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10 Recommencled Priorities for Nuclear Physics Federal funding for basic nuclear-physics research in the United States began in the late 1940s, first by the Office of Naval Research and then under the auspices of the Atomic Energy Commission. It contin- ues today under joint sponsorship of the Department of Energy (DOE) and the National Science Foundation (NSF). Without the support of these organizations, this vital discipline could not have made the many significant contributions to basic and applied research that have helped to place the United States in a position of world leadership in science and technology. It is the perception of the Panel on Nuclear Physics, however, that American leadership in our discipline is eroding, owing in part to the aggressive pursuit of major research programs in Europe and Japan. Decisive steps must be taken if the United States is to maintain a position in the vanguard of international research in nuclear physics. In October 1977, the DOE/NSF Nuclear Science Advisory Commit- tee (NSAC) was established in answer to the need for a committee of experts to oversee the general activities and trends in the various subfields of nuclear physics and to make appropriate recommendations to the funding agencies. In 1979 NSAC produced its first Long Range Plan for Nuclear Science; its second Long Range Plan was completed in 1983. The purpose of these studies is to review previous and ongoing programs, evaluate current requirements, and anticipate future needs; they also seek to ensure that existing facilities are maintained and 169
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170 NUCLEAR PHYSICS upgraded appropriately and that new ones are developed to provide the capabilities required for continuing major scientific advances. The Panel met independently and also joined with NSAC during its week-long Workshop in July 1983, when the major draft of its 1983 Long Range Plan was formulated. The recommendations that follow are a result of these extensive interactions and discussions. ACCELERATORS IN NUCLEAR PHYSICS Because accelerators are the basic tools of nuclear physics research, we will briefly review their current status. The probes needed to examine the atomic nucleus are projectile beams of nuclei and subnuclear particles, which must be accelerated to sufficiently high energies to be able to penetrate into or scatter from target nuclei. The projectiles must arrive as a focused beam in the target area, which is often located far from the point at which the beam emerges from the accelerator. One or more detectors are used to record and measure the particles produced by the nuclear interactions. The planning, design, and construction of first-rate accelerators and their associated experi- mental 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. An accelerator's capability for providing beams of a given particle with a specific energy can be described by three parameters: the beam intensity, or the number of particles striking the target per second, expressed as beam current; the energy resolution, or the narrowness of the energy spread of the beam, usually expressed as percent of total energy; and the duty factor, or the fraction of time that particles actually strike the target. Some beams, for example, are pulsed: the duty factor is then the ratio of the pulse duration to its repetition time. Optimizing all three parameters is desirable but seldom possible, so designing a particular experiment requires that decisions be made regarding which of them can or must be optimized. A low beam intensity or a low duty factor can greatly increase the time required to accumulate the number of events (nuclear interactions) necessary to make statistically meaningful measurements. Poor energy resolution restricts the accuracy of measurement attainable. Often a trade-off is made; for example, beam intensity might be optimized at the expense of energy resolution, or vice versa. Accelerators range in size from large, multiuser facilities designed to serve the needs of both resident physicists and users from other
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 171 institutions (both domestic and foreign) to smaller, dedicated univer- sity accelerators. Although the latter are generally also available to outside users, they are more closely tailored to the special require- ments of their own faculties. All of these facilities make it possible to conduct forefront research in nuclear physics while providing for the education and training of undergraduate and graduate students and postdoctoral fellows. Existing Facilities The accelerators in use today provide a wide range of projectiles, energies, and beam intensities for a great variety of research programs. The type of projectile and its energy determine the nature of the information that the experiment will yield. Some experiments require electrons, with their particularly well-understood interactions; others require intense beams of protons or secondarily produced mesons; still others require high-energy heavy ions. The ability to bring such complementary experimental techniques to bear on a variety of re- search