Special instrumentation  Facilities and Instrumentation: How  European Stable-Beam Facilities

European Radioactive Beam Facilities

Making radioactive beams

    The two basic ways of making radioactive beams (see figure ), offer unique research opportunities in nuclear physics and nuclear astrophysics. It is important to note that the two techniques address two different energy regimes with a very small overlap around 25 MeV/u. They thus are complementary. The ISOL method, which was the first to be developed, uses the radioactive ions produced by the beams of a primary accelerator or by the neutrons from a nuclear reactor. The target/catcher arrangement stops the recoils and the activity is transported from there into an ion-source (diffusion, jet transport...). Chemical selectivity in the transfer process to the ion-source can be obtained by a suitable choice of the target material, of its operating temperature and of the ``connection'' to the source. A variety of ion source techniques is available today which can offer additional selectivity. Furthermore, much promising R&D is under way. After the extraction of the desired charge-state from the ion-source, and mass-separation, the radioactive species can be used for experiments at low energy (a few tens to a few hundreds of keV) or may be re-accelerated by a second accelerator. Louvain-la-Neuve is the first operational facility of this kind in the world. Here the radioactive beams are accelerated to energies of 0.65-5 MeV/u. Other projects, discussed in more detail below, will soon offer radioactive ion beams of a large variety of isotopes and/or with still higher energies.

 

Figure: The two complementary ways of producing exotic nuclear beams.

 

Radioactive beams of energies above 20 MeV/u have so far been made at heavy-ion accelerators using in-flight separation of the recoils; a technique which relies on the forward focusing present in peripheral nuclear reactions. The concept of ``fragment-separators'' was pioneered with relativistic heavy ion-beams at Berkeley. The first dedicated spectrometer, ``LISE'', was built for GANIL, followed by the construction of other large instruments, the ``FRS'' at GSI, ``A1200'' at MSU, and ``RIPS'' at RIKEN and, most recently the ``SISSI'' device at GANIL. At present, these fragment separators have a very heavy workload with some facilities investing up to 80% of their available primary beam time to secondary beam experiments.
 

    Fragmentation facilities

At GSI (Germany), (LINAC + Synchrotron), the whole mass range of heavy ions is available with relativistic energies (up to 2 GeV/u), feeding the high-transmission fragment separator ``FRS''. Storing and cooling the secondary beams by means of the ``ESR'' provides unique possibilities. Within 3 years the heavy-ion synchrotron SIS can be operated at its space charge limit, providing a considerable increase in intensity. Discussion of a long-term upgrade has started including a project for very intense relativistic heavy-ion beams. GANIL (France), (two coupled K=380 cyclotrons), provides intermediate-energy heavy ion-beams up to 95 MeV/u. For rare, isotopically enriched beams world-record intensities are available and a further increase up to 6 kW of beam power will soon be made. Fragment separation is carried out with LISE (including a WIEN-filter for high purification) and SISSI (large solid angle). The fragment separators ETNA and COMBAS will soon be commissioned at the K=800 cyclotron in Catania (Italy), and the K=450-630 cyclotron U400M Dubna (Russia), respectively.
 

