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.