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Executive Summary NUCLEAR PHYSICS TODAY Nuclear physics deals with the properties of atomic nuclei, their structure and interactions, and the laws governing the forces between their constituents. The interactions in nuclei have their roots in the interactions of elementary particles, the quarks and gluons that to- gether constitute nuclear matter. But additional dynamical forces, long known to exist in nuclei, cannot be understood with elementary particles alone, just as new cooperative interactions, not recognizable in nuclei or atoms, are known to exist in macroscopic materials. The basic questions facing nuclear physics today span a broad range, including strong and electroweak interactions, and cover the properties of the physical world from the microscopic scale of nuclear forces to the large-scale structure of the universe. Nuclear physics deals with many-body aspects of the strong interaction. It also deals with tests of fundamental theories and symmetries. Furthermore, nuclear physics plays an important role in the fields of astrophysics and cosmology. Our understanding of nuclear structure and nuclear dynamics con- tinues to evolve. New simple modes of excitation have emerged, new symmetries are appearing, and some completely new phenomena are being discovered. In the 1970s, for example, several new modes of vibration of nuclei were discovered, using the technique of inelastic scattering of charged particles from target nuclei. One of these vibrations, the giant mono 1
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2 NUCLEAR PHYSICS pole, is particularly significant because of its direct relation to the heretofore unmeasured compressibility of nuclear matter. In similar studies using pions as projectiles, important information on the relative roles of protons and neutrons in nuclear vibrations has been gained, as well as that of nucleon excited states called deltas. The use of high-energy electron scattering from nuclei has revealed unprecedented levels of detail of nuclear structure, in terms not only of the nucleons but also of the mesons present in nuclei and, to a rudimentary degree, of the quarks that compose all of these particles. Such studies represent one of the major frontiers of nuclear physics today. At the opposite extreme of projectile size, heavy ions have come into increasingly widespread use, particularly as versatile probes of nuclear dynamics. Their massive impact on target nuclei can cause a great variety of excitations and reactions, analyses of which are invaluable for understanding different kinds of motions of the nucleons within a nucleus. Heavy-ion collisions have also been indispensable for produc- ing many exotic nuclear species, including four new chemical elements (numbers 106 through 109) during the past decade. It is noteworthy that almost all nuclear-physics research to date has been possible only within the very limited domain of nuclei under conditions of low nuclear temperature and normal nuclear density. The vastly greater domain of high-temperature, high-density nuclear phys- ics has just recently begun to be explored, using heavy-ion projectiles at relativistic energies. This too is currently a major frontier of the field. Inevitably, fundamental new problems arise to challenge our under- standing of nuclear physics. For example, although we now know how to explain certain nuclear phenomena in terms of the presence, within nuclei, of mesons in addition to protons and neutrons, we are not yet able to solve the corresponding equations of quantum chromodynamics (the quantum field theory that is believed to govern the manner in which these particles interact) to describe the effects in question. Current efforts to solve this problem are particularly important because they hold the promise of new insights into one of the fundamental forces of nature, the so-called strong force. Indeed, the nucleus in general represents a uniquely endowed laboratory for investigating the relationships among the fundamental forces as well as the symmetry principles underlying all physical phenomena. Its key role in shaping our view of the cosmos is evident in the field of nuclear astrophysics, which provides information vital to our understanding of the origin and evolution of stars and of the universe itself. On the Earth, meanwhile, nuclear medicine (including the development and use of specifically tailored radioisotopes and accelerator beams for
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EXECUTIVE SUMMAR Y 3 both diagnostic and therapeutic procedures), nuclear power (both fission and fusion), materials modification and analysis (for example, ion implantation and the fabrication of semiconductor microcircuits), radioactive tracers (used in a number of research areas ranging from geophysics to medical physics), as well as many routine industrial applications (including, for example, well-logging in test bores using miniaturized nuclear accelerators, food preservation by irradiation, and die hardening by ion implantation to reduce wear), and even the analysis of art objects are just a few examples of how the fruits of nuclear-physics research have found a multitude of useful and some- times surprising applications in other basic sciences and in modern technologies, many of which have direct and significant impacts on society at large. Much of this research is done with particle accelerators of various kinds. Some studies require large teams of investigators and high- energy accelerators, typically operated by national laboratories, while other, lower-energy studies continue to be performed at colleges and universities typically by a professor and a few graduate students- using smaller accelerators or laboratory-scale equipment. Both pro- duce fundamental advances in nuclear physics. This very wide range of facilities and manpower requirements is among the unusual characteristics of nuclear physics. Maintaining the proper balance between the research programs of large and small groups is essential for overall progress in the field. Equally important is the balance between experimental and theoretical research, as well as the availability of state-of-the-art instrumentation and computers for the respective programs. The major advances of the past decade of nuclear-physics research and the exciting prospects for its future as well as some of the myriad ways in which nuclear physics has an impact on the other sciences and on society at large~onstitute the subject of this nuclear-physics survey. RECOMMENDATIONS FOR THE FUTURE OF NUCLEAR PHYSICS In formulating the recommendations for the future of nuclear phys- ics, as presented below, the Panel on Nuclear Physics has profited from extensive interactions between its members and the participants in the 1983 Long Range Planning Workshop of the Nuclear Science Advisory Committee (NSAC) of the U.S. Department of Energy and the National Science Foundation.