ISOL facilities

ISOL-type beams produced bombarding thick targets with 1-2 GeV protons are available at ISOLDE/CERN. Radioactive ion beams from heavy ion reactions in thin targets are produced at the on-line mass separator at GSI (Germany) using a thermal ion source and at LISOL/Louvain-la-Neuve (Belgium) and at IGISOL/Jyväskylä (Finland) using ion guide systems. The situation with regard to ISOL-laboratories featuring post-acceleration was summarised in 1993 in a detailed report from a NuPECC study group chaired by R.H. Siemssen. One important recommendation of this report was that a major R&D effort had to be completed before the technical case for the ultimate ``2<sup>nd</sup> generation'' facility could be made. This R&D, mostly of a complementary nature, would be assured by the ``1^st generation'' facilities which are, from a strictly financial point of view, remarkably modest investments. In this sense, one may presently enumerate the following European facilities or projects. Louvain-la-Neuve (Belgium) operates an intense low-energy proton driver (30 MeV, 500 $\mu$A) and a K=110 cyclotron post-accelerator. It will be complemented, in early 1998, by a new post-accelerator, the cyclotron CYCLONE 44. In this way, secondary beams close to stability, in the energy range for nuclear astrophysics (0.2 to 0.8 MeV/u) will become available with very good isobaric separation and an order of magnitude increase in intensity. The SPIRAL facility at GANIL (France), is scheduled to begin operation in late 1998. The existing GANIL cyclotrons will be used as the ``driver'', which allows a great variety of production reactions; the K=265 cyclotron CIME under construction will deliver exotic beams over a wide energy range (2-25 MeV/u), including nuclei far from the stability line. REX-ISOLDE at CERN is also expected to be operational in 1998. This project relies on the long experience gathered at ISOLDE in the production of low energy beams of nuclei far from stability. A novel concept for post-acceleration, bunching and cooling in a Penning trap prior to charge-state breeding and injection into a linear accelerator, will initially provide ions covering the energy region up to 2 MeV/u. The EXCYT project at Catania (Italy) will be operational around 1999, and will place special emphasis on secondary beams with well defined energies provided by the tandem post-accelerator; in the longer term a new 200 MeV proton driver is under consideration. A project exists at Dubna (Russia), which relies on the two existing U400 cyclotrons. The PIAFE programme at Grenoble (France) proposes the use of the reactor of the Institut von Laue-Langevin as a prolific source of very neutron-rich fission products made by the interaction of thermal neutrons with a ²³⁵U target. The first stage consists in the extraction of the secondary beams in the 1+ charge state; the second stage, including charge-state breeding and post-acceleration of these beams, is being studied in detail. The outcome of the R&D, associated with PIAFE will be of prime importance for a similar project at the FRM-II reactor at Munich (Germany); the reactor has been under construction since 1996.
 

    Future opportunities in a world-wide context

The scheduled intensity up-grades at GSI and GANIL and the availability of beams from the ``first-generation'' ISOL post-accelerator facilities will ensure the leading position of European nuclear structure physics for the next five years. Beyond this period, only a major step forward towards a next generation fragmentation facility will allow a thorough investigation of all the exciting aspects of exotic nuclei discussed earlier. European efforts will be in competition with the fragment beams from the MSU upgrade in the United States and the RIKEN project in Japan. The RIKEN project is particularly ambitious, adding to the present facility two super-conducting cyclotrons, a booster synchrotron, three fragment separators, a double storage ring and an electron accelerator. The latter would allow, inter alia, electron/exotic beam collisions and synchrotron-radiation excitation of atomic levels of exotic species.

    Thus, there is an urgent need for concerted European action to explore all possibilities to maintain European leadership for radioactive fragment beams, e.g. by significantly upgrading GSI and/or GANIL.

    In the same way, a major European effort is needed to develop second generation, ISOL based, post-acceleration facilities in order to maintain a leading position for Europe. It is important to note that the US DOE/NSF NSAC long-range plan ``strongly recommends development of a cost-effective plan for a next generation ISOL-type facility and its construction when RHIC construction is complete''. Furthermore, Japan plans to build such a facility at the intense driver accelerator (1GeV protons, 100$\mu$A) of the Japanese Hadron Project (JHP). To maintain European leadership it is therefore vital to design and build a European ``second-generation'' RNB facility or facilities, capable of handling much higher activities for the radioactive nuclei produced in the primary target than is available at the ``first-generation'' facilities. As to the production method, detailed investigations have to be carried out on the characteristics of production by the intense primary beams, be it protons, deuterons, heavy ions, fast or slow neutrons. In this respect, e.g. the Radioactive Ion Source Test (RIST) project and the design study of the second generation SIRIUS facility at the Rutherford-Appelton Laboratory (UK), where 0.1 mA of 800 MeV protons is available, are of interest. A second important initiative, supported by a European network (GANIL, Jyväskylä, Louvain-la-Neuve, KVI Groningen, Orsay) has been started to investigate the use of fast neutrons. Based on the R&D carried out at existing facilities, substantial technical developments will be required, to produce adequate targets and deal with their remote handling, their efficient coupling to the ion source and with the ion source systems themselves. Ultra-high current accelerators, developed in various contexts, e.g., for nuclear waste treatment, hybrid reactors and the European Spallation Source, will be very important. Design-studies for such high-current accelerators are now under way at Legnaro (Italy), at CERN (where they consider the re-use of the LEP cavities), and in France where the project IPHI has been launched.

    As a first step towards European ``second-generation'' facilities we recommend the formation of a study group to consider the technical proposals, taking into account the European R&D efforts.