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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.
Representative terms from entire chapter:
nuclear matter
