Post-acceleration of ~7Be at the Louvain-la-Neuve radioactive ion beam facility

摘要

The development of an intense and pure post-accelerated ~7Be beam at Louvain-la-Neuve will be discussed. Given its properties (metallic nature, long half-life (53 days)) and the special beam parameters required (multi-charge ions, high purity), a range of special techniques had to be investigated. At Louvain-la-Neuve, ~7Be is produced by irradiating a lithium target with 30 μA of 27 MeV protons and is extracted using offline chemical separation techniques. Because of the large amounts of activity required, the chemistry has to be adapted for use in hotcells. The ionization is performed with an Electron Cyclotron Resonance ion source with the ~7Be injected in the source by means of sputtering. Special techniques have to be used to prevent the beryllium atoms from being lost on the plasma chamber walls. A dedicated heated plasma chamber for the ion source was developed. The ionization efficiency was increased by studying the chemistry involved in the ion source. The atoms are ionized to the 1+ or 2+ charge state and injected in the CYCLONE cyclotron for acceleration. To further reduce the ~7Li contamination, the cyclotron is tuned as a mass separator, suppressing the Li intensity by about 10~5. The accelerated beam is then stripped to the 4+ state to eliminate the remaining ~7Li. This method allowed us to produce a ~7Be beam with intensities up to a few 10~7 pps and with a very high degree of purity. Experiments with energies ranging from 4.4 MeV (1+) to 56 MeV (2+) have been performed.