Nuclear Structure Research at Argonne
Gamma-Ray Spectroscopy Studies
The major part of the work done in gamma-ray spectroscopy at Argonne
deals with the study of all facets of superdeformation, primarily in
the mass 190 region, but also in the mass 150 and 80 regions. Our work
covers not only the nature of states inside the SD minimum, but also
elucidates the physics related to the feeding into and decay from this
minimum. There is also a diverse program on non-superdeformed nuclei,
which covers aspects such as: tests of the cranked shell model and
other theories describing high-spin phenomena, quenching of pairing
with temperature, a search for double octupole-phonon states in 208Pb,
conservation of the K-quantum number at high spin, phase transitions in
mesoscopic systems, structure of high-lying states in actinide nuclei,
spectroscopy of fission fragments, and structure of nuclei far from
stability (conducted with the FMA ).
New large gamma-ray detector arrays are currently under construction in
the United States (Gammasphere) (GS) and in Europe (Eurogam). These
new arrays provide new opportunities for nuclear structure research.
Argonne is participating vigorously in the construction of Gammasphere
and has performed experiments with the so-called "early implementation
phase" of the device in 1993-1995. The group is also collaborating in
several experiments at Eurogam. Results from these experiments show
clearly the promise and power of these devices. At Argonne we
developed a battery of programs to analyze the new high-fold data from
these instruments.
The main research tool at ATLAS for this program is the Argonne Notre
Dame BGO gamma-ray facility which consists of 50 hexagonal BGO detectors
(used mainly as a sum-energy/multiplicity filter) surrounded by 12
Compton-suppressed Ge detectors. Auxiliary equipment include: a
scattering chamber, constructed by the University of Kansas, for
coincidence measurements between gamma rays and particles; a plunger
apparatus, developed by Notre Dame, for recoil-distance measurements of
nuclear lifetimes; and dedicated chambers for special experiments
(g-factor measurements, fission-fragment coincidence measurements,
etc.). A rare capability exists at ATLAS for performing gamma-gamma
coincidence experiments with the Fragment Mass Analyzer ( FMA (Fragment Mass Analyzer) for this purpose. A support for up to 7
Compton-suppressed spectrometers at the magnetic spectrograph is also
available.
Superdeformation Studies:
The occurrence of an excited secondary minimum at large deformation
provides a rare opportunity to study nuclear states which are cold,
although highly excited with respect to the normal yrast line. Within
the superdeformed well, isolated from normal states with smaller
deformation, there is a cold "ground" state, as well as low-lying
excited states which can give rise to sharp equally-spaced
transitions. With increasing excitation energy, the coupling with
states outside the well grows until the separate identity of SD states
melts away; in addition, the collective properties may be altered.
When the SD "yrast" state lies high enough above the true (normal)
yrast state, then a coupling occurs between a cold system with a hot
normal one, causing the SD band to decay.
Research on superdeformation at Argonne addresses the physics
associated with states within the SD well and their coupling with
states outside the well. Investigation of the decay out of SD states
into lower-lying normal states allows us to examine the coupling
between a cold, ordered system and a hot, chaotic one. Discrete line
spectroscopy investigates the cold SD bands and tries to determine
their properties, in particular the nature of the excitations in the SD
well. Study of the feeding of SD bands and of the associated
quasicontinuum gamma rays probes the nature of excited states and their
increasing mixing with normal states.
Decay of SD Bands
We solved the long-standing problem of decay from SD bands by using a
novel approach. Instead of trying to decipher the fragmented decay
pathways, we instead measured the complete spectrum of gamma rays
decaying out of the SD band. We were able to characterize the decay
mechanism, and define experimental excitation energies and spins for a
SD band in 192Hg. We have extracted the complete spectrum of the gamma
rays linking states in two separate wells in a number of mass 190
nuclei. The spectra, which have a quasicontinuous distribution with
superimposed broad structures and sharp peaks, establishes the decay
mechanism as due to mixing of a SD state with some of the sea of normal
states in which it is embedded. We propose that a conspicuous
clustering of gamma strength between 1.4 and 2.2 MeV is due to a
rearrangement of the level densities by pair correlations. A model was
developed to calculate levels from all quasiparticle excitations, as
well as the ensuing statistical spectrum from a highly excited state.
The calculated statistical spectra reproduce the observed features of
the decay spectra, including the differences in even-even and odd-even
nuclei. Thus, the decay spectra from SD states are serendipitous
probes for the quenching of pairing with temperature.
