Scientific Opportunities with a Rare-Isotope Facility in the United States (2007) / Chapter Skim
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1 Introduction and Background
1 Introduction and Background
Pages 5-28
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From page 5... ...
In the past decade, however, nuclear structure scientists have learned how to build high-beam-power facilities for producing useful beams of short-lived, radioactive nuclei. With these new beams of unstable nuclei, they can make and study many thousands of exotic nuclear species -- most of which have never existed before or are only fleetingly created in the hot interiors of stars.
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From page 6... ...
rare iso topes themselves play an important role in physical environments that are hot, dense, or highly interacting, such as those within neutron stars, stellar fusion cycles, nuclear reactions in reactor fuel cycles, and so on. communities proposed that such a new rare-isotope accelerator be built in the United States.
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From page 7... ...
The Rare-Isotope Science Assessment Committee (RISAC) was charged to define the science agenda for a next-generation rare-isotope beam facility.
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Or, going the other way, can the study of nuclei lead us to new forces and new symmetries, new insights into the world of quarks? How do nuclear reactions power quiescent stars like the Sun and lead to stellar catastrophes like supernovae?
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From page 9... ...
The first models of the nucleus, Niels Bohr and John Wheeler's "liquid-drop" or "compound nucleus" model and Eugene Wigner's "supermultiplet" model of light nuclei, began to apply new quantum ideas to nuclear structure. Enrico Fermi wrote his famous paper proposing a theory to explain beta decay, an early step on the path to the discovery of the Standard Model of fundamental physics.
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From page 10... ...
Recently, nuclear physicists have directly confirmed his theory of the Sun's energy source by a quantitative measurement of the flux of neutrinos from the Sun. Bethe's work not only led to an understanding of the energy sources that power the universe but also initiated the field of nuclear astrophysics, which now includes the study of supernovae where heavy nuclei are created and of degenerate collapsed stars such as neutron stars, which are, in essence, gigantic nuclei of stellar proportions.
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From page 11... ...
A large and vibrant community continued the study of nuclear physics after the birth of elementary particle physics. There was much to understand about nuclear structure, nuclear reactions, and other nuclear phenomena.
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From page 12... ...
The shell model in its original formulation, however, had little success describing the spectra of nuclei far from closed shells or in regions of N and Z, where the overall nuclear shape deforms away from spherical symmetry. The unified model combined the early picture of the nucleus as a deformable, rotating, and vibrating object -- a picture that had grown out of Bohr and Wheeler's liquid-drop model -- with the shell model.
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From page 13... ...
This deformed-shell model implemented important principles implicit in the unified model by coupling independent particle models to the collective description. Significant progress was made during this period in nuclear reaction theory, and the ability to interpret the results of nuclear reactions quantitatively added much to the knowledge of nuclear structure.
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From page 14... ...
SCIENTIFIC OPPORTUNITIES RARE-ISOTOPE FACILITY 14 WITH A FIGURE 1.1 Various shapes of nuclei either observed or expected. Exotic orbitals that appear in regions far from the stability line may provide some new types of deformation.
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From page 15... ...
Most of the spectroscopic properties of nuclei described by the nuclear shell and unified models are determined by the interactions of the least-bound nucleons in the nucleus, the analogue of the valence electrons in an atom or the particles at or near the Fermi surface in a degenerate Fermi liquid. Thus, neighboring nuclei would often exhibit quite different spectra and reveal very different behavior in low-energy nuclear reactions.
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From page 16... ...
The worldwide activity in this field produced an extensive body of data on the nucleon-nucleon and pion-nucleon interactions, mounted sensitive tests of the Standard Model, and was essential to the develop ment of a relativistic nucleon-nucleus potential based on a mesonic description of the nucleon-nucleon interaction. This model provided a natural explanation for the strong nuclear spin-orbit force required to account for the observed nuclear shell structure; the origins of the spin-orbit force to that point in time had been obscure.
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From page 17... ...
Even though it was soon recognized that QCD would be extremely difficult to implement on the scale of hadrons and even more so on the scale of nuclei, the emergence of a fundamental underlying theory changed the way that nuclear physicists thought about nuclei and changed the criteria for an "explanation" of nuclear phenomena. Ideally, one would like to be able to trace the properties of nuclei back to the fundamental structure of QCD.
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From page 18... ...
