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Towards a Network of Complementary Research Facilities in Europe

The scientific achievements and visions of nuclear physics outlined in the present report are intimately connected with the availability and development of the corresponding research facilities in Europe. The exciting broad research programme foreseen for the coming years requires a variety of complementary small and large-scale facilities, with accelerators covering a wide range of particle species and energies, high intensity neutron sources and versatile underground laboratories. It is one of the main goals of NuPECC to help in creating a network of research facilities for nuclear physics in Europe that includes local initiatives, national facilities, as well as European laboratories, which are complementary and guarantee international access. In the present report NuPECC identifies as future directions of research the following areas: nuclear structure under extreme conditions; nucleus-nucleus collisions and the phase transitions of nuclear matter; quark and hadron dynamics; nuclear and particle astrophysics; neutrino physics and fundamental interactions. The status of the requisite facilities that are available will be reviewed in this chapter, with the focus on both present and future aspects of their programmes.

 

Facilities for the Study of Nuclear Structure under Extreme Conditions

For the study of nuclei at high angular momenta and extreme shapes as well as for the investigations of their thermal response, high efficiency, high resolution multi-$\gamma$ detector arrays, in combination with heavy ion accelerators with energies around the Coulomb barrier are needed. Such facilities are the Tandem-Linac combination (ALPI) in Legnaro (Italy), the present site of EUROBALL, which is a world-wide unique 4$\pi$-$\gamma$ detector array constructed by six European countries, the Tandem Accelerator Vivitron in Strasbourg (France), and the cyclotron in Jyväskylä (Finland). These facilities are complementary in the type of heavy ion beams supplied and in the availability of additional special equipment needed to perform modern nuclear structure studies. For the study of fundamental nuclear excitations with high energy resolution, the superconducting cyclotron AGOR at Groningen (The Netherlands) provides unique high energy light ion beams including 200 MeV polarised proton and deuteron beams. The nuclear structure programmes with stable ion beams rely on the continuing availability of accelerators providing high quality beams throughout the periodic system; these facilities, which already form a complementary subnetwork, will need to be further supported and improved (see Figure [*]). Intensive studies of the structure of nuclei far from stability are in progress at several European laboratories. These are GANIL in Caen (France), GSI in Darmstadt (Germany), ISOLDE at CERN (Switzerland) and the facilities in Jyväskylä (Finland) and Louvain-la-Neuve (Belgium). By making use of modern ISOL-type sources, recoil or projectile separators in combination with highly sophisticated detector devices, different approaches can be exploited to reveal the properties of nuclei with extreme isospin. The UNILAC of GSI is devoted to a very successful synthesis programme of the heaviest elements. First generation radioactive beam facilities are in operation at GANIL, GSI, and Louvain-la-Neuve and under further development in these and other laboratories. They are complementary in the production process used, the isotopes available, the energy range and in particular in the instrumentation needed. In Louvain-la-Neuve, the first radioactive beam facility with low energy beams for nuclear astrophysics is being upgraded. Using modern trapping and charge-breeding methods, the REX project at CERN-ISOLDE aims at the acceleration of the already available radioactive beams of low energies and charge states to 2-3 MeV/u for nuclear structure studies of light nuclei far from stability. The SPIRAL-facility, under construction at GANIL, will use ISOL techniques and post-acceleration to 3-30 MeV/u in a cyclotron to pursue a broad, low to medium energy nuclear structure and reaction programme. Within the EXCYT project in Catania (Italy), negatively charged radioactive beams are being developed for further acceleration by a Tandem for high-resolution spectroscopy. GSI is undertaking an intensity upgrade of its existing high energy radioactive beam facility based on projectile fragmentation of heavy ion beams at energies up to 1 GeV/u, with also the possibility of storage ring cooling for various high energy reaction studies, precise mass and life-time measurements on exotic nuclei. Within the PIAFE-project at ILL in Grenoble (France), the production of high intensity beams of neutron-rich fission products is being considered, and at the Rutherford Appleton Laboratory (England), a 100 kW test-bed ISOL facility is being developed for high intensity ISOL-based radioactive beams. These facilities and projects are highly complementary and already form a subnetwork oriented to the future for studies of nuclei far from stability and for nuclear astrophysics (see Figure [*]). As these various challenging topics will ultimately require the highest conceivable luminosities, the main options for second generation radioactive beam facilities in Europe will need to be investigated. 

Facilities to Study Nucleus-Nucleus Collisions and the Phase Transitions of Nuclear Matter