problems in nuclear structure and nuclear reactions has been a crucial element in many of the major advances in nuclear physics during the past decade. There are currently nine large, multiuser, national accelerator facilities spanning this experimental range; the two largest are the Los Alamos Meson Physics Facility (LAMPF), a proton linear accelerator at the Los Alamos National Laboratory, and the Bevalac Complex, a relativistic heavy-ion accelerator at the Lawrence Berkeley Laboratory. In addition, 13 dedicated university accelerators are supported primarily for nuclear-physics research and provide specialized probes for their quite diversified research programs. These 22 accelerators (many of which have been substantially upgraded in recent years), their capabilities, and examples of the kinds of research problems for which they are used are summarized in Appendix A. With continuing advances in both physics and technology, it is inevitable that accelerators eventually become obsolete as primary research facilities. Since 1976, federal funding by DOE or NSF for basic nuclear-physics research has been withdrawn from 17 accelera- tors. Although invariably painful and often accompanied by a substan- tial disruption of graduate-student and postdoctoral training, judicious attrition has been necessary for the evolution of the field, in order that pioneering new machines can be built and operated at maximal efficiency. The 22 accelerators described in Appendix A constitute, for the near future, a vital, highly productive, and balanced force for our development of modern nuclear physics. The imperative to push the
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172 NUCLEAR PHYSICS frontiers ever further also demands, however, that major new initia- tives be undertaken. Several of these are described in the following sections. The Planned Continuous Electron Beam Accelerator Facility The electron accelerators designed and built in the 1960s for nuclear- physics research contributed much to our understanding of the distri- bution of electric charge in nuclei, the coherent collective excitations of the nucleus, and the incoherent electrodisintegration of the nucleus. These accelerators, however, had relatively low energy, poor energy resolution, and poor duty factor. In the last decade, a new generation of electron accelerators has produced electrons with energies of up to 750 MeV with excellent energy resolution and with duty factors of 1 to 2 percent an order-of-magnitude increase over those of the earlier machines. Experiments at these facilities have had an enormous impact on our knowledge and understanding of nuclear spectroscopy, meson production, and meson-exchange currents. Over the same period of time, experiments on the lightest nuclei done at the very-high-energy but low-duty-factor machine at the Stanford Linear Accelerator Center suggested the need for a broader view of nuclei, encompassing the quark structure of the nucleons. Significant connections between nuclear physics and elementary- particle physics have emerged from these electron experiments, and it appears that a smooth transition in the behavior of the nucleus occurs with increasing energy. This behavior is well described at low energies by independent-particle models of nuclear structure, which take into account only the nucleons as constituents; at higher energies, account must also be taken of the effects of baryons and mesons and, eventu- ally, of quarks and gluons. Coincidence measurements, in which significant results come from only a small fraction of the total number of events, are of extreme importance in these studies and require accelerators with much higher duty factors than now exist. Higher energies and higher beam intensities are needed to extend investiga- tions to the scale of very short distances, where the nucleus can best be described in terms of its fundamental quark and gluon constituents. This research frontier can be reached by an accelerator producing 4-GeV electrons, an energy that is also sufficient for studying the production of baryon resonances (excited states of nucleons), heavy mesons, and "strange" particles in the nuclear medium. On the basis of both the DOE/NSF Joint Study of the Role of Electron Accelerators in U.S. Medium Energy Nuclear Science (the
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 173 Livingston report, 1977) and its own deliberations, NSAC, in its 1979 Long Range Plan, found a critical need for a high-duty-factor electron accelerator with variable beam energies of up to several GeV. Subse- quently, in the 1983 report of the NSAC Panel on Electron Accelerator Facilities, a specific recommendation for such a machine, to be operated as a national facility, was made: a 100 percent-duty-factor, 4-GeV linear-accelerator/stretcher-ring complex now called the Con- tinuous 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. We conclude this section by quoting from the NSAC 1983 Long Range Plan (A Long Range Plan for Nuclear Science: A Report by the DOE/NSF Nuclear Science Advisory Committee, December 1983, page 751: It is clear that electromagnetic probes will play an increasingly important role in many areas of