Permanent Magnets under Irradiation and Radioactive Alkali Ion Beam Development for Spiral 1

Up to now, eighteen Target Ion Source Systems (TISSs) have been built and used for the production of radioactive ion beams on SPIRAL 1 facility, based on the IsotopeSeparator-On-Line (ISOL) method. The TISSs are composed of thick carbon targets and of fully permanentmagnet Electron Cyclotron Resonance Ion Sources (ECRISs) of the Nanogan III type. After irradiation and a decay period of two years, the irradiated TISSs are dismounted and if their magnetic fields are still suitable, the ECRIS are used with a new target. Thereby thirty-two runs have been performed using new or renewed TISSs. , After irradiation, the measured magnetic field sometimes reveals magnet damage. Our experience is reported here. In the second section, we present the progress on the NanoNaKE setup, which aims to extend the radioactive ion beams in SPIRAL 1 to the alkali elements, by connecting a surface-ionization source to the Nanogan III ECRIS via a compact 1+ ion beam line. The main issues and difficulties are discussed and the preliminary solutions are described. INTRODUCTION The use of high-energy fragmentation as well as the ISOL methods for exploring the structure of nuclei far from the stability has become one of the major activities at GANIL (Grand Accelerateur National d’Ions Lourds). The ISOL method, used in SPIRAL, provides radioactive ion beams, with subsequent acceleration by a K=265 cyclotron CIME, (Cyclotron d’ Ions à Moyenne Energie). Three cyclotrons are used to produce the primary beam which bombards the target of the TISS (Figure 1) placed in a heavily shielded cave. Exotic nuclei produced by nuclear reactions are released from the high temperature target (2000°C), effuse through a cold transfer tube up to a multi-charged ECR ion source. After extraction from the ECRIS at low energy (≤ 34 q.keV), the beam of interest is selected by a magnetic spectrometer (m /Δm = 250) and injected into CIME. The exotic beams can be accelerated in an energy range of 1.7 to 25 MeV/u and, after extraction, the proper magnetic rigidity is selected by GANIL’s modified alpha spectrometer and directed to one of the existing experimental areas. Two kinds of carbon target are used for the radioactive ion beam production, one dedicated to the production of He isotopes, the other to heavier gaseous element up to Krypton. The restriction to gaseous elements is provided by a cold transfer tube, situated between the target cavity and the source chamber. Available intensities are given on the GANIL web site [1] PERMANENT MAGNETS UNDER IRRADIATION Figure 1: angular distribution of neutrons downstream from the target The carbon target is placed close to the permanent magnets of the ion source (Nanogan III), which can then be damaged by neutron irradiation, leading to losses in axial and radial confinement of the source. For this reason, after a radioactive decay period of some years, the irradiated TISSs are placed in a glove box and dismounted. The target part is discarded as nuclear waste, and the magnetic field of the ion source is measured: if it is still acceptable, the ion source is reassembled with a new target. During recent years, magnetic measurements have revealed more and more damage. Among the eighteen TISSs constructed since 2001, two types of degradation have been observed: (a) Degradation of the injection magnet (close to the target): Figure 2 shows an example of the decrease of the axial magnetic field on the injection side, with no modification at the opposite (or extraction) side. Measurements all around the injection magnet (Figure 3) show an average loss of magnetic field of about 20%, with a maximum of 40% on the target side. MOPO-15 Proceedings of ECRIS08, Chicago, IL USA Radioactive Ion Beams 102 in 2p 300 33 18 09 , v er si on 1 23 J un 2 00 9 Author manuscript, published in "18th International Workshop on ECR Ion Sources ECRIS2008, Chicago : United States (2008)"