simpler probes, which can add more energy and momentum to nuclear excitations; and with more precision, exploring the quantitative limits of our understanding of the nucleus.

A currently very lively frontier of nuclear physics is the one it shares with its younger and precocious sister fieldparticle physics. The quark-gluon theory of elementary particles that has evolved over the past two decades has been enormously successful in describing the properties of these particles. Since protons and neutrons consist of three quarks each, and the mesons, which are the carriers of the nuclear force, are quark-antiquark pairs, the implications of these new ideas about the structure of nucleons on our understanding of the nucleus must be explored.

The fundamental forces governing the physics of nuclei and all subatomic phenomena have recently been embedded in a consistent theoretical framework involving the basic building blocks-quarks and leptons--and their interactions. It is of paramount importance to determine whether this theory is correct or is in need of modification or extension. This requires experiments at high energies in elementary particle physics and precision experiments at low and intermediate energies using nuclei and the tools of nuclear physics. Nuclear physics has provided a testing ground for many implications of the structure of matter and of the basic forces of nature. Some recent examples are limits on the existence of stable free quarks, of light particles such as axions or light Higgs bosons, of neutrino oscillations and neutrino masses, to name only a few.

The expansion of our horizons and knowledge in nuclear physics has had a unique impact on society. The influence of nuclear fission on the modern age is universally known, as is the continuing search for a major new energy source in the form of nuclear fusion. But the applications of nuclear techniques are much more widespread. Nuclear medicine is the fastest growing subfield in medicine, nuclear accelerators find widespread use in fields beyond medicine, ranging from food preservation to the fabrication of integrated circuits and the measurement of the ages of paleolithic artifacts. Nuclear systems measure environmental contaminants with unprecedented accuracy, and nuclear techniques have given life scientists exquisitely sensitive and selective probes. Most important of all, a good fraction of each generation of U. S. nuclear scientists serve society in industry and government, by applying their skills and training in the techniques of the field to further help solve societal problems. For this contribution to continue, and for the continued scientific vitality of the whole field, it is important that the training of young nuclear scientists receive the high priority that it deserves.

The field of nuclear physics is on the threshold of a new phase in its development. A number of new techniques are in the process of being applied to the investigation of areas of the science where our knowledge is at best fragmentary. The next decade is certain to uncover new results, better understanding and improved insights into this major area of our physical universe.

In 1979 the DOE/NSF Nuclear Science Advisory Committee proposed its first Long Range Plan. It is gratifying to see the extent to which the overall recommendations of that Plan have been and are being implemented, even though the level of funding has been severely constrained and some difficult priority decisions have had to be made in order to allow the research programs to continue productively within the available, very limited, resources. The severe lag in accelerator construction of the mid 1970's has been partially corrected and several modest but important construction p.ojects are either underway or just completed. These new capabilities open fresh areas for investigation and should provide us with a new harvest of knowledge over the coming decade.

Especially encouraging is the prospect of a 4 GeV CW electron accelerator which has been a gleam in the eyes of the nuclear physics community for a decade and which was the key goal of the 1979 Long Range Plan. Since 1979, that goal has evolved into specific facility proposals and, in 1983, into the specific recommendation of this Committee for the national electron accelerator laboratory. This facility will play a crucial role in extending the frontiers of nuclear physics to encompass studies of the nuclear dynamics based upon quark degrees of freedom on the interface between nuclear and high-energy physics.

In this chapter we formulate some of the basic questions facing nuclear physics today. These questions span a broad range, including both strong and electroweak interactions and the properties of the physical world from the scale of nuclear forces to the large-scale structure of the universe. Nuclear science deals with the many-body aspects of the strong interactions. It also deals with tests of fundamental theories and symmetries. In particular, the limits of the electroweak theory must be explored and nuclear science has important contributions to make to this effort, as it does also in connection with the role of nuclei and nuclear processes in determining stellar struc ture and in constraining cosmological models.