nuclear physics. Questions about the nucleon-nucleon interaction, about connections to QCD and the quark structure, about the hadronic structure of nuclei, elementary excitations, and nuclear-structure symmetries, all require electromagnetic probes. The new 4-GeV electron facility at NEAL National Electron Accelerator Laboratory, the original name for CEBAF] is clearly the major near-term new initiative in nuclear physics. The Panel on Nuclear Physics endorses the construction of CEBAF THE NEXT MAJOR INITIATIVE: THE RELATIVISTIC NUCLEAR COLLIDER . As discussed in Chapter 7, our increased understanding of the strong interaction between hadrons has led us to believe that, under condi- tions of greatly increased temperature and density in nuclear matter, there will be a transition from excited hadronic matter to a quark-gluon plasma, in which quarks, antiquarks, and gluons will no longer be confined inside individual hadrons but will be free to move about (for about 10-22 second) within a much larger volume. This extreme state of matter is believed to have occurred in nature at the very beginning of the universe, 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. Its production and analysis in controlled laboratory experiments would provide us with scientific information cutting
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174 NUCLEAR PHYSICS across the traditional boundaries of nuclear physics, elementary- particle physics, and astrophysics and would create a common ground on questions relevant to cosmology the universe and our place in it. Present theoretical estimates suggest that collisions of heavy nuclear projectiles with energies of the order of 30 GeV per nucleon can generate temperatures and densities high enough to liberate the quark and gluon constituents of the nucleons and more importantly- to create large numbers of quarks, antiquarks, and gluons from the energy of the collision. At such relativistic energies, the head-on collision of two heavy nuclei will create an extremely hot, dense region of nuclear matter encompassing hundreds of cubic fermis in volume. The enor- mous energy density achieved throughout this large volume will constitute a unique combination of conditions not available in the collisions of electrons, protons, or light nuclei-for creating the quark-gluon plasma. The accelerator needed to produce these condi- tions, a relativistic nuclear collider (RNC), would be the world's highest-energy accelerator capable of providing nuclear beams over the full range of the periodic table, from hydrogen to uranium. Although the production of the quark-gluon plasma- in the regions of both high energy density (the central region) and high baryon density (the fragmentation regions)- would represent a major focus of re- search at the RNC, this accelerator would provide many additional new research opportunities in nuclear physics, including the following: · Extension of the study of quantum chromodynamics (QCD) to large distances (roughly the diameter of a nucleus), complementing its study at very short distances (less than the diameter of a nucleon), in which electrons or hadrons are used as probes. · The possibility of studying conditions under which the masses of the light quarks go to zero (as predicted by QCD) and the states of the system of quarks obey a right-hand/left-hand symmetry (chiral sym- met~y). · The first opportunity for investigating the dynamics of extended objects with very-high-energy density conditions that can be achieved only in relativistic nuclear collisions. · The possible production of exotic objects, such as free quarks (with fractional electric charge), quark "globs" with unique topological (structural) properties or exceptionally high strangeness, and Centauros mysterious events, observed in very-high-energy cosmic- ray studies, that produce few or no neutral pions, which suggests a hitherto unknown kind of nuclear interaction.
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 175 In addition to producing colliding nuclear beams for a dedicated program of study of the quark-gluon plasma, the RNC should also have the capability for a variety of fixed-target experiments at energies of the order of 30 GeV per nucleon. Some examples demonstrating the breadth of this fixed-target research program are the following: · Production and study of radioactive nuclei far from the valley of stability and their use as exotic secondary beams. · Development of a rich program of nuclear physics with very heavy systems at relativistic energies, using intense beams to investigate rare processes, such as coherent pion production (from a pion condensate, for example). · Investigations of highly excited hadronic matter (in which the quarks and gluons are confined), providing new opportunities for deducing the equation of state of nuclear matter under conditions far from normal. · Creation of the maximum possible baryon density achievable in a laboratory experiment, thereby opening a new avenue of experimental research in nuclear astrophysics. · Studies of few-electron, very heavy ions, enabling new domains of quantum electrodynamics to be tested. Recommendations from the NSAC 1983 Long Range Plan Because