## 4 THGEM Characteristics
### Dielectric Charging Behaviour  
For each image, as detailed in Figure 9, the pixel intensities inside the ROI were summed. These summations were then binned into 1 s intervals shown in Figure 10a and also 20 s intervals shown in Figure 10b. The shorter 1 s intervals better show gain fluctuations throughout the THGEM charging process, whereas the longer 20 s intervals provide improved clarity for charging behaviour and shape. The most apparent difference in charging behaviour between the G-THGEMs and the traditional FR4 THGEM is that the S2 light intensity of G-THGEMs increases over time, whereas the S2 light intensity of the conventional THGEM decreases over time. The distinct charging behaviours may be explained by differences in hole geometry. As a consequence of the abrasive machining process, G-THGEMs have bi-conical holes. The effect of bi-conical holes on dielectric charging behaviour is well understood for GEMs [46; 47], and the theory can be extended to G-THGEMs. The insulating glass substrate provides a surface on which electrons may accumulate over time. Consequently, this accumulated charge \begin{table}
\begin{tabular}{c c c} \hline
**THGEM Type** & **Charging Study Bias (kV)** & **Breakdown Voltage (kV)** \\ \hline FR4 circ & 1.60 & 1.80 \\ FS circ & 1.60 & 1.90 \\ BF circ & 1.60 & 1.85 \\ BF hex & 1.70 & 1.80 \\ \hline \end{tabular}
\end{table} 
Table 2: A Table of the THGEM bias during the charging test and the measured breakdown voltage, in 1100 mbar N6.0, flowing at a rate of 5 L/min. The naming convention in the THGEM type is consistent with Table 1.  
will distort the electric field within the G-THGEM hole. In the case of bi-conical holes, this accumulated charge causes an increase in electric field within the exit side of the hole (closer to the top electrode). This results in an increase in gain and therefore increased light production. In contrast, THGEM holes with straight walls exhibit dielectric charging effects which result in the decrease of THGEM gain (and S2 light intensity) over time. This is primarily explained by a differential charge distribution along the walls of the holes [48]. All datasets shown in Figure 10b are fitted with Equation (1) [49; 50], where G is the total light intensity per 20 s. The parameters of Equation (1) can be found in Table 3. The parameter \(p_{0}\) is the initial gain at time \(T_{0}\). The direction and amplitude of charging are governed by \(p_{1}\), where a negative (positive) \(p_{1}\) value represents charging up (down)  
Figure 10: The charging behaviour, manifesting as variation in S2 light intensity, of the various types of bi-conical hole G-THGEMs and cylindrical hole FR4 THGEM. (**a**,**b**) represent time binnings of 1 s and 20 s, respectively. All functions in (**b**) have been fitted using Equation (1), the parameters of which can be found in Table 3. The data colour is consistent with the legend on Figure 11.  
Figure 9: An EMCCD image of approximately 30 alpha particle GAr interactions, induced by a 30 kBq \({}^{241}\)Am alpha source, collimated to 500 Bq. The EMCCD was configured in 4 \(\times\) 4 binning, with a 50 ms exposure, and a gain of 1000. This was a direct image taken of the WLS S2 light produced within a fused silica substrate G-THGEM holes, at a field of 1600 V/mm. This image has been background subtracted. The red overlay is an ROI cut for analysis. or, more specifically, greater (less) S2 light production over time. The rate of charging is dictated by parameter \(p_{2}\). \[G=p_{0}+p_{1}e^{-t/p_{2}} \tag{1}\]  
Despite similar bi-conical hole geometries, clear differences in dielectric charging behaviours are discernible between all G-THGEMs. The Borofloat 33 substrate G-THGEMs, with both circular and hexagonal hole geometries, had similar S2 light production at \(T_{0}\). Additionally, both hole layouts resulted in maximal charging time after around 10 mins.