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Filomena Nunes and Vladimir Zelevinsky
Frontiers of nuclear physics in the XXI century moved in the direction of nuclei far from the valley of stability. There exist about 300 isotopes which are stable or have enormously long lifetimes. Other known isotopes (there are about 3000 of them) are unstable but with a lifetime that is long compared to the nuclear scale - they decay mainly by beta decay, alpha decay or spontaneous fission. Meanwhile, the estimates show that the full nuclear chart delineated by the driplines (where the nuclei become particle-unstable and decay in about 10-20 s or faster) should contain more than 7000 isotopes. In fact, we do not accurately know the properties of many of the recently discovered isotopes, and the limit of nuclear existence continues to be an open question in our field.
Why is this of great scientific interest? We list a few of the primary reasons for our quest. First of all, this is a testing ground of our ideas about nuclear structure in general. The new isotopes are loosely bound and we can expect many surprises concerning nuclear shells, shapes, and reactions. It is not clear if our standard models and theoretical approaches will work under such conditions. Second, nucleosynthesis in the Universe mostly proceeds through nuclei far from stability and the knowledge of such nuclei is the main instrument for understanding crucial astrophysical problems. Third, nuclear physics had already provided many ideas and theoretical tools for mesoscopic science, and now we can approach problems of complexity and many-body chaos for open and marginally stable quantum systems, problems of great importance for quantum information, atoms in traps, soft condensed matter, and even biophysics. Fourth, one can hope to find among the new isotopes the best candidates to look for the violation of fundamental symmetries in nature. Finally, but not the least in importance, one can anticipate many practical applications of rare isotopes.
There are a number of large world centers where the experimental effort focuses on rare isotope science. In the US, the leading institution is Michigan State University with its National Superconducting Cyclotron Laboratory (NSCL). The new Facility for Rare Isotope Beams (FRIB) approved by the Department of Energy is under design with construction scheduled to start later this year. NSCL/FRIB will be the largest university-based laboratory in the country with full staff exceeding five hundred people, including more than sixty graduate students (the graduate nuclear physics program at MSU is ranked No. 1 in the US) and more than thirty faculty. The number of users for FRIB is expected to approach a thousand per year, from all over the world. The rare isotopes will be created in the fragmentation reaction of the primary beams (from protons to uranium with energy about 200 MeV/nucleon) on various targets. The high production rate of rare isotopes will make possible their separation, collimation, stopping for laser spectroscopy or reacceleration to energies around and above the Coulomb barrier. The completion of the FRIB project is expected in 2018 - 2020.
Similar efforts are underway in several countries around the globe. The new international accelerator center FAIR (Facility for Antiproton and Ion Research) for intense, high-energy beams of ions and antiprotons is under construction at the GSI (Darmstadt, Germany). This facility will operate at higher energies and partly be directed to antiproton and hadron physics, still with great perspectives for nuclear physics, especially due to the cooling rings planned in cooperation with the Budker Institute of Nuclear Physics (Novosibirsk, Russia). Another major player in this arena is Japan with its RIKEN Nishina Center for Accelerator Based Science (RIKEN is a broad scientific organization spread over the country similarly to the Max-Planck Institutes in Germany). Here there are opportunities to use various beams at energy 300-400 MeV/nucleon for fragmentation reactions or in flight uranium beams for fission products. The TRIUMF center in Vancouver, Canada, with its world’s largest cyclotron and Isotope Separator and Accelerator (ISAC) facility uses the isotope separation on-line (ISOL) technique to produce rare isotope beams. Grand Accelerateur National d’Ions Lourds (GANIL) in France is going to finish its new project SPIRAL-2 in 2013 and this, together with CERN, can serve in the future as a seed for the huge EURISOL proposal (European Isotope Separation On-Line).
Nuclear theory plays an extremely important role in the study of rare isotopes. It provides frameworks to understand new phenomena, as well as new directions to explore, and enables a translation of the enormous nuclear complexity contained in the data into a tangible language. As in experimental nuclear physics, the technological developments are changing the paradigm for nuclear theory. The complexity of the problems we are trying to solve requires varied expertise. Because resources are limited, it is vital to coordinate the efforts and find effective solutions. The role of collaborations has thus never been more important in nuclear theory, collaborations that cross borders and oceans.
Informal collaborations have always existed in nuclear theory. However, over the last decade we have seen a true shift in the way nuclear theorists organize their work. Already in the early nineties, a formal recognition of the importance of collaborations in theory was made when establishing the INT (Institute for Nuclear Theory at the University of Washington in Seattle) and the ECT* (The European Center for Theoretical Studies in Nuclear Physics and related areas) in Trento (Italy). Since their creation, these institutions have enabled and nurtured many fruitful collaborations on a variety of theoretical topics, bringing together theorists from all over the world for workshops or programs. The new direction in nuclear physics, the study at Brookhaven National Laboratory of relativistic nuclear collisions at extreme energies, when the nucleons liberate their constituents, quark and gluons, and form new states of matter, was strongly supported by the efforts of the BNL-RIKEN theoretical center. Over the last decade, additional focused investments have been made: JUSTIPEN (Japan - US Theory Institute for the Physics with Exotic Nuclei) was established in 2006, to foster collaborations between nuclear theorists in the US and in Japan; FUSTIPEN was created in 2010, with a similar purpose to strengthen collaborations between nuclear theorists in France and in the US and now CUSTIPEN is being proposed to develop collaborations between China and the US. No doubt, funding agencies are also recognizing the need for large collaborations in the theory for rare isotopes, one example being UNEDF, a Department of Energy initiative which brought together close to 50 theorists in the US to develop a Universal Nuclear Energy Density Functional.
Despite the variety of collaborations worldwide, very often efforts are not internationally coordinated nor aligned with the experimental programs. With the construction of FRIB, the local nuclear theory group at Michigan State University feels that such a coordination should be a priority. The NSCL nuclear theory group has eight faculty members, four postdocs and twelve doctoral students. Reflecting the importance of international collaborations, the group also has seven adjunct faculty from all over the world. (For more detail see http://groups.nscl.msu.edu/theory/). At this time, it is important to go beyond small unstructured collaborative work, and provide a reliable framework to establish strong collaborations, bringing together the variety of expertise that can effectively tackle a relevant problem within a relevant timescale. It is with this mission in mind that, working together with other large rare isotope laboratories in the world, including GSI and RIKEN, the NSCL nuclear theory group hopes to establish ICNT (International Collaborations in Nuclear Theory).
Last year the world scientific community celebrated the first century of nuclear science that began with the discovery of the atomic nucleus by Rutherford. Nuclear theory now connects far reaching frontiers from fundamental problems of the microworld to cosmology, and from practical energy problems to quantum information and mesoscopic physics. The science of rare isotopes is an extremely interesting experimental and intellectual enterprise that should unite physicists of different countries in their quest for new and deep knowledge.
Filomena Nunes and Vladimir Zelevinsky are at the National Superconducting Cyclotron Laboratory and the Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan