influence of an applied electric field, the electron-hole pairs separate and drift to the respective electrodes, and on the way, can recombine with the trapped charges, leading to a shift in threshold voltage toward the positive direction, as shown in Figure 5a. This behaviour is accompanied by an increase in drain current at a fixed gate voltage of \(-15\) V because of the shift induced in the IV curve. Therefore, the shift in drain current and/or threshold voltage can be considered as radiation dose metrics. In this work, the drain current shift at V\({}_{\rm g}=-15\) V is used as a sensing metric, as shown in Figure 5. In a wireless readout configuration, devices were programmed and scanned then irradiated and scanned again to check the current shift in response to doses between 1 and 15 Gy (in 1 Gy increments). The data shown in Figure 5b indicate that the response is linear, following irradiation with a 100 kVp beam. The linearity of transistor response was also observed at variable dose steps (1 Gy to 6 Gy per step) up to an accumulated dose of 29 Gy, as shown in Figure 5c,d. These devices, like many solid state radiation sensors, exhibit a change in sensitivity at low dose levels which then stabilize; thus, the 1 and 2 Gy points have not been included in the fit for this reason. The device sensitivity can be expressed as the change in drain current per unit dose: \(S=\Delta I_{d}/D\). Figure 6 shows the I\({}_{\rm d}\) vs. dose relationship following a duplicate series of programming and irradiation steps. The data demonstrate the potential of these devices to be reused across multiple irradiations. The sensitivity of the device used in the 6 MV beam remained essentially unchanged. The device used for the 100 kVp irradiations showed a small change in sensitivity, though it preserved its linear response with dose. After irradiation with a kilovoltage 100 kVp photon beam and megavoltage 6 MV photon beam, programed charges were depleted. Some devices were then reprogrammed for additional experiments. For the wireless readout configuration in the megavoltage photon beam, the average sensitivity of the devices studied was approximately 60 \(\pm\) 5 nA/Gy. At 100 kVp, the sensitivity of the devices was approximately 80 \(\pm\) 10 nA/Gy. Figure 4: (**a**) Transfer curves of the pentacene-based OTFTs with PS electret before and after programming by applying the \(-80\) V gate bias for 3 s; (**b**) the stability of the drain current after programming with negative \(-80\) V gate bias for 3 s measured at V\({}_{\rm d}=-15\) V, V\({}_{\rm g}=-15\) V. ‘First’ refers to the I–V curve in the absence of programming and ‘prog’ refers to the I–V curve following the programming at \(-80\) V for 3 s.  
Figure 5: Transfer characteristics of pentacene OTFTs with PS electret after first programming and consequent accumulated irradiation with (**a**) 1 Gy steps and (**c**) variable dose steps. (**b**,**d**) The corresponding drain current value at V\({}_{\rm g}=-15\) V as a function of accumulated dose. Irradiations were performed with a 100 kVp photon beam. The 1 and 2 Gy points have been excluded in the fit, as the sensitivity was more variable at these low doses. ’Prog1’ refers to the I–V curve following the programming at \(-80\) V for 3 s.  
Figure 6: I\({}_{\rm d}\) vs. dose for a device across two cycles of programming and irradiation at both (**a**) 6 MV and (**b**) 100 kVp. The sensitivity remained constant beyond doses of 3 Gy. The pentacene OTTFs with polymer electret were also investigated for wired real-time readout of the 6 MV photon beam.