## 4 THGEM Characteristics
### S2 Light Production THGEM Bias Scan  
A THGEM bias scan study was also performed, detailing the S2 light intensity as a function of the THGEM field. Similar to the method described for the dielectric charging behaviour, an ROI cut was taken around the alpha particles. This was done for all of the 2000 (background subtracted) EMCCD images collected for each THGEM type at a given bias. For each event, within this ROI, the pixel intensities were summed and a distribution produced of the total light intensity, in ADU, for the respective G-THGEMs/THGEM fields. The results of the study are shown in Figure 11.  
Nominal THGEM behaviour can be seen for both the G-THGEMs and the FR4 THGEM. At a low field (below approximately 1000V), the THGEMs are in the linear gain regime, and the S2 light production is purely via electroluminescence. As the field increases, the THGEM enters the electron multiplication regime, and the S2 light intensity is dominated by exponential effects. The overall THGEM S2 light production behaviour can be described by Equation (2) (a convolution of both linear and exponential regimes). The fit parameters for Equation (2), describing the data sets in Figure 11, can be seen in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline
**THGEM Type** & \(\mathbf{p_{0}}\) & \(\mathbf{p_{1}}\) & \(\mathbf{p_{2}}\) \\ \hline FR4 circ & \(1.30\times 10^{9}\pm 1.26\times 10^{6}\) & \(6.60\times 10^{8}\pm 1.42\times 10^{7}\) & 0.752 \(\pm\) 0.0210 \\ FS circ & \(1.02\times 10^{10}\pm 2.36\times 10^{7}\) & \(-7.48\times 10^{9}\pm 4.12\times 10^{7}\) & 0.04808 \(\pm\) 0.00560 \\ BF circ & \(2.07\times 10^{9}\pm 6.63\times 10^{6}\) & \(-1.30\times 10^{9}\pm 1.23\times 10^{7}\) & 0.149 \(\pm\) 0.00346 \\ BF hex & \(1.49\times 10^{9}\pm 3.05\times 10^{6}\) & \(-7.42\times 10^{8}\pm 6.81\times 10^{6}\) & 0.162 \(\pm\) 0.00312 \\ \hline \end{tabular}
\end{table} 
Table 3: A Table of the fit parameters for Equation (1), describing the fits in Figure 10b. \[I=p_{0}x(1+p_{1}e^{p_{2}x})+p_{3} \tag{2}\]  
By far, the greatest light intensity comes from the fused silica G-THGEM. Both Borofloat 33 and fused silica G-THGEMs, which have hole sizes comparable to the FR4, produce increased S2 light intensity relative to the FR4 THGEM. This can be attributed to the distinct charging characteristics of the G-THGEMs. The Borofloat 33 G-THGEM with a hexagonal hole pattern produces the least S2 light intensity. This can be attributed to a larger hole, meaning that for the same bias, the electric field within the hole is lower, therefore resulting in relatively less gain and thus reduced S2 light intensity. Both fused silica and Borofloat 33 G-THGEMs were found to be more resilient to damage than FR4, where irreparable carbonisation can occur after repeated discharges. As noted previously, carbonisation has the potential to severely degrade the performance of the THGEM. Nominally, the highly conducting copper electrode of classical THGEMs can allow a large current flow during discharges. The resistive nature of the ITO electrode coating of the G-THGEMs may possibly be quenching sparks by limiting current flow [51]. With regards to G-THGEM applications within TPCs, the benefits of increased S2 light intensity are obvious. Firstly, for the same applied bias, increased light intensity could possibly result in a lowered energy threshold of a TPC. Functioning at a lower bias, while giving comparable light intensity, reduces the possibility of discharges, both directly on the THGEM and on connectors, feedthroughs or cabling. Detector stability is an important consideration for long-term, large-scale experiments.