## 3 Cyclotron-Produced \({}^{68}\)Ga Using a Liquid Target  
Further developments in the production of \({}^{68}\)Ga using medical cyclotrons have been attempted, with the most recent success with liquid targets [35; 36; 50; 51]. These results will encourage the routine production of \({}^{68}\)Ga, particularly in facilities unable to produce via solid targets. Currently, three notable works [35; 36; 37] use the PETtrace cyclotron described in Table 5. In contrast to the solid target, the \({}^{68}\)Zn target was prepared in nitric acid ([\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\)) in the liquid form. Previous experience with irradiation of zinc salt, especially ZnCl\({}_{2}\), reported gas formation that subsequently increased the pressure inside the target [52]. This reaction is caused by the radiolysis of water after ionizing radiation and forming radicals [53]. The presence of chloride ions further enhances gas formation, whereas nitrates induce the opposite. Thus, nitric salts are used to reduce gas formation, whereas the addition of nitric acid prevents the formation of a solid precipitate [35]. Tables 6-8 represents an overview of studies on cyclotron-produced \({}^{68}\)Ga with a liquid target. Riga and co-workers [35] produced 4.3 GBq \(\pm\) 0.3 \({}^{68}\)Ga activity using 1.7 M [\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\) in 0.2 M nitric acid after 32 min of irradiation at 46 \(\upmu\)A. More than 75% of the initial activity was retained during post-processing. [PERSON] and group [36] reported a higher EOB of 9.85 \(\pm\) 1.6 GBq using 1.42 M [\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\) in 1.2 M nitric acid as starting material when irradiated with 40 \(\upmu\)A for 60 min as described in Table 6. The group further experimented with the effect of [\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\) concentration and irradiation time on yield activity [51; 53]. They found that reducing the [\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\) concentration to 0.6 M would decrease the yield to 3.94 GBq \(\pm\) 0.20. Moreover, shortening the irradiation time to 30 min and lowering the beam current to 30 \(\upmu\)A reduces the activity, as 0.6 M [\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\) only produces 1.64 GBq \(\pm\) 0.07 of \({}^{68}\)Ga, whereas 1.2 M [\({}^{68}\)Zn]Zn(NO\({}_{3}\))\({}_{2}\) produces 3.37 GBq \(\pm\) 0.17 of \({}^{68}\)Ga at EOB. In addition, the lower yield owing to the longer irradiation time could be avoided by increasing the nitric acid concentration. This effect was mainly due to the high nitric acid consumption at a long irradiation time (<30 min) and high beam current. Nonetheless, increasing the nitric acid concentration can potentially damage the materials connected to the target. Thus, it is important to properly evaluate the equipment and materials used to potentiate high concentrations of nitric acid in the preparation of the target material [51; 53]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Internal Volume** & **Degrader** & **Support** & **Ref.** \\ \hline
2.5 mL & 250 \(\upmu\) foil of niobium & 25 \(\upmu\)m Havar\({}^{\text{@}}\) foils for & \\  & liquid niobium-body & helium cooling chamber & \\  & target & and the helium cooling & \\ \hline
2.0 mL & Dual foils of 200 \(\upmu\)m & Havar (40 \(\upmu\)m) separated & \\  & aluminum & by helium cooling. & \\ \hline  & & 25 \(\upmu\)m Havar foil for & \\  & & support and 25 \(\upmu\)m & \\
2.2 mL & 200 \(\upmu\) aluminum foil & niobium foil for chemical & \\  & & inertness with the target & \\  & & media & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Target preparation or modification.