the long-range plans for nuclear physics were reviewed by the Nuclear Science Advisory Committee in 1983, it is important to state the Committee's major recommendation for new facility con- struction, taken from the summary (page vi) of its 1983 Long Range Plan: Our increasing understanding of the underlying structure of nuclei and of the strong interaction between hadrons has developed into a new scientific opportunity of fundamental importance-the chance to find and to explore an entirely new phase of nuclear matter. In the interaction of very energetic colliding beams of heavy atomic nuclei, extreme conditions of energy density will occur, conditions which hitherto have prevailed only in the very early instants of the creation of the universe. We expect many qualitatively new phenomena under these conditions; for example, a spectacular transition to a new phase of matter, a quark-gluon plasma, may occur. Observation and study of this new form of strongly interacting matter would clearly have a major impact, not only on nuclear physics, but also on astrophysics, high-energy physics, and on the broader community of science. The facility necessary to achieve this scientific breakthrough is now technically feasible and within our grasp; it is an accelerator that can provide colliding beams of very heavy nuclei
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176 NUCLEAR PHYSICS with energies of about 30 GeV per nucleon.... It is the opinion of this Committee that the United States should proceed with the planning for the construction of this relativistic heavy-ion colliderfacility expeditiously, and we .~ I' n.c the hi~hest-Drioritv new scientific opportunity within the purview of our science. The Panel endorses the NSAC 1983 Long Range Plan in recommend- ing the planning for the construction of an accelerator that can provide colliding beams of very heavy nuclei at energies of the order of 30 GeV per nucleon with which to create the extreme conditions of nuclear matter described above. The cost of this facility, including initial major detectors, is estimated to be $250 million (in FY 1983 dollars), with a construction period of 4 to 5 years. Operating and research costs are estimated at $35 million per year. Research and development will be needed to refine the design of this accelerator and specify its costs. Once designed, construction should begin as soon as possible, consis- tent with that of the 4-GeV 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 increase in nuclear-physics operating funds suffi- cient to support the necessary accelerator research and development as well as the operations and research programs at these two new facilities as they come into being. Complementary Aspects of CEBAF and the RNC Both of the new accelerators being planned by the United States nuclear-physics community the Continuous Electron Beam Acceler- ator Facility (CEBAF) and the relativistic nuclear collider (RNC - will address extremely important questions concerning the quark aspects of nuclear matter. The theoretical and experimental research programs at these two accelerators will be dramatically different, however (see Figure 10.14. Using intense beams of high-energy electrons, CEBAF will probe the short-range behavior of quarks in nuclei with surgical precision. It will do this by implanting a localized, well-understood electromagnetic disturbance in the nucleus and measuring the response of the nuclear environment to this stimulus. Electrons, being pointlike particles, are well suited to such studies. They will act as a powerful microscope, able to focus on the ways in which the quark substructure affects the properties and interactions of nucleons residing inside the target nucleus.
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 177 (a) Continuous Electron Beam Accelerator Facility (CEBAF) Nucleus - (b) Relativistic nuclear collider (RNC) .. q ~ mu. Nucleon "- ~ ,~, \~e' r e FIGURE 10.1 The complementary aspects of CEBAF and the RNC. (a) CEBAF will test the response of nuclei to high-energy, pointlike disturbances caused by the interaction of electrons with quarks, over distances much less than 1 fermi. (b) The RNC will test the response of heavy nuclei to the high energy densities created throughout large volumes (hundreds of cubic fermis) when they collide head-on at relativistic velocities. The RNC, on the other hand, will cause beams of heavy nuclei to collide violently with each other. These nuclei are relatively large objects, with volumes of up to several hundred cubic fermis. When they collide head-on, all the nuclear matter can interact and be heated to such enormous temperatures and energy densities that the quarks and gluons become Reconfined from the nucleons, and large numbers of quarks, antiquarks, and gluons are created. These particles can then move about inside a relatively large volume the quark-gluon plasma. It is expected that the macroscopic behavior of quarks will be revealed under these conditions. Thus, to see how quarks will modify and extend our understanding of nuclear physics, both of the accelerators are needed-to elucidate both the microscopic and the macroscopic aspects of quarks in nuclear matter.