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4 NUCLEAR PHYSICS Accelerators are the basic tools of nuclear-physics research. The planning, design, and construction of first-rate accelerators and their associated experimental facilities have become increasingly important to the nuclear-physics community at large. Designs must be optimized to support those programs most likely to produce new results in critical research areas and to satisfy the needs of the largest number of users. There are currently two major accelerators, of complementary natures, whose construction has been recommended by NSAC. The Planned Continuous Electron Beam Accelerator Facility In April 1983, NSAC recommended the construction of a 100- percent-duty-factor, 4-GeV linear-accelerator/stretcher-ring complex now called the Continuous Electron Beam Accelerator Facility (CEBAF), which was proposed by the Southeastern Universities Research Association. The research and development funding for this machine began in FY 1984, and construction funding is proposed for FY 1987. A total accelerator cost of $225 million (in actual-year dollars) is projected; this includes $40 million for the initial experimental equipment. The Panel on Nuclear Physics endorses the construction of CEBAF. A major focus of nuclear-physics research at CEBAF will be investigations of the microscopic quark-gluon aspects of nuclear matter (the regime of high energies, high momentum transfers, and small distances), using the electron beam to probe the detailed particle dynamics within an entire nucleus with surgical precision. Of great importance also, however, will be investigations of baryon-meson aspects of nuclear matter (the regime of lower energies, lower momen- tum transfers, and larger distances). In particular, it will be most valuable to study the nature of the transition from the low-energy regime of nucleon-nucleon interactions (best described by indepen- dent-particle models of nuclear structure) to the intermediate-energy regime of baryon resonances and meson-exchange currents (described by quantum field theories of hadronic interactions in nuclei) and the ensuing transition to the high-energy regime of quarks and gluons (described by quantum chromodynamics). For these and other studies, the variable beam energy of CEBAF, from 0.5 to 4.0 GeV, is necessary. Also necessary is its 100 percent duty factor (continuous-wave operation), so that coincidence measure- ments can be made; these are vital for isolating particular channels and variables for study. The unique capabilities of CEBAF will thus provide unprecedented opportunities for examining nuclear matter at different levels of structure in great detail.
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EXECUTIVE SUMMARY 5 The Next Major Initiative: The Relativistic Nuclear Collider In NSAC's 1983 Long Range Plan (A Long Range Plan for Nuclear Science: A Report by the DOE/NSF Nuclear Science Advisory Com- mittee, December 1983), the construction of a variable-energy, relativ- istic heavy-ion colliding-beam accelerator is recommended. Such a machine is seen by NSAC as the highest-priority major new initiative in nuclear science after the completion of CEBAF. The recommenda- tion is for a collider with an energy of about 30 GeV per nucleon in each beam; its estimated cost would be roughly $250 million (in FY 1983 dollars). A major scientific imperative for such an accelerator derives from one of the most striking predictions of quantum chromodynamics: that under conditions of sufficiently high temperature and density in nuclear matter, a transition will occur from excited hadronic matter to a quark-gluon plasma, in which the quarks, antiquarks, and gluons of which hadrons are composed become "reconfined" and are able to move about freely. The quark-gluon plasma is believed to have existed in the first few microseconds after the big bang, and it may exist today in the cores of neutron stars, but it has never been observed on Earth. Producing it in the laboratory will thus be a major scientific achieve- ment, bringing together various elements of nuclear physics, particle physics, astrophysics, and cosmology. The only conceivable way at present of producing the conditions necessary for achieving quark Reconfinement is to collide the very heaviest nuclei head-on at relativistic energies, thereby creating enor- mous nuclear temperatures and energy densities throughout the rela- tively large volume of the two nuclei. The ability of quarks and gluons to move about within this volume will enable fundamental aspects of quantum chromodynamics at large distances to be tested. It is believed that various exotic features of Reconfined quark matter, such as the production of many "strange" particles and antibaryons, may be observed. In addition to colliding-beam experiments, operation of such a relativistic nuclear collider (RNC) in a fixed-target mode with a variable-energy beam would provide a diversity of important research programs in high-energy nuclear physics, nuclear astrophysics, and atomic physics. Among the most valuable of these would be studies aimed at providing new information on the fundamentally important nuclear matter equation of state at high temperature and density. The Panel endorses the NSAC 1983 Long Range Plan in recommend- ing the planning for construction of this accelerator. Construction should begin as soon as possible, consistent with that of the 4-GeV
