Design and tests for the new CERN-ISOLDE spallation source: an integrated tungsten converter surrounded by an annular UCx target operated at 2000 °C

The production of high intensity and isobarically pure neutron-rich fission fragments is of high importance for the physics research program of the ISOLDE facility at CERN. This is typically done in a two-step method where a tungsten converter, positioned parallel and below the UCx target, is irradiated with 1.4 GeV protons. This will produce spallation neutrons which irradiate a UCx target producing the isotopes of interest. Currently, the intarget production is limited by the geometrical overlap of the neutron fluence and the target material and suffers from low production yield. In this work, a prototype is proposed where the tungsten converter is positioned in the center of an annular UCx target. FLUKA simulations were conducted to optimize the geometry, maximizing the production of isobarically pure neutron-rich fission fragments which determined that a large diameter target is necessary (5 cm). Thermo-electric ANSYS® simulations were conducted in order to develop a large Ta target oven which can reach 2000 °C and tests were conducted to benchmark the simulations. A prototype design was validated, for ISOLDE operation, with offline tests which shows that the tungsten-graphite-tantalum assembly is fully stable up to 2200 °C.