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178 NUCLEAR PHYSICS FURTHER RECOMMENDATIONS In evaluating the prospects and promise for nuclear-physics research in the next decade, it is also vital to consider facilities and opportunities beyond the construction of the two major new accelerators discussed above. Our analysis of the current state of nuclear physics leads us to make the following recommendations for other important aspects of the field. - Additional Facility Opportunities A number of additional opportunities are under discussion in the nuclear-physics community. The most important ones are listed in Table 10.1. Here it is again appropriate to quote from the summary (page v) of the NSAC 1983 Long Range Plan: The major questions facing nuclear physics point to a number of important scientific opportunities beyond the reach of the facilities in existence or under construction. Many of these opportunities may be attained by a variety of possible upgrades and additions to the capabilities of present facilities. Among these are the capability for high-resolution continuous (COO) electron operation below 1 GeV, substantially enhanced kaon beams, improved medium-energy neutnno capability, antiproton beams, improved proton beams of variable energy between 200 and 800 MeV, and also above 800 MeV, intense neutron sources with energies up to a few hundred MeV, capabilities for accelerating very heavy ions with easily varied energy between 3 and 20 MeV per nucleon, a high-intensity pulsed muon facility, and a number of other options. We estimate that a reasonable fraction of these opportunities can be realized within the currently envisioned base program. Decisions on relative priorities should be made at a later time and with more specific proposals in hand. It should be noted that a number of the capabilities listed in Table 10.1 (specifically, the second, fifth, sixth, and eighth items), addressing many of the physics topics mentioned above, could be encompassed by another major new multiuser accelerator. As currently envisioned, such an accelerator might comprise a synchrotron producing 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 improvement in the precision and sensitivity of a large class of important experiments at the interface between nuclear physics and
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 179 TABLE 10.1 Additional Facility Opportunities for Nuclear Physicsa Research Program (Examples) Structure of elementary nuclear Capability Required excitations; form of nuclear momentum distributions; nature of long-range and medium-range nuclear interactions Spin dependence of the nuclear interaction; fundamental symmetry tests; nuclear structure at high- momentum transfer Microscopic optical model; nuclear structure and nuclear shape transitions; studies of Gamow-Teller resonances Nuclear spectroscopy of isotopes far from stability; nuclear astrophysical reaction rates; search for exotic nuclei and superheavy elements Hypernuclear physics; rare kaon decays and other weak interaction studies; exotic atoms Tests of electroweak interactions; weak interactions of leptons with nuclei; muon spin resonance studies of solids Energy dependence of nuclear-reaction mechanisms; multiparticle decay of highly excited compound nuclei; giant resonances Nuclear physics with antinucleons; antinucleon-nucleon interactions to study few-quark dynamics; anti- nucleon atomic systems Nuclear astrophysics solar neutrons measurements; neutr~no oscillations High-duty-factor electron beams with good energy resolution at energies below 1 GeV High-quality, high-intensity polarized proton beams spanning in stages the energy range from 50 MeV to several GeV Secondary neutron beams (polarized and unpolarized) with good intensity and energy resolution at energies of up to several hundred MeV Intense secondary beams of radioactive nuclei Intense kaon beams of high purity Intense muon and neutrino beams of high quality Heavy ions through uranium, at energies between 10 and 100 MeV per nucleon Low-energy and medium-energy antinucleon beams Solar neutr~no detector sensitive to low- energy (less than 300-keV) neutrinos a The sequence of items is not intended to suggest relative priorities. particle physics. In particular, many experiments that are currently impractical because of low count rates or cosmic-ray backgrounds would become possible. In this context, we quote once more from the NSAC 1983 Long Range Plan (pages 74-751: A major new "Kaon Factory," a 10-30-GeV proton accelerator with 10~4-lO`5 protons per second, would provide substantial opportunities for
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18O NUCLEAR PHYSICS physics in all of these areas. This physics is clearly very fundamental, important, and exciting. Given our commitment to the construction of the National Electron Accelerator Laboratory [now called the Continuous Elec- tron Beam Accelerator Facility] and the heavy-ion collider discussed above, the financial assumptions of this report preclude a major additional facility. But as circumstances change, we want to keep this important option readily available: it clearly presents many unique opportunities. 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 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. Examples of the need for new equipment abound. Obtaining infor- mation about the de-excitation of high spin states formed in heavy-ion- induced reactions requires the use of large, spherical arrays of scintil- lation detectors called crystal balls. The study of relativistic heavy-ion collisions requires large-mass, fine-grained detectors that allow the simultaneous localization, tracking, identification, and energy detec- tion of large numbers of emitted particles. Magnetic spectrometer systems have been steadily improving in performance, and even greater improvements (as well as significant cost reductions) can be made by using superconducting magnets. Studies of effects arising from the aligned spins of particles require both polarized targets and ion sources that will efficiently produce high-intensity polarized beams. Equally pressing is the need for advances in data reduction techniques, as the number of measured parameters grows with the increasingly complex experiments. Research and development programs are also necessary to deter- mine the most effective solutions for the rapidly increasing require- ments for sophisticated instrumentation. Higher-energy beams, for example, will require the development of detector systems whose capabilities far exceed those that have been used in nuclear physics to date. An extensive research and development program for the imple- mentation of detectors at the CEBAF will be needed, as well as a
