Scientific Opportunities with a Rare-Isotope Facility in the United States (2007) / Chapter Skim
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2 Key Science Drivers for a Rare-Isotope Beam Facility
2 Key Science Drivers for a Rare-Isotope Beam Facility
Pages 29-67
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From page 29... ...
This chapter presents the committee's view of the principal scientific drivers in nuclear structure physics, nuclear astrophysics, fundamental symmetries, and some important technical applications. However, it is often the case with new world-class facilities that their most important scientific discoveries are not foreseen.
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From page 30... ...
The phenomena that arise -- shell structure, pairing, super fluidity, collective motion and its connections with many-body symmetries, and spectral transitions from order to chaos -- and the methods that nuclear physicists employ are also fundamental to fields such as atomic and con densed-matter physics and quantum chemistry. Nuclear structure theory has made significant progress in recent years by adapting numerical tech niques for high-performance computing and through conceptual advances such as effective field theory and improved density functionals.
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From page 31... ...
. With an improved understanding of strongly interacting matter in finite nuclei with large neutron excesses, scientists will be better equipped to model neutron stars: giant reservoirs of neutron matter.
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From page 32... ...
While an exotic-beam facility would not directly probe the high densities of neutron stars, it would be able to constrain the isospin dependence of the nuclear equa tion of state that determines neutron star structure. Moreover, using charge-exchange reactions on the most critical neutron-rich nuclei along the electron-capture chains that produce the critical nuclei in the crusts of neutron stars, a FRIB could enable the study of the central questions con cerning the composition and energetics of their upper mantles.
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From page 33... ...
NUCLEAR STRUCTURE A quantitative understanding of nuclear structure is important in problems ranging from the origin of the elements to the use of nuclei as laboratories for probing new interactions. Yet a general theory of nuclear structure remains elusive: the classical formulation of this problem, protons and neutrons interacting through a strong, short-range potential, is difficult to solve except for the lightest nuclei.
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From page 34... ...
Figure 2.1 illustrates some of the progress that has been made in solving the classical nuclear physics problem, protons and neutrons interacting through a potential derived from two-nucleon scattering data, augmented by three-nucleon forces also constrained by experiment. The results were obtained from computationally intensive variational and Green's function Monte Carlo calcula tions.
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From page 35... ...
The nuclear shell model, perhaps the most widely used microscopic nuclear model, superimposes such correlations on the shell structure, thereby directly accounting for that part of the residual interaction most important to the long-distance structure of the nucleus. The effects of short-distance correlations can also be treated, though indirectly.
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From page 36... ...
In astrophysics, unstable nuclei play crucial roles in explosive environments such as supernovae and colliding neutron stars. In fact, it is believed that roughly half of the stable nuclei heavier than iron were synthesized as unstable nuclei in the core of an exploding supernova, then ejected into the interstellar medium.
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This repulsion grows as the square of the number of protons. begins, for light nuclei, with N ~ Z, then later veers toward nuclei with N > Z as the repulsive Coulomb force begins to favor heavy nuclei with fewer protons than neutrons.
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From page 38... ...
One important goal of a FRIB is to produce new neutron-rich, doubly magic nuclei, that is, unstable nuclei where N and Z are both magic. If the shell gaps are unusual, this will demonstrate that the mean field, and thus the interaction of valence nucleons with the rest of the nucleus, differ from that of stable nuclei.
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From page 39... ...
It is not surprising, therefore, that pairing plays an important role in nuclear structure. As the number of nucleons can be precisely controlled at a FRIB, exotic nuclei accessible with a FRIB would offer many new opportunities to study pairing, including its influence on the structure of the diffuse, neutron-rich skin found in nuclei far from the valley of stability.
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From page 40... ...
In many cases, these regularities arise from underlying symmetries that govern the systems, from which the relevant and usually simple collective coordinates can then be deduced. The goals of nuclear structure physics include identifying the relevant collective coordinates, understanding their connections to the approximate symmetries governing nuclear motion, and then understanding how these symmetries arise from the underlying microscopic theory based on the degrees of freedom of nucleons.
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From page 41... ...
These nuclei, and their neighbors in the expected transition regions, would be available for study at a FRIB, given beam intensities ranging from a few to 10,000 ions per second. Such beams would allow experimenters to determine masses and lifetimes, and, for the
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From page 42... ...
To the extent that the understanding of strongly interacting matter with large neutron excesses is improved, it will also be more possible to model the exotic neutron-rich environment of neutron stars. One expects to find new collective modes that are a consequence of this ex tended neutron skin.
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superheavy nuclei are due to a neutron deficiency, and that more-neutron-rich isotopes of the same elements might have very long lifetimes. However, theories disagree in their predictions for the location and extent of the region in (N,Z)
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NUCLEAR ASTROPHYSICS The nuclear physics of unstable nuclei is fundamentally important in three astrophysical contexts: in making determinations of the abundances of the ele ments and isotopes produced in stars and stellar explosions; in generating and releasing energy in such environments; and in helping develop the understanding of the behavior of matter at the extremes of neutron excess found in neutron stars and supernovae. Each of these areas poses robust problems in nuclear physics that have eluded solution for decades.
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KEY SCIENCE DRIVERS RARE-ISOTOPE BEAM FACILITY FOR A 45 FIGURE 2.6 Deformations and shapes for the heaviest nuclei calculated in nuclear density functional theory. The Z = 110-113 alpha-decay chains found at Gesellschaft für Schwerionenforschung (GSI)
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From page 46... ...