Cold States
We found 23 SD bands in the mass 190 region from work done at ATLAS,
Gammasphere and Eurogam. We were the first to establish this region
as a new "island" of superdeformation, and have focused much of our
effort on superdeformation here. This large body of data was vital in
helping to identify the occurrence of "identical" bands, i.e. SD bands
in neighboring nuclei which have transition energies with DE < 1/500,
or which have identical dynamic moments of inertia J(2). The identical
bands, which were not anticipated, are still not explained, but may
imply a symmetry which has yet to be identified. Another striking
observation is a staggering of alternate levels in three SD bands in
194Hg, which suggests the presence of a Y44 symmetry (i.e. four-fold
symmetry in a plane perpendicular to the symmetry axis). In addition,
we discovered a band in 151Dy, with energies midway between those in
152Dy, which provides additional evidence for pseudospin symmetry. In
154Dy we found a SD band which has energies identical to those of an
excited SD band in 153Dy and is the first SD band found to decay to
prolate collective normal states.
Identification of vibrational states in the SD well can serve to
establish the rigidity of the deformation with respect to beta, gamma or
octupole distortions. Theory has pointed out that SD nuclei may
manifest octupole instability. We found the first indications for an
octupole vibrational band in 190Hg (from Gammasphere data) at a
surprisingly low energy (~ 600 keV) above the yrast SD band.
We measured lifetimes of individual states of SD bands in 192,194Hg
which prove that the deformation is indeed large and that it is stable
with respect to spin and particle excitation.
Excited SD States
From Eurogam and Gammasphere data we established that excited SD bands
give rise to a pronounced E2 bump in the gamma spectrum. This feature
allows us to probe the collective properties of excited SD states.
There are preliminary indications that the quadrupole moment and moment
of inertia of the excited SD states are larger than those of the yrast
SD band in 192Hg. We studied the coupling of excited SD and ND states
and the mechanism for the unexpectedly large population of SD states by
both experiment and theory. We are able to reproduce by Monte Carlo
simulations all observables connected with the feeding: band
intensities, variation of intensity with spin, entry distribution (in
spin and energy) of states leading to trapping in the SD well, and the
spectra of feeding gamma rays.
Shape Changes in Nuclei
Research on the evolution of the nuclear shape as a function of spin
and excitation energy along the yrast line and its vicinity
concentrated mainly on nuclei near A = 190, i.e. in the region where
most of the superdeformation studies by the Argonne group are carried
out. This region is of particular interest because, as one gets close
to the Z = 82 closed shell, the occupation of specific orbitals is
expected to have a large effect on the overall nuclear shape. This
region is also one of the very few where the cranked shell model can be
tested in the limit of oblate collective rotation. In fact, in some
nuclei of this region, collective bands associated with prolate and
oblate collective shapes were shown to coexist. Thus, the cranked
shell model can be tested in both the prolate and oblate limits in a
single nucleus. Recently, our studies concentrated on the Hg isotopes
with A = 188-191 which involve mainly neutron excitations and on Pb
nuclei with A = 192,195,196 where proton excitations turn out to be
particularly intriguing. Indeed, sequences of M1 transitions were
observed in these nuclei. Insight into the associated quasiparticle
configurations was obtained not only from the usual level properties
(spins, excitation energies etc..), but also from the detailed
measurement of lifetimes.
The Fragment Mass Analyzer ( FMA ) is now fully operational at ATLAS and
can be used in conjunction with 10 Compton-suppressed Ge detectors for
spectroscopy studies in nuclei located far from the valley of
stability. Experiments this year yielded results on 189,187Pb, 179Au,
181Hg, 194,195,197Po and 202,204Rn. A common theme in these cases is
shape coexistence. All projects at the FMA involve strong
collaborations with outside users.
Finally, we list other aspects of the research program, many of which
involve major efforts by collaborators from outside institutions.
These include (1) the search for 2-octupole phonon excitations in
208Pb, (2) the study of the decays of high-K isomers in 176W, (3) the
study of neutron-rich nuclei from the prompt radiation of fission
fragments, (4) the study of quasiparticle excitations in neutron-rich
Sn nuclei following complex heavy-ion-induced reactions involving the
exchange of several nucleons between the target and the projectile, and
(5) the study of shape-driving orbitals in Pt-Ir-Au nuclei through
lifetime measurements.