Our understanding of nuclear structure and nuclear dynamics continues to evolve. Under the impact of improved facilities, new techniques in instrumentation and computing, and fresh ideas we have made substantial progress in the last five years. New simple modes of excitation have emerged, new symmetries are appearing and some completely unexpected new phenomena have been discovered. We may expect this trend to continue in the next decade as new facilities with qualitatively new capabilities will become available for use by nuclear scientists. The identification and characterization of simple modes is a difficult challenge requiring a multiplicity of experimental techniques. But as our knowledge of these modes increases and becomes more complete, it can confirm or alter our understanding of the structure of the atomic nucleus very profoundly. The study of the symmetries inherent in the nucleus has been very rewarding and the insights gained of the nature and the limits of the symmetries have considerable overlap with the study of symmetries in other branches of science. There are many aspects of nuclear symmetries that require further work and hold promise of qualitatively new results, particularly outstanding among these is the pursuit of nuclear structure symmetries in very rapidly rotating nuclei.

Our microscopic approach to a multiparticle system seeks to identify its constituents, to discover and study its elementary modes of excitation, and to describe the

elementary excitations in terms of the constituent nucleons and, to some extent, the other hadrons. But as experimental evidence accumulates to confirm quantum chromodynamics (QCD) as the correct theoretical framework for hadronic phenomena, it becomes clear that the basic degrees of freedom in multihadron systems are those of the constituent quarks. The theoretical description of the nucleus in terms of quark dynamics is not yet tractable. Nor is it the whole story. Many-body problems can only be solved approximately and in building such approximate solutions qualitative physical insight is as important to our understanding as numerical prediction. In studying a given mode of excitation, the preferred description reproduces the essential physics in the simplest most readily-visualized way and the selection of the proper degrees of freedom contains a crucial element of qualitative judgment.

In most low-energy nuclear phenomena, the quark dynamics appear to be well approximated in terms of three-quark nucleonic clusters with meson exchanges. The validity of this approximation resides in the fact that in the center of nuclei the typical internucleon separations seem to be substantially larger than the diameters of the nucleon "bags" within which the quarks are confined. The appropriate description here is then in terms of nucleons and the effective interactions between them. This nucleons-only regime is well known and far reaching: the remarkable success of "conventional" nuclear models in correlating nuclear phenomena has profound importance for QCD-based dynamics. The richness and beauty of the excitations and phenomena it encompasses are central to nuclear physics; the pursuit of the detailed characterization and understanding of these nucleonic excitations will remain of crucial importance even as we dig deeper towards the underlying quark structure.

At somewhat higher energies and for high-precision treatment of electroweak and hadron-induced processes, the description in terms of inert, structureless nucleons breaks down, and a level of description intermediate between that based on structureless nucleons and that based

on quarks becomes valuable. At this level, the nucleons (bound states of three quarks) can exist and propagate in their ground and excited states, and their effective interactions are described in terms of the exchange of mesons (bound states of quark-antiquark pairs). This intermediate level of description has had impressive successes, but its place in many-particle strong-interaction physics is still imprecisely defined, although a deeper level of understanding appears to be tantalizingly close. At still higher energies, as the substructures of baryons and mesons become important, so also does the quark content of nuclei, and the basic features of QCD should begin to emerge. It will certainly be of considerable interest to understand such distinctions more precisely as more experimental results emerge.

One of the greatest challenges facing nuclear physics today is to find and follow the implications of quarks and of QCD in nuclei. This challenge is experimental and theoretical in equal measure. We must design experiments that will reveal the relevant degrees of freedom as clearly and unambiguously as possible; we must attempt to find experimental signatures for modes of excitation in which the quark degrees of freedom participate individually, not merely as the underlying structure of nucleons and mesons. The 4-GeV CW electron accelerator recently recommended for funding will play a crucial part in beginning this endeavor.

The more macroscopic approach to nuclei views them as aggregates of nuclear, hadronic or quark matter. Experimental studies focus on the flow of matter and energy in collisions between such aggregates. What matter densities are achieved in such collisions? How much energy can be deposited in regions of high density and over how large a volume is this energy distributed? As answers to those questions are found, it becomes possible to study the nuclear equation of state over a wide range of density and temperature. We may look for phase transitions in which new forms of hadronic matter are produced and the many-body aspects of QCD are realized as they were in the early Universe, new forms in which quarks are no longer confined into individual nucleons and mesons. In this report we identify this physics as presenting a major new opportunity of fundamental significance.

