## 1 Introduction  
Recent years have witnessed growing interest in radiopharmaceuticals labeled with several radiometals of clinical interest, such as \({}^{68}\)Ga and \({}^{64}\)Cu [1, 2]. With the aim of improving routine supply of these relevant radiometals with enhanced ease, efforts have been made from several authors of the field in last few years to develop processes for the production of radiometals from the irradiation of liquid targets in biomedical cyclotrons [3]. Production of radiometals in liquid targets has proven to be a cost-effective, reliable technique with several practical advantages, presenting itself as a complementary alternative to the more conventional irradiation of solid targets. For instance, the technique has been successfully implemented for GMP-production of \({}^{68}\)Ga-based radiopharmaceuticals [4]. However, as the optimal impinging energy to minimize or even prevent the co-production of undesired radionuclide impurities depends on each production route, the fixed energy of biomedical cyclotrons is usually inadequately too high, since it was originally intended for the routine production of \({}^{18}\)F. Therefore, in order to decrease the beam energy down to the optimal energy characteristic of each production route, there is sometimes a necessity to degrade the initial energy of the beam (e.g., the production of \({}^{68}\)Ga or \({}^{89}\)Zr [5, 6]), by implementing thicker target windows with respect to the conventional case of the \({}^{18}\)F liquid target [5, 7]. For instance, the IBA Nirta(r) Ga-68 liquid target (IBA, Louvain-La-Neuve, Belgium), developed for the production of \({}^{68}\)Ga from the proton irradiation of a \({}^{68}\)Zn-based liquid target solution, uses thick niobium target windows instead of the conventional thin havar window used for the production of \({}^{18}\)F. Niobium was used since its chemical inertness avoids the introduction of additional metallic impurities in the target solution [5]. The irradiation of such thicker target windows also comes, unfortunately, at the expense of significantly higher heat generated in the window, due to an increased loss of beam energy, with the potential to lead to fast material degradation or even rupture [3]. Besides, niobium is also mechanically less resistant than havar [8]. As a result, when increasing the target current, the overall target system performance can potentially be limited by the beam current withstood by these more sensitive thick target windows, rather than by high pressure generated inside the liquid target cavity. It is thus crucial to study and understand the thermal performance of the liquid target system in order to improve its capacity to remove heat, which will allow for the increase in the beam current reaching the target, and therefore will improve the overall target performance. However, available data on the subject is scarce, due to the difficulty to experimentally measure the temperatures of the target components during irradiation. One way to overcome this problem is to develop simulations representing the thermodynamic performance of the target system by coupling the heat generated by the beam energy deposition to the temperature volumetric distributions. With the purpose of studying solid/liquid targets in cyclotrons, several works developed thermal models relying on computational fluid dynamics (CFD) software, coupled with the energy deposited by incident particle calculated through Monte Carlo simulations [9; 10; 11; 12; 13; 14; 15]. The main thermal and physical properties of the target components are determined, and, in some cases, the influence of the coolant mass flow rate is studied. In the present work, a simulation platform representing the thermal performance of the IBA Nirta(r) Ga-68 liquid target system was developed to study the influence of the use of thick Niobium target windows when compared to the standard use of thin havar.