OPPORTUNITIES IN NUCLEAR SCIENCE A Long-Range Plan for ...

THE SCIENCE • ATOMIC NUCLEI: STRUCTURE AND STABILITY
nuclear properties with increasing excitation energy provides
a fascinating quantum analog to the general problem of transitions
from ordered to chaotic motion. Very neutron-rich
nuclei, far from the common isotopes found in nature, open
the door to investigations of the often-unusual properties of
weakly bound quantum systems. For instance, such nuclei
can include regions characterized by entirely new forms of
low-density, spatially extended, nearly pure neutron matter
akin to that on the surfaces of neutron stars.
Recent advances in understanding the nucleus. Major
conceptual and technical advances have revolutionized the
study of nuclear structure over the past ten years, and
nuclear structure studies have flourished with measurements
and calculations aimed at nuclei both near and far
from the valley of stability. Recent significant advances in
several areas can be briefly summarized:
• Shell structure in exotic nuclei. Investigations of
nuclear shell structure far from stability are fundamental
to our understanding of nuclei and their synthesis
within the cosmos. Recent landmark experiments
include the observation of the doubly-magic unstable
nuclei 48 Ni (Z = 28, N = 20) and 78 Ni (Z = 28, N = 50).
In lighter neutron-rich nuclei, spectroscopic studies
have demonstrated clear evidence for a reordering of
nucleonic shells; for a weakening of the familiar shell
closures around N = 8, 20, and 28; and for the emergence
of a new shell gap at N = 16 in the most neutronrich
nuclei. These studies provide the first indications
that our models of nuclear structure, developed near
beta stability, are not adequate for nuclei with large
neutron excesses. In the most proton-rich nuclei,
where, counterintuitively, a strong Coulomb force
inhibits charged-particle emission, proton decay has
rapidly evolved from an exotic phenomenon to a powerful
spectroscopic tool. First signatures of a new
form of pairing have been seen in nuclei with equal
numbers of protons and neutrons, and a new decay
mode, nonsequential two-proton radioactivity, has
been discovered.
• Collective excitations. We gain insight into the properties
of nuclei by establishing and studying their
basic modes of excitation. Recent advances include
the discovery of the first candidates for the new collective
modes of chiral rotation and wobbling motion
in triaxial nuclei. Dynamical symmetries of the
nuclear Hamiltonian have been explored, and the
properties of nuclei with very elongated (superdeformed)
shapes have been elucidated. In particular,
light superdeformed nuclei have been discovered.
They provide a unique opportunity to study the
underlying microscopic structure of collective rotations.
Definitive excitation energies and quantum
numbers have been determined for the first time in
the key superdeformed nuclei 152 Dy and 194 Hg.
• Synthesis, structure, and chemistry of the heaviest
elements. The discovery and investigation of the
heaviest nuclei test our understanding of which combinations
of neutrons and protons can give rise to
long-lived superheavy nuclei, and extends the periodic
table, fundamental to all of chemistry.
Significant achievements within the last several years
include the synthesis of new superheavy elements;
the first chemical studies of seaborgium (Z = 106),
bohrium (Z = 107), and hassium (Z = 108); and the
first in-beam gamma-ray spectroscopy of the transfermium
nucleus nobelium (Z = 102).
• Nuclear structure theory. Enormous progress has
been made in the microscopic description of nuclei,
including ab initio calculations for light nuclei and
advances in the shell model and mean-field theory to
include improved effective interactions and coupling
to the continuum for studies of weakly bound systems.
These advances enabled theorists to make
detailed quantitative predictions of structure and
reaction aspects of nuclei.
Looking ahead: A concise roadmap. Current progress in
understanding the properties of nuclei is impressive, and the
field is poised for significant breakthroughs over the next
decade. A deeper understanding of the atomic nucleus will be
achieved by efforts in both nuclear structure theory and
experiment, where each discipline takes its inspiration from
the other. But the impetus in both areas lies increasingly with
studies of nuclei under extreme conditions, especially those
with extreme proton-to-neutron ratios. In the near and intermediate-term
future, complementary studies both near and
far from stability will be pursued at stable-beam accelerators,
such as those at Argonne and Berkeley Lab and at a number
of universities, as well as at existing dedicated exotic-beam
facilities, especially HRIBF at Oak Ridge and the new projectile
fragmentation facility of the NSCL at Michigan State. In
the longer term, the properties of the new and currently inaccessible
rare isotopes that inhabit the very boundaries of the
29