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 181 program to develop detectors with large solid angle, high segmentation, and good particle identification for the RNC. Nuclear Theory In nuclear physics, as in all other branches of physics, theoretical work provides both interpretation and guidance. Although in every field of science there are always some experiments that produce significant and sometimes dramatic progress in and of themselves, steady progress is made for the most part through the informed choice of experiments. Theorists working closely with experimentalists can provide direction in the best choice of experiment by suggesting what the most critical test of a concept would be and the measurements or conditions that would make a complete theoretical analysis feasible. The closer the link between theory and experiment, 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 current 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 19794. 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. Among these are the study of the behavior of nuclear states ever farther from stability, the study of the nonnucleonic substructure of nuclei, the search for the quark-gluon plasma, and the increasing interaction between nuclear physics and particle 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 meeting the need for new advanced facilities and should be appropri
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182 NUCLEAR PHYSICS ately supported. One of the most important recent breakthroughs has been the successful use of superconducting materials in accelerators. Radio-frequency (rf) superconductivity is now an established technol- ogy, with numerous applications to electron acceleration and to heavy-ion beam bunching and acceleration. Other superconducting structures are also currently being investigated. For example, the University of Illinois Nuclear Physics Laboratory is using a superconducting linear accelerator (developed at Stanford) in a microtron, and two superconducting rf linear accelerators are now in operation as postaccelerators at Argonne and at SUNY-Stony Brook. In a related area, the extremely strong magnetic fields obtained from superconducting magnets reduce the size, the power requirement, and hence the cost of cyclotrons that use them for the main field. Two superconducting cyclotrons were begun in the mid-1970s. One is now in operation at Michigan State University; the other, at the Chalk River Nuclear Laboratory in Canada, will be operating in the near future. A fundamentally new type of accelerator for low-velocity ions, the radio-frequency quadrupole, has been pioneered at the Los Alamos National Laboratory. Based on a theory originally developed in the Soviet Union, it makes use of advanced techniques to capture more than 90 percent of the beam from the ion source. It is an extremely efficient preaccelerator for a larger accelerator and is currently being developed at various laboratories in the United States and around the world. Borrowing a technique developed by elementary-particle physicists, scientists at the Indiana University Cyclotron Facility are adding a beam cooler a storage ring in which the accelerated beam will be circulated and "cooled" via interaction over part of the ring with a collinear electron beam of the same velocity to reduce greatly its energy spread. This will provide a previously unmatched level of precision for experiments with high-energy protons. The technique represents a cost-eiTective way to achieve unusual capabilities at other accelerators as well, and it is likely to be extensively developed in the near future. Studies are in progress to devise elective methods for producing beams of short-lived radioactive nuclides with intensities that are adequate for nuclear-physics and astrophysics experiments. For exam- ple, radioactive beams can be obtained by methods in which the desired nuclide is produced as a low-energy fragment from the target of a primary beam in a bombardment reaction, captured in an ion source, ionized, and finally accelerated toward a second target. In another, more direct method, the radioactive nuclides emerge at relatively high
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 183 energy from a suitable primary target in the form of a secondary beam that can be used as is or accelerated or decelerated to different energies. The development of new ion sources has been rapid in the last decade. The electron-cyclotron-resonance ion source and the electron- beam ion source, both of which underwent their pioneering develop- ment in Europe, are currently being put to use in the United States. Along with various schemes for laser-driven ion sources and polarized ion sources, they will be important elements of future nuclear-physics research programs. Training New Scientists The Gardner report on excellence in education (A Nation at Risk: The Imperative for Educational Reform, The National Commission on Excellence in Education, U.S. Government Printing Office, Washing- ton, D.C., 1983) points out that for the first time in U.S. history, the educational skills of a generation not only do not surpass those of the previous generation, they do not even approach them. These