Speaker
Description
The SPIRAL1 facility (Système de Production d'Ions Radioactifs Accélérés en Ligne) at GANIL (Grand Accélérateur National d'Ions Lourds) produces radioactive ion beams via the Isotope Separation On-Line (ISOL) method. Primary heavy-ion beams from ¹²C to ²³⁸U at up to 95 MeV/u currently impinge on graphite targets to induce projectile fragmentation. To extend and enhance isotope production, a new target based on fragmentation of a heavier target material is under development at SPIRAL1. Using a high-power ¹²C beam (up to 3 kW) on higher-Z materials, the long-term objective is to deliver a wider range of exotic beams for nuclear physics and interdisciplinary research, including future experiments at the SPIRAL2–DESIR low-energy facility.
The design of these new ISOL targets must reconcile enhanced isotope production, a high operating temperature to favour isotope release, and management of the thermal load. Target material selection was guided by EPAX-estimated in-target production and material properties (melting point, vapour pressure, manufacturability, diffusion). Niobium and the composite oxides ZrO₂ and Y₂O₃ emerged as the most promising candidates within the Z ≤ 41 regulatory limit at GANIL.
Unlike proton-driven ISOL facilities, where energy is deposited over several hundred millimeters, the ¹²C beam stops within ~4.7 mm in niobium. The volumetric energy density is several orders of magnitude higher, shifting design priorities from global heat dissipation towards local heat spreading, while preserving short diffusion paths.
To address this, a bent-foil geometry has been developed: foils mounted at an inclined angle provide a larger apparent beam thickness, an enlarged radiative surface and self-supporting stacking with minimal inter-foil contact, mitigating large-area sticking that would impede release. A first preliminary design comprises: each 50 µm-thick foil is bent with a 45° included angle, repeated periodically to form a corrugated profile, providing an apparent beam thickness of 130 µm; 34 such foils are then stacked into the final assembly. Energy deposition profiles calculated with TRIM (Transport of Ions in Matter) coupled with steady-state ANSYS simulations code show that this configuration, within an enclosure held at 2000 °C, keeps the maximum foil temperature within the operational limits of niobium and below that of a flat-foil equivalent.
The geometrical parameters of the stack (foil angle, height, inter-foil spacing) directly influence both the temperature distribution and isotope release. A parametric optimisation is therefore being implemented to identify configurations that combine improved temperature homogeneity with favourable release rates.
In support of this design effort, niobium bent-foil samples have been tested offline inside a graphite resistance furnace, maintained at nominal operating temperature without observable damage, providing a first offline qualification of the mechanical and thermal behaviour. The next steps include the fabrication of a complete target prototype, full-scale heating tests on a dedicated offline platform, and ultimately online irradiation tests under the high-power ¹²C beam.