Energetic nucleus-nucleus collisions allow atomic nuclei to be heated and compressed. Thus they provide a unique tool to study the phase diagram of nuclear and hadronic matter over a wide range of parameters. Such investigations require complementary accelerators providing nuclei with kinetic energies ranging from the Fermi energy of several ten MeV/u up into the TeV/u region. For studies around the Fermi energy, GANIL in Caen (France) has a long-term programme, highlighted by the successful operation of the second generation 4$\pi$ particle detector INDRA. At GSI in Darmstadt (Germany), a comprehensive reaction programme is carried out on the SIS accelerator at energies up to 1 GeV/u, of which a recent highlight is the first experimental indication for a liquid-gas phase transition. The heavy-ion acceleration capabilities of the new superconducting cyclotrons in Groningen (The Netherlands) and in Catania (Italy) add to the opportunities to study medium energy nucleus-nucleus collision dynamics using advanced instrumentation. At higher energies, properties of the resonance matter and chiral symmetry restoration may be favourably studied, the dilepton spectrometer HADES presently under construction at GSI being a key instrument for the latter question. With the heavy-ion beams available at the SPS at CERN (Switzerland), a glimpse at the quark-gluon plasma phase transition seems to be possible. At the beginning of the next century, the LHC at CERN will come into operation which -- with ALICE as a dedicated detector for the heavy-ion programme at energies of 7 TeV/u -- will become the most suitable facility to create and study the quark-gluon plasma. For the study of these complex hadronic processes, large sophisticated detectors are needed which can only be built within large collaborations involving various European laboratories. In fact, European networking in the construction and use of large detector systems in this field has long been introduced to ensure the proper scientific exploitation of the world-class heavy ion physics facilities already available or under construction in Europe (see Figure [*]). To achieve these goals the variety of beams of existing facilities needs to be maintained and the existing detectors be further developed. The realisation of ALICE in time for the start of LHC is identified as the highest priority of the high-energy heavy-ion community. 

Facilities to Study Quark and Hadron Dynamics

The study of hadrons, their structure, dynamics, and properties in a nuclear medium is a major theme of present day nuclear physics. The corresponding investigations are at present carried out mainly with photon, lepton, and light hadron probes (see Figure [*]). At MAMI in Mainz (Germany) and ELSA in Bonn (Germany), successful programmes to study the structure of nucleons and light nuclei using CW beams of electrons of 1 and 3 GeV, respectively, are in progress. Polarised real photons of energies up to 1.5 GeV are available from the GRAAL facility at the ESRF in Grenoble (France) and at MAMI and ELSA for precise photoabsorption and photoproduction experiments. With the storage cooler rings CELSIUS in Uppsala (Sweden) and COSY in Jülich (Germany), high quality proton and light-ion beams are now available in the GeV-energy range and are being employed mainly in complementary precision studies exploiting the polarisation degree of freedom in scattering and meson production experiments. In Frascati (Italy), DA$\Phi$NE, a high luminosity e+e- collider, will become operational soon. DA$\Phi$NE will produce intense beams of kaons, which will be used for low-energy scattering as well as to study their rare decay modes. At higher lepton energies, HERMES at DESY in Hamburg (Germany) has started its programme to study the spin structure of the nucleon using 26 GeV polarised positrons and polarised internal targets. The COMPASS experiment, presently being built up at the SPS at CERN (Switzerland), will use muons and hadrons of several hundred GeV to measure the gluon polarisation and to investigate light and charmed hadrons. In order to address in more detail the structure of hadrons in terms of quarks and gluons, there is a requirement for high energy, high duty cycle lepton beams with a luminosity much beyond that presently available world-wide. A high luminosity and high-duty cycle electron facility of at least $\sqrt{s} = 7 GeV$ (E>25 GeV for fixed target experiments) is needed to explore this frontier. 

Nuclear and Particle Astrophysics, Neutrino Physics and Fundamental Interaction Studies

The impact of nuclear physics on other fields of science is particularly strong in astrophysics, neutrino physics and in fundamental interaction studies. The continuing availability of the European facilities mentioned previously in connection with such studies is of great importance in order to pursue the exciting programmes discussed in chapter [*] of this report. With the return to operation of the high flux reactor at the ILL in Grenoble (France), Europe has again an excellent source of neutrons, which will be complemented by the new high flux reactor FRM II in Munich (Germany) coming into operation around the turn of the century. For the study of fundamental symmetries and rare decays, high intensity pion and muon beams are well suited and are presently available at PSI in Villigen (Switzerland). For various projects concerning the solar neutrino problem and neutrino oscillation, double $\gamma$-decay and astrophysical relevant reaction cross sections, the underground laboratory at GRAN SASSO (Italy) provides an excellent environment. A dedicated underground accelerator for background free measurements of important astrophysical cross-sections at thermal energies is needed. 

Summary and Conclusion

There are in Europe a few large facilities that attract large nuclear physics communities from various subfields due to both the unique research opportunities that they offer at present and to the proposed new options for enhancement. These facilities are presently CERN in Geneva, GANIL in Caen, GSI in Darmstadt, and the GRAN SASSO underground laboratory near Rome. In addition, as described in the previous sections, there are in Europe specialised facilities of great scientific impact, which complement the large facilities. Taken together, they form a comprehensive and well-balanced European network for creative nuclear research. This network rests and relies on a number of regional nuclear physics laboratories spread throughout Europe, which are of great importance not only because they fulfil an educational need by attracting students from universities but also because they are the roots of the programmes carried out within the network. The European network of nuclear physics facilities is both competitive and complementary to current and projected installations outside of Europe. At the same time we should be alert to opportunities in Europe for innovative development and new facilities in the future. As discussed and substantiated in the present report, several actions should be taken now to ensure that the new challenging problems of nuclear physics can be addressed properly and successfully.  

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