Development of a Liquid-PbBi Target for ISOL@MYRRHA

Radioactive Ion Beams (RIBs) are used as a tool by physicists to study nuclei far from stable nuclei in the nuclide chart (see Figure 0.1). The RIBs used in these studies are produced via different methods, among which the Isotope Separation Online (ISOL). At current ISOL facilities, part of the requested RIBs cannot be delivered because they necessitate R&D. In order to meet the requirements of certain experiments, it is necessary to increase RIB-intensities by several orders of magnitude. The project EURISOL aims at increasing the RIB intensity by up to four orders of magnitude. However, because of the substantial technological developments required for such a facility, intermediate-generation facilities like ISOL@MYRRHA have been planned. A common feature of these facilities is their high-power driver beam which results in an increased heat deposition in targets. This induces a need for development of a new generation of targets capable of operating under this high-power condition. A molten metal target concept, capable of addressing this need, was proposed within the EURISOL Design Study (see Figure 0.2). Because such a target is of interest for different ISOL facilities, a project entitled LIEBE (LIquid lEad Bismuth eutectic loop target for EURISOL) was initiated for the detail design and construction of a prototype Pb-Bi loop target setup. This work comprises the conceptual design and simulations of the target. Because the target material flows in a loop equipped with a heat exchanger, this target concept is capable of handling high power primary-beams. In addition, liquid targets typically have a higher number of target atoms per unit area of beam spot, as compared to solid targets. This usually results in higher in-target production rates. Also, as in this concept the irradiated liquid is fractionated into small droplets, good release properties can be envisaged. Effects like pressure drop, cavitation, liquid-metal recirculation, instabilities and non-uniform flows are studied for the design of this target. These phenomena are most crucial inside the irradiation volume, as they can significantly affect the performance of the target. A major design requirement is a complete evacuation of the irradiated lead-bismuth-eutectic (LBE) within 100 ms after the impact of a proton pulse. The dynamics of LBE in the irradiation volume was analyzed, with CFD simulations, in order to ensure a proper design. Starting from a simple cylindrical geometry, several improved configurations of the irradiation volume have been studied. In each of the proposed satisfactory geometries, the inlet-jet effect was solved with a combination of two approaches: (1) increasing the size of the inlet sections in order to reduce inlet velocities; (2) positioning one or two feeder grids to distribute the inlet jet over the length of the irradiation volume. A parallelepiped-shape feeder proved to be the most robust of the satisfactory concepts, with regards to risks of clogging. Upon irradiation by a proton pulse, the liquid target material is fractionated into droplets inside the release volume. This release volume must be optimized for the reference nuclide of interest, 177Hg, of which the half-life is only 0.118 s. A sound engineering of the release volume is therefore crucial in order to minimize decay losses during the release of these nuclides. In this objective, a proper modeling of the release of nuclides out of the target is a requisite. A computational approach to predict the release of nuclides out of the target and assess its efficiency was thus developed. The proper test case for validation of the proposed release model is a loop-type target. However, in the absence of experimental data for the liquid loop-target, measured data on several targets operated online were used. These are the ISOLDE-SC Pb target and two Ta foil targets with different internal geometries. In all these cases, a good match between computed and experimental release curves and efficiencies has been observed for a variety of isotopes of different elements. The method proposed, to predict the release of nuclides, was then applied to optimize the design of a molten LBE loop target. Optima of different parameters have been determined and it was found that the optimum size of the target is species and half-life dependent. In addition, the shorter the half-life of the isotope of interest, the more compact is the optimum target design. When passing through the target material, the driver beam can deposit significant amounts of energy in the target through Coulomb interactions. The energy loss profile was computed with FLUKA, a Monte Carlo code for particle transport in matter. This profile was then used to calculate initial temperature and pressure distributions in the target using Fluent. These calculations provide insight into effects generated by proton-induced shocks in this liquid metal target under a highly pulsed beam. The simulations show that strong pressure waves develop in the target but no severe constructive interference was found when using the 16.2-�s bunch spacing of the beam. The prototype will be constructed and tested at CERN-ISOLDE. Offline tests are foreseen this fall and the prototype is planned for online tests in November 2016.