Physics with Reaccelerated ISOL Beams

Abstract

Since the pioneering work of Cockcroft and Walton, one of the central tools in experimental nuclear physics has been reactions induced by accelerated beams. Over the last 25 years or so, following the experiments at Berkeley and elsewhere which saw the uncovering of evidence for the neutron halo, reactions induced by high-energy beams (≳ 30 MeV/nucleon) of radioactive nuclei produced via in-flight fragmentation have becomean essential probe of nuclei far from stability. Radioactive ions have, however, long been available at very low energies, through the Isotope Separation On-Line (ISOL) method. Indeed, the technique maybe traced back as far as the work of Kofoed-Hansen and Nielsen in Copenhagen in the early 1950s.The first, and longest running dedicated ISOL facility is, of course, ISOLDE at CERN, which firststarted operation in October 1967. Given the combination of intense beams coupled to thick targets, the ISOL technique hasthe capability of producing very high yields of nuclei far from stability. Extractingand separating the nuclei of interest from the targets requires, as experience at ISOLDE and elsewhere has shown over the years, a broad range of techniques both in both chemistry and ion-source technology, as well as separation techniques. Nevertheless, the routine production of a broad range of elements and isotopeshas become possible over the years.Many nuclear reaction studies require beams with energies considerably lower than those produced by in-flight fragmentation, and often with superior optical qualities and much smaller energy spreads (ideally with the characteristics of stable beams). Such needs have lead to the development of(re)accelerated ISOL beams, whereby the on-line target-ion source systems and separators are coupled toa post-accelerator. The first such facility was developed atthe Cyclotron Research Centre at Louvain-la-Neuve in the late 1980s based on the existing Cyclone-110 accelerator. Whilst originally concentrating on the development of a 13N beam fornuclear astrophysics, the Louvain-la-Neuve facilitywent on to furnish a range of other beams over the years for nuclear structure and astrophysics.Louvain-la-Neuve was followed in the mid-1990s by the Holifield facility at Oak Ridge, which combined anexisting cyclotron with the 25MV tandem, to provide reaccelerated beams for nuclear structure and astrophysics, as well as some reaction and decay studies. The turn of century saw three other facilities come on line: ISAC at TRIUMF in Vancouver, Canada; SPIRAL at GANIL in Caen, France; and REX-ISOLDE at CERN. Whilst ISAC and REX employ linacs as the post-accelerators with energies up to a few MeV/nucleon, the SPIRAL project chose a cyclotron as the post-accelerator, providing, depending on the ion, energies up to some 25 MeV/nucleon. In the last few years the ISAC post-acceleration has been upgraded (ISAC-II) to provide for beams with energies up to some 10 MeV/nucleon.As many readers will be aware, a new generation of reaccelerated ISOL facilities is now being conceived and constructed, including the ARIEL project at TRIUMF, SPIRAL2 at GANIL, the HIE-ISOLDE upgrade at CERN, as well as the SPES project in Legnaro, Italy. Given these developments and the recent termination of nuclear physics experiments with radioactive beams at Louvain-la-Neuve, it seemed timely to look back on the first two decades of physics using reaccelerated ISOL beams. Hopefully the readers of the contributions to this section will appreciate the physics insights that have been gained over this time, the vitality of the field and the prospects that the new facilities hold. Finally, whilst this Focus Section concentrates on the physics with post-accelerated ISOL beams, the efforts over many years of those involved in the development of the beams used in the experiments described heredeserve to be recognised.