Another interface area with elementary particle physics stems from the fact that nuclear physics can provide tests of the theory of the electroweak interactions which together with QCD forms the "Standard Model." The electroweak theory has been enormously successful. Its prediction and its limits of validity must be explored and understood, and nuclear physics and the tools of nuclear physics can provide some of the crucial experiments for this purpose.

A nucleus is a quantal system, built from a number of strongly interacting particles, mainly nucleons, but with important admixtures of pions, other mesons, as well as excited states of the nucleons such as the delta isobar. A consequence is the coexistence and interplay in nuclei of an astonishingly rich spectrum of motions, some involving one or a few nucleons, some involving the coherent motion of many nucleons, and still others involving simultaneously pions, deltas and nucleons. But there is considerable order within this complexity. Studies with a variety of projectiles, targets and energies have led to remarkably successful models of nuclear structure. Two aspects of the nuclear many-body system have made this possible. First, there are simple modes of motion of the nucleus which serve as the elementary degrees of freedom for describing more complex motions; and second, reaction probes are available which selectively excite the simple modes. In the past decade many new elementary excitations have been discovered and their properties characterized. The behavior of these excitations, as a function of mass, proton-neutron ratio, angular momentum and energy provides a controlled laboratory in which to test and develop models of nuclear structure.

The Mean Field

Perhaps the most fundamental and conceptually simple of the elementary modes is the motion of a single nucleon in a nucleus. To a good approximation each nucleon moves almost independently in a mean field created by its average interaction with all other nucleons. The goodness of this approximation is the basis of the shell-model

description of nuclear phenomena. Self-consistent Hartree-Fock calculations have now put this picture on a firm theoretical foundation and predict nucleon densities in nuclei that are extremely accurate, deviating significantly from experiment only deep in the nuclear interior.

A similar picture should describe the interaction of a nucleon scattering from a nucleus, but the mean field, or optical model potential, must now contain an imaginary part to account for absorption of the projectile by the nucleus. Accurate measurements of elastic proton scattering cross sections and spin analyzing powers for energies up to 800 MeV have now been made for many nuclei. New experimental techniques yield similar data for neutrons and permit estimates of the difference in the mean field for neutrons and protons, a difference that affects many nuclear phenomena. These data span a broad range of energy and momentum transfer to provide stringent tests for theories of the mean field.

Progress in the derivation of the mean field from a fundamental theory has been made at three levels: in terms of neutrons and protons alone; in a relativistic theory involving implicitly not only the nucleons but also mesons; and finally at the most fundamental level, in terms of the quarks and gluons of QCD.

It is certainly an oversimplification of the nuclear system to consider only nucleon coordinates. Mesons play a central role for the nuclear force; a more complete theory of the nuclear many-body system must explicitly include them and also include relativistic effects. Manybody and multiple scattering theories based on the relativistic Dirac equation are being considered; they have already provided a simple and accurate description of some nuclear properties and scattering. The spin-orbit interaction, crucial to the success of the nuclear shell model, arises naturally from the relativistic treatment, as it does in atoms.

The detailed implementation of relativistic effects in mean field theories awaits future developments. It will be necessary to understand the status of a relativistic field theory based on composite objects rather than on the presumably fundamental quarks; the origin of certain of the mesonic degrees of freedom in the theory; and the meaning of the implied large decrease in nucleon mass in the relativistic nuclear field. Studies of nuclear structure and reactions within this framework will form a major task for the future.

Single Particle and Collective Excitations

In its simplest form, the shell model describes nucleons moving independently, it is not well suited to the descrip

tion of more coherent modes. Allowing for a residual pairwise interaction between valence nucleons yields an interacting shell model with greatly increased predictive power. It provides a remarkably accurate description of low-lying nuclear energy spectra in light nuclei and near closed shells where detailed calculations are practical and its predictions have a general validity even for nuclei far from the valley of stability. A recent measurement of the charge density difference between 205T1 and 206 Pb determined the shape of a specific proton orbital in the deep interior of the lead nucleus as shown in figure II. 1-A; it was found to be well described by the shell model.

