## 3 Results
### Thick Niobium Target Windows  
As previously mentioned, thick niobium windows were implemented on the IBA Nirta(r) Ga-68 liquid target system, therefore bringing performance challenges associated with larger amounts of heat generated at the target windows. Hence, with the aim to  
Figure 19: Maximum temperatures of the target window, liquid target, niobium insert (full lines) and temperature of the cooling helium at outlet (dashed line) as a function of the flow rate of the cooling helium. Figure 20: Maximum temperatures of the target window as a function of the flow rate of the cooling helium, for two distinct powers of 250 and 1000 W.  
Figure 21: Temperature of the liquid target for niobium target windows of 75, 125, 250, 325, and 375 \(\upmu\)m (from **top** to **bottom**). Figure 22: Temperature of the liquid target at the axis of the cavity, along the beam patch, for the different target windows considered. For all windows, the maximum temperature inside the horizontal plane of the liquid target is still always located at the end of the niobium conical insert (Bragg peak location). Figure 22 also shows that the greater the thickness, the closer the temperature of the liquid target in the vicinity of the target window is to the maximum temperature obtained. As illustrated in Figure 24, the temperature profiles of the different windows along the vertical axis and on the surface in contact with the liquid target are very different. Figure 24 shows that the asymmetric profile of temperature observed in the havar foil due to gravity no longer occurs for thicker niobium windows. This result can be explained by the much larger heat generated that overlaps the effect of gravity, thus exhibiting a behavior similar to that observed for lower mass flow rates of He (Figure 11). Figure 24 also shows that, contrary to the case of the havar window for which the temperature drops rapidly outside the area of the window intersected by the beam, the temperature decreases more gradually in the case of thick niobium windows. Such behavior is not only due to the larger temperatures achieved with thicker niobium windows, but also to the fact that larger heat transfer by conduction from the window to the window spacer then occurs, since niobium presents larger thermal conductivity than havar, as discussed below. Figure 25 presents the average and maximum temperatures of the target window and the liquid target for the distinct niobium windows considered. It is possible to verify that both the average and maximum temperatures of the liquid target decrease as the thickness of the window increases. Regarding the temperatures of the niobium windows, although both the average and maximum values increase with the window thickness, only the maximum temperature is strongly influenced This confirms the fact that the heat generated in the window leads to a local increase of the temperature of target window, in the beam patch, whereas the rest of the window remains almost unaffected. Such effect is also evidenced in Figure 21, since the surface of the liquid target in contact with the target window also shows such radial temperature gradient. Figure 25 also shows that the maximum temperature of the window increases with the thickness to a point that it even surpasses the temperature average of the liquid target, and almost reaches the maximum temperature of this latter, showing that the target foil becomes a more and more critical component for the performance of the target system for thicker target windows. Additionally, simulations with multiple mass flow rates of cooling helium were conducted for all niobium windows, considered with the purpose of studying the influence of the cooling helium system as the thickness of the niobium window increased. As expected, decreasing the mass flow rate of the cooling helium leads to a significant increase in the maximum temperature for all the windows.