The QCD paradigm changed the way that nuclear physicists think about nuclear matter produced at very high temperature or density. Confinement of quarks and gluons within hadrons is regarded as a (relatively)
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A large community of experimental and theoretical nuclear physicists has launched an ambitious program to explore this very dense, hot, strongly interacting form of matter, often referred to as the quark-gluon plasma (sometimes called QGP)
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study of this matter is expected to explain much about QCD and the dynamics of the very early universe. The connection between nuclear reactions and astrophysics goes back to Bethe's pioneering work on the energy source of stars.
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From page 21... ...
In addition to the relevance of nuclear structure to astrophysics, there is widespread interest in the nuclear physics community in investigating the many phenomena encountered with large neutron excesses and nearly unbound systems. Until the 1990s it was not clear that it might be possible to create a viable experimental program to investigate these issues.
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From page 22... ...
Subsequent research in Japan and Canada (see Figure 1.4) has confirmed this deficit, showing that it is due to neutrino oscillations and that the character ization of the nuclear reactions driving the Sun is correct.
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From page 23... ...
These measurements have also provided positive evidence for such particle physics landmarks as conserved vector currents, the unitarity of the Cabibbo-Kobayashi-Maskawa matrix that describes the interactions of quarks, and parity conservation by the strong interactions. The past decade has witnessed significant developments in experimental studies of nuclei and nuclear astrophysics, driven largely by qualitative advances in technology, including high-resolution particle separators, large arrays of gammaray or particle detectors, a variety of traps, storage ring and laser spectroscopy techniques, and especially the development of first- and second-generation facilities for the production and use of nuclei far from stability.
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24 I Nobel Prize Nobel Prize Nobel Prize Nobel Prize Nobel Prize
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NOTE: ANL -- Argonne National Laboratory; ATLAS -- Argonne Tandem Linear Accelerator System; BBHF -- Burbidge Burbidge Hoyle Fowler, referring to a team of scientists who wrote a landmark paper on nucleosynthesis; CERN -- European Organization for Nuclear Research; GANIL -- Grand Accélérateur National d'Ions Lourds (Great Heavy-Ions National Accelerator) ; GSI -- Gesellschaft für Schwerionenforschung mbH; HRIBF -- Holifield Radioactive Ion Beam Facility; IGISOL -- Ion Guide Isotope Separator On-Line; ISAC -- Isotope Separator and Accelerator; ISOL -- Isotope Separator On-Line; ISOLDE -- On-Line Isotope Mass Separator, a facility at CERN; ISOLTRAP -- Tandem Penning trap mass spectrometer at ISOLDE; LLN -- Laboratoire Louis Néel; NMR -- nuclear magnetic resonance; NSCL -- National Superconducting Cyclotron Laboratory; PET -- positron emission tomography; PS -- proton synchrotron; REX-ISOLDE -- Radioactive Beam Experiment at ISOLDE; RIBs -- rare-isotope beams; SPIRAL -- Système de Production d'Ions Radioactifs en Ligne; TRINAT -- TRIUMF Neutral Atom Trap; TRISTAN -- Terrific Reactor Isotope Separator to Analyze Nuclides; TRIUMF -- Tri-University Meson Facility.
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From page 26... ...
The experimental study of exotic nuclei involves three separate stages: produc tion and preparation of the rare isotopes for research and the observation of the final products by means of the end-station instrumentation. Broadly speaking, there are two basic approaches to producing radioactive beams for use in nuclear physics experiments; they are called the in-flight technique and reacceleration.
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From page 27... ...
The second approach, reacceleration, takes the exotic nuclei formed in the production target and prepares a beam by bringing the exotic nuclei to rest and then injecting them into a second accelerator. This method produces high-quality, reaccelerated beams at the lower energies traditionally used for nuclear structure and nuclear astrophysics experiments, so these well-tested and well-understood techniques can be exploited in investigating their subsequent interaction with the target.
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From page 28... ...
An essential additional development in facilitating the study of exotic nuclei is advances in experimental instrumentation that now allow measurements to be carried out with beams as weak as a few hundred particles per second or, in special cases, as low as 1 particle per day, whereas traditionally, nuclear structure and astrophysics experiments have usually been carried out with beams on the order of 108 to 1013 particles per second. Thus, it appears that the technological advances are now available to allow the construction of rare-isotope facilities of enhanced capability that would permit the execution of experiments unimaginable a decade ago.
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