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6 NUCLEAR PHYSICS electron accelerator discussed above. Since current funding levels are barely adequate to respond, with the present facilities, to the exciting scientific opportunities confronting the field, we recommend an in- crease in nuclear-physics operating funds sufficient to support the necessary accelerator research and development as well as the opera- tions and research programs at these two new facilities as they come into being. Additional Facility Opportunities The major questions currently facing nuclear physics, including nuclear astrophysics, point to a number of important scientific oppor- tunities that are beyond the reach of the experimental facilities either in existence or under construction. Many of these opportunities might be realized through a variety of upgrades and additions to the research capabilities of existing facilities, and it appears that a reasonable fraction of them could be achieved within the base program envisioned at present. Decisions regarding the relative priorities must be made at the appropriate later times. It should be noted that a number of these important research opportunities could be encompassed by another major new multiuser accelerator, comprising a synchrotron that would produce very intense proton beams at energies of up to tens of GeV, followed by a stretcher ring to produce a nearly continuous spill of protons that would yield secondary beams of pions, kaons, muons, neutrinos, and antinucleons. The intensities of these beams could be typically 50 to 100 times greater than those available anywhere else, allowing a substantial improve- ment in the precision and sensitivity of a large class of important experiments at the interface between nuclear physics and particle physics. Although funding for such an accelerator was not recommended by NSAC, given its commitments to the electron and heavy-ion facilities discussed above, the accelerator remains an important option for future consideration because of the unique scientific opportunities that it would address. Nuclear Instrumentation A serious national problem exists in the area of appropriate contin- ued support for nuclear-physics instrumentation. The NSAC 1983 Long Range Plan notes that the amount spent by the United States for basic nuclear-physics research relative to its Gross National Product is
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EXECUTIVE SUMMARY 7 less than half of that spent in Western Europe or Canada. The effects of this disparity can readily be seen in the quality and sophistication of European instrumentation, which in many instances far surpasses that found in American universities and national laboratories. An increase in dedicated funding for instrumentation at both large and small facilities is therefore deemed essential. Nuclear Theory The closer the link between theory and experiment, and the better the balance in the effort, the more effective they both become in synthesizing a coherent and elegant body of knowledge. Although the NSAC 1979 Long Range Plan stressed the need for increased support of nuclear theory, a comparison of the FY 1984 budget for nuclear physics with the FY 1979 budget shows that during the intervening 5 years, funding for nuclear theory has remained essentially constant as a percentage of the whole (5.8 percent in FY 1984 versus 6.0 percent in FY 19791. We believe that there is still a clear need for a substantial relative increase in the support of nuclear theory, especially in light of the new and challenging frontiers that are opening up in nuclear physics. Progress in current theoretical research depends on substantial access to first-class computational facilities. Extensive calculations based on the complex models describing today's experiments require the large memories and rapid processing capabilities of Class VI computers. Access by nuclear theorists to a major fraction of the time available on a central, well-implemented Class VI computer could initially meet this need. Accelerator Research and Development Accelerator research and development continues to be vital in making progress toward new advanced facilities, and it must be appropriately supported. Among the important new accelerator tech- nologies that are deserving of such support are superconducting materials for various accelerator structures (including main-field mag- nets), the radio-frequency quadrupole pre-accelerator for low-velocity ions, beam coolers for reducing the energy spread of accelerated beams, beams of short-lived radioactive nuclides with intensities that are adequate for nuclear-physics and astrophysics experiments, and a variety of advanced ion sources.
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8 NUCLEAR PHYSICS Training New Scientists Nuclear physics is among the most fundamental of sciences. The applications of its principles and techniques are vital to such diverse areas of the national interest as energy technology, military prepared- ness, health care, environmental monitoring, and materials engineer- ing. To meet these needs and to continue to explore the basic research opportunities in nuclear physics, a steady influx of first-rate young scientists to our universities, national laboratories, and industries is essential. The Panel is concerned about the continuing decline in the number of students pursuing graduate courses in physics, and nuclear physics in particular. The decline has various causes. Its remedy must lie in large measure in the vigorous support of nuclear-physics education from undergraduate to postdoctoral by the federal government. Enriched Stable Isotopes The Calutron facility at Oak Ridge National Laboratory is the major U.S. source of stable isotopes, which are used both in scientific research and in the production of radioactive isotopes needed for biomedical research and clinical medicine. Acute shortages of stable isotopes now exist (some 50 are currently unavailable), and severe funding insufficiencies forecast rapid deterioration in the supply. The worsening shortages could have disastrous consequences in many areas of scientific research as well as in clinical medicine, where stable isotopes are indispensable tools. An important priority is there- fore to replenish the supply of separated isotopes before much nuclear- physics research is crippled. To ensure that the problem is solved, corrective steps must continue to be vigorously pursued, both by the scientific communities affected and by the funding agencies. Nuclear-Data Compilation For more than 40 years, compilers and evaluators have attempted to keep scientists abreast of detailed nuclear data as they become available. With the rapid experimental advances of the last two decades, however, nuclear-data compilations have begun to fall be- hind. Because the costs of this program are relatively small, a modest increase in funding would greatly enhance the ability to maintain a thorough compilation/evaluation effort and to ensure the timely publi- cation of these results in the various formats required both by nuclear physicists and by applied users of radioactive isotopes.
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