These single particle excitations are the basis for the descriptions of more complex phenomena, and must be understood for nuclear excitations to be described on a fundamental basis. At present the location and width of hole states is reasonably well understood only for the two outermost shells, and little is known about deeper lying states. The coming generation of high duty-cycle electron accelerators will allow one to form these in a particularly clean fashion by electron induced knockout of a nucleon, and may even permit definitive studies of short range pair correlations between nucleons. Proton induced knockout reactions provide complementary information through their sensitivity to neutrons and their control of the spin degree of freedom.

The macroscopic collective model is often used to describe phenomena in which large numbers of nucleons move coherently. In this model, the nucleus is treated as a liquid drop, capable of vibrating and rotating. The structure of low lying collective states in most nuclei is dominated by the prolate spheroidal shape. Coulomb excitation is a selective probe for this mode, exciting states with probabilities directly related to this quadrupole deformation. New experimental and analysis techniques involving comparisons of Coulomb excitation with light and heavy projectiles allow an essentially complete determination of the quadrupole structure of nuclei. Other studies have found independent-particle and collective excitations coexisting at low excitation in many nuclei.

New technical developments in gamma-ray spectroscopy involving germanium detectors with antiCompton shields and many-detector arrays, the so-called "crystal ball" multiplicity filters, should allow extension of these studies to still higher spin and excitation. Unusual new collective modes may be discovered in these unexplored regions. Already there may be some indication of octupole, or pear-shaped deformations for nuclei with mass numbers between 220 and 230. In general one hopes to delineate better the interplay of single particle and collective degrees of freedom, and to determine the limits of the macroscopic collective model.

Giant Resonances and Selective Probes

Over the past few years our knowledge of giant resonances has increased dramatically. These resonances are states of the nucleus in which all available nucleons participate coherently. One classifies them into multipoles by the angular momentum L characterizing their vibration (L0, monopole; L = 1, dipole; L = 2, quadrupole; etc.). Since the nucleus has four components, protons and neutrons with spin up or down, each multipole vibration can be of four types, neutrons and protons with particular spin orientations moving in or out of phase. These are classified as isoscalar (neutrons and protons in phase), isovector (neutrons and protons out of phase), electric (no spin coherence), and magnetic (spins parallel).

Because these resonances are expected to be broad and often overlapping, it is imperative to use probes which selectively excite the specific modes. The history of the discovery of the giant resonances - the isovector dipole in the mid 1940's, the isoscalar quadrupole in the early 1970's and many others in the past six years reflects our success in developing such selective probes. Systematic data on the location, width and strength of the electric giant resonances now exist for the isoscalar monopole, quadrupole and octupole and for the isovector monopole, dipole and (possibly) quadrupole. Similar

data exist for the spin excitation or magnetic monopole (Gamow-Teller transition) and higher magnetic multipoles. In addition, a variety of data clearly show that the giant dipole resonance exists even when built upon excited states of the nucleus or states with very high spin. The location, width, strength, and systematic variation with nucleus of these simple giant resonance vibrations provide an excellent testing ground for the unification of macroscopic models based on the bulk properties of the nucleus and microscopic descriptions based on the shell model. The location of the isoscalar electric monopole states, in which only radial or breathing motion occurs, provides by far the best measurement of the resistance of nuclear material to compression. This compressibility is important for testing nuclear matter calculations, for understanding shock wave phenomena in heavy-ion collisions and supernova explosions, and for establishing bounds on the sizes of neutron stars.

An important discovery followed from a property of the nucleon-nucleon force around 200 MeV. When a 200-MeV proton projectile knocks a neutron out of a nucleus in a charge exchange (p,n) reaction, it almost always transfers a unit of spin to the nucleus. This process is then a uniquely selective probe for the spin structure of nuclei. A striking application of this probe is the discovery of the giant spin-vibration or Gamow-Teller resonance in