Similarly, theory predicts that part of potassium was made in supernovae as radioactive calcium, manganese from cobalt, cobalt from copper, and so on. Explosive events such as novae, supernovae, and x-ray bursts tend to produce unstable nuclei either because they quickly fuse fuels that have equal numbers of neutrons and protons (as in the 56Ni example)
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The primordial abundance pattern The abundance pattern in the oldest The solar abundance pattern observed stars HE1017 & HH1037 FIGURE 2.7 The history of the universe is depicted in this time sequence, starting from the Cosmic Dark Age (top left panel) , displaying the formation of the first galaxies as breeding grounds for the first stars developing to the first supernovae (top center panel)
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. The rp-process can also occur in that neutrino-powered wind and additionally is the power source for Type I x-ray bursts on the surfaces of accreting neutron stars.
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Most of the important branchings for beta-decayed neutron emission and the relat ed nuclear mass measurements are also within reach. With these measurements, astrophysical models would have a solid nuclear physics underpinning for investigating the synthesis of r-process nuclei in the region of the A ~ 195 peak and beyond to explain the production of the heaviest nuclei found in nature.
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Using isotope harvesting, an exotic-beam accelerator facility could also enable neutron-capture cross-section measurements of long lived unstable nuclei produced in the slow neutron-capture process (s-process)
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How Would an Exotic-Beam Accelerator Facility Help Improve Understanding of Neutron Star Structure, Supernovae, and Gamma-Ray Bursts? There are roughly one billion neutron stars in the Milky Way Galaxy, yet their structures and crusts are very poorly understood.
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These electron capture rates can be studied with an exotic-beam accelerator facility using charge exchange reactions on the most critical radioactive neutron-rich nuclei along the dominant electron-capture chains between A = 56 and A = 104. The measurement of the Gamow-Teller strength distribution will also provide information about the neutron release and the subsequent neutronization of neutron star crust matter.
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From page 53... ...
With an exotic-beam accelerator facility, the range of neutron excesses available would be much larger, so the neutron-to-proton ratio dependence of the nuclear equation of state could be determined. Exotic Beams: An Urgent Need of the Nuclear Astrophysics Community The key feature of an exotic-beam accelerator facility (such as a FRIB)
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Careful simulation and analysis suggest that their ejecta are also rich in the nuclei produced in the rapid neutron-capture process. Courtesy of the Max Planck Institute for Astrophys ics.
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• Charge-exchange reactions on unstable nuclei in the iron group to get the nuclear matrix elements for use in electron-capture rates in presupernova stars of all types. • Proton- and alpha-capture cross sections on heavy, proton-rich nuclei up to lead for use in studies of the p-process (or gamma process)
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Understanding the properties of the universe at a deeper level than the Standard Model is one of the greatest challenges facing science. Historically, many features of fundamental interactions have been discovered in nuclear physics experiments.
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From page 57... ...
However, the presence of an analogous electric dipole moment in their ground state violates time-reversal and CP symmetry and has never been observed. At the level of present experimental sensitivity, an EDM could be a signal of the excess CP violation beyond that allowed by the Standard Model to explain the matter-antimatter asymmetry.
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From page 58... ...
While current EDM searches are very susceptible to various environmental noise sources and often have to contend with significant systematic effects, experi ments using radioactive isotopes with large intrinsic sensitivity to CP violation will be much less affected by these problems. Therefore, there is a strong expectation that they will be able to make clean EDM measurements; optimistic forecasts suggest that these results might only be limited by the statistical uncertainty deter mined by the number of available atoms and the integration time.
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This situation led to the consideration of the role for advanced 5In addition to the question of the accuracy of radiochemical inferences on device performance, there are potentially relevant nuclear data uncertainties in basic cross sections such as D + T → α + n. Here, however, only those nuclear physics issues addressable by exotic-isotope production are discussed.
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The surro gate method is useful in cases both in which the target lifetime is too short for practical scattering experiments and in which a neutron-scattering source is un available. A facility capable of isotope production rates significantly greater than those now available could improve this situation in two powerful ways.
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of importance to nuclear kinetics are also shown. The indicated charge-particle-out reactions are sets of nuclear reactions that alter the number of protons in the nuclei; in order to conserve electric charge, an electrically charged particle is emitted.
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At present, while stockpile stewardship has a continuing need for people conversant with the phenomenology of nuclear physics, homeland security's nuclear physics and 7For additional information, see E.M. Campbell, W.M.
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The "m" that follows some of the isotopes is the standard nuclear physics notation for an isomer. Isomers are excited states of an isotope with a significantly long half-life.
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Unless there is an increase in the number of nuclear physicists, perhaps spurred by a new U.S. initiative in low-energy nuclear physics, there is likely to be a surge in unfulfilled demand before 2010 in the number of such applied scientists and engineers.8 Medical and Biological Research Applications of Radionuclides The applications of radionuclides to the medical sciences and biological re search fall into the three overlapping categories of imaging, targeted therapy, and radiotracers.
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at high specific activity and the complete coverage of almost all candidate nuclei. Given the enormous production rates, parasitic harvesting of appropriate radioisotopes may be attractive.
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Forkel-Wirth, "Exploring Solid State Physics Properties with Radioactive Isotopes," Rep.
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Many of the required cross sections could be measured at a rare-isotope facility in a manner analogous to needs for stockpile stewardship and astrophysics, either by using direct neutron reactions (if available) or by application of the surrogate method.
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