educa- tional deficiencies, coming at a time when the demand for high technical skills is accelerating, can result in the loss of America's place of world leadership in intellectual achievement, technical innovation, and material benefits. The report contends, furthermore, that the security of the United States depends on the government's nurturing of its intellectual capital. To maintain the highest level of achievement by their students, colleges and universities must offer the best possible learning tools. The report states that: "The Federal Government has the primary responsibility to identify the national interest in education. It should also help fund and support efforts to protect and promote that interest." It recommends that the government provide student finan- cial assistance and research and graduate training with a minimum of administrative burden and intrusiveness. In addition to the general decline of trained personnel, a marked decrease in the number of students pursuing graduate courses in physics, and nuclear physics in particular, has become evident since the early 1970s. If this trend continues, it promises to leave the field seriously deficient in skilled scientists. The causes of the decline, although varied, must certainly include as contributing factors the severe financial problems faced by many colleges and universities. This results in diminished financial aid for students, the loss of dedicated, on-site accelerator facilities (indispensable tools for the teaching of
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184 NUCLEAR PHYSICS nuclear physics), and the reduction of new academic positions (which is intensified by the current low retirement rate in university faculties). Futhermore, many who do obtain higher degrees in physics are attracted by the much higher salaries in industry and are thus lost to basic research. Some recommendations to offset these tendencies are the following: · Attract students to nuclear physics by funding undergraduate nuclear-science research programs and by arranging for the participa- tion of secondary school students in introductory studies. · Increase National Science Foundation predoctoral fellowships in general, and establish a specific program of Department of Energy fellowships in nuclear physics. · Increase the emphasis on support of new research initiatives by awarding 3-year funded grants for proposals submitted by young scientists past the postdoctoral stage. · Increase the funding for university research groups to enable them to hire their own nonacademic staff, such as scientists or engineers specializing in technical problems. · Instigate a program of temporary support of tenure-track faculty positions to sustain nuclear physicists during the present period of low university retirement rates. · Consider the educational aspects of new facilities where practica- ble; they should attract the highest-caliber graduate students and give them the best possible training. Enriched Stable Isotopes The Calutron facility at Oak Ridge National Laboratory (ORNL) 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. Several stable isotopes can occur in a chemical element; the isotope of interest, which may constitute only a minute fraction of the total material, must be carefully separated and purified from contamination by other isotopes. The electromagnetic separation method used at ORNL is notable for its ability to respond to changing demands; it represents an invaluable national as well as international resource. The only comparable elec- tromagnetic separation facility is in the Soviet Union. Acute shortages of stable isotopes now exist (some 50 are currently unavailable from ORNL), and severe funding insufficiencies forecast rapid deterioration in the supply. The worsening shortages could have
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RECOMMENDED PRIORITIES FOR NUCLEAR PHYSICS 185 disastrous consequences in many areas of scientific research as well as clinical medicine, where stable isotopes are indispensable tools. The importance of enriched isotopes in nuclear-physics research derives from the specific properties of the isotope in question. Virtually all nuclear studies require separated isotopes, because the properties of a nucleus can change drastically with the addition or removal of a single nucleon. Consequently, an important priority is to replenish the supply of separated isotopes before much nuclear-physics research is crip- pled. 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 behind. The continuing need for timely, cost-effective, and high-quality evaluations led in 1976 to the formation of an international evaluation network under the auspices of the International Atomic Energy Agency. The network consists of 16 data centers in 11 countries; each center is responsible for the evaluation of specified information in order to avoid costly duplication of effort. All evaluated data are published in Nuclear Data Sheets or Nuclear Physics and are entered into the computerized Evaluated Nuclear Structure Data File maintained by the National Nuclear Data Center at Brookhaven National Laboratory. These data do not include a comprehensive compilation of charged- particle cross sections, however; the need for such a compilation exists in many areas of research, both basic and applied. In addition to participating in the international network, the five United States data centers coordinate their activities through the U.S. Nuclear Data Network. These activities are funded primarily by the Department of Energy (DOE) and are reviewed annually by the National Academy of Sciences' Panel on Basic Nuclear Data Compi- lations, which is advisory to DOE. 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 publication of these results in the various formats required both by nuclear physicists and by applied users of radioactive isotopes.
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Representative terms from entire chapter: