Session DG - Nuclear Structure II.
ORAL session, Friday morning, October 06
General Jackson's Redoubt, Fort Magruder Conference Center

[DG.001] First Evidence of Excited States in ^126Pr

Excited states in odd-odd ^126Pr have been observed for the first time making it the lightest odd-odd praseodymium nucleus in which rotational structures have been reported. Two experiments were performed using the same ^40Ca + ^92Mo reaction. In one experiment, the recoil mass seperator at Oak Ridge National Laboratory was combined with the new CLARION Ge and HYBALL CsI arrays to investigate nuclei near the proton drip line. Another experiment utilized Gammasphere, in conjucntion with the Microball, to observe the highest spin states possible in these proton-rich nuclei. Evidence for two bands, one doubly decoupled and the other strongly coupled, based on the \pi h_11/2\nu h_9/2 and \pi h_11/2\nu h_11/2 excitations, respectively, has been observed. Systematics of the lightest odd-odd Pr nuclei will be addressed.

[DG.002] Study of the low-lying structure of ^128Ba by gamma-gamma angular correlations

The low-lying structure of the proton-rich even-even Ba isotopes is of special interest for nuclear structure studies, as these nuclei are situated in a transitional U(5)-O(6) region. In this work we report measurements of gamma-gamma angular correlations in the ^128Ba nucleus, following beta decay of ^128La and using the Yale Moving Tape Collector. The ^128La parent nuclei were produced by the reaction ^116Sn(^16O,1p3n) at the WNSL tandem accelerator at Yale University. The reaction products were deposited on an aluminized Kapton tape, and transported for out of beam measurement to the center of an angular correlation setup consisting of three clover HPGe detectors. The clover leaves were considered as separate detectors, so in fact a twelve-detector system in compact geometry was obtained. The E2/M1 mixing ratios for several transitions between low-lying states as well as new spin assignments will be discussed and compared to theoretical models. This work was supported by the US DOE under grants DE-FG02-91ER-40609, DE-FG02-88ER-40417 and DE-AC02-98-CH10886.

[DG.003] Chiral doublet search in odd-odd N=73 isotones in the A\sim130 region

Through a series of experiments on N=75 isotones of ^134Pr, namely ^130Cs, ^132La, and ^136Pm, we have systematically identified positive-parity \DeltaI=1 bands which are proposed as chiral partners of the yrast \pih_11/2\nuh_11/2 bands[1]. In order to map out the extent of the chiral conditions, N=73 odd-odd isotones are being investigated. Searches for the nonyrast chiral doublet bands are being carried out on ^130La and ^132Pr via the ^124Te(^10B,4n) at 51 MeV and ^117Sn(^19F,4n) at 88 MeV reactions, respectively. Six Compton-suppressed HPGe detectors together with a 14-element BGO multiplicity filter measured \gamma-\gamma coincidences at the tandem/linac facility at SUNY at Stony Brook. In ^130La, a new side band was found, which shared common characteristics with those in the N=75 isotones. In the case of ^132Pr, five transitions were identified as being associated with a similar, but less developed, side band; some of these \gamma rays had been seen before. The DCO analysis supports the \pih_11/2\nuh_11/2 configurations for the side bands. An experiment in the ^128Cs isotone is in progress. Results will be compared with the N=75 isotones, and discussed in terms of the restoration of chiral symmetry, which is broken in the intrinsic frame.

[1]K. Starosta et al., (to be published).

[DG.004] Chiral doublets and \gamma-softness in the A \sim 130 region

Systematic properties of doublet bands related to chiral symmetry restoration in odd-odd A\sim130 nuclei were recently established by experimental studies [1]. In the case of ^134Pr, the doublet \pih_11/2\nuh_11/2 bands become nearly degenerate starting at spin \sim 14\hbar, while in other N=75 and N=73 isotones the bands are separated by \sim 300 keV. The degenerate bands are predicted for this region from the coupling of the valence h_11/2 proton and neutron to a triaxial rotor core with \gamma=30^\circ. Current work explores effects of \gamma-softness on the properties of these bands. The calculations were conducted using a core-particle-hole coupling model. The Hamiltonian consists of the collective core, the single-particle valence nucleons, and qQ interactions. The Kerman-Klein method was used to find eigenstates, which provided a convenient way for exploring core effects. Calculations were made for triaxial cores with various \gamma-softness using the General Collective Model keeping \langle \gamma \rangle =30^\circ. The degeneracy in the \pih_11/2\nuh_11/2 bands calculated for the triaxial rotor of fixed \gamma=30^\circ is indeed lifted for \gamma-soft cores, but consistent with the values observed experimentally. The effects of \gamma-softness on transition rates and other observables will be discussed. [1mm] [1] K. Starosta et al (to be published) [1mm] [Supported by NSF-PHY9870280; DOE-DE-FG02-91ER-40609]

[DG.005] High-spin states in ^122_~56Ba

High-spin states in ^122Ba have been studied via the ^64Zn(^64Zn,\alpha2p) reaction. A 260-MeV ^64Zn beam was provided by the Argonne Tandem/Linac Accelerator System. The emitted \gamma rays were detected using the Gammasphere array of 101 suppressed Ge detectors. In addition, the Microball charged-particle detector array was used to select the \alpha 2p channel, thus isolating ^122Ba. The level scheme of ^122Ba has been substantially extended from the previously published level scheme of only six \gamma rays(J.~Conrad, Nucl. Phys. A234), 157 (1974).. Six decoupled bands and two strongly-coupled bands have been observed to spins of up to 40\hbar. Configuration assignments are made with the aid of the systematic alignment properties of ^122Ba and its neighbors. Octupole deformation is predicted to develop in Z \approx 56 nuclei due to the occupation of \Delta l = \Delta j = 3 proton orbitals. Systematics of proposed negative-parity side bands in the A \approx 120 barium isotopes will be discussed. Comparisons with cranked Nilsson-Strutinsky calculations will also be presented in light of possible smooth band termination properties.

[DG.006] High-K Rotational Structures in Light Praseodymium Nuclei

Strongly-coupled bands at high excitation energy have been observed in the proton-rich ^128-131Pr nuclei. The high-spin states of ^128,129Pr were populated in the ^40Ca + ^92Mo reaction at a beam energy of 184 MeV, while the heavier ^130,131Pr nuclei were produced in the ^40Ca + ^94Mo reaction with a beam energy of 180 MeV. In both experiments, Gammasphere and the Microball were utilized for the detection of gamma rays and light charged particles, respectively. The bands have similar characterists (e.g. large B(M1)/B(E2) ratios and no signature splitting) to structures in nearby nuclei that are associated with oblate deformations [1]. An investigation of whether the new sequences in the Pr nuclei are oblate or prolate has been conducted. Tilted axis cranking calculations and an analysis of the rotational alignments aid in determining the nuclear shape and configurations of these bands.

[1] D. B. Fossan et al., Nucl. Phys. A520, 241c (1990).

[DG.007] Octupole correlations in ^141Ba

From the new Gammasphere \gamma -\gamma -\gamma data in the spontaneous fission of ^252Cf, the energy level scheme in ^141Ba has been extended. Two new band structures and new high spin states are identified. Based on the connecting transitions from the new band-4, the parity of band-3 starting at 1341.2 keV is changed to negative. Now one has two opposite parity doublet bands, each connected by enhanced E1 transitions. Using the measured intensities, the B(E1)/B(E2) ratios are calculated and found to range from 0.19 to 0.41 \times10 ^-6 fm ^-2, with an average value of 0.27 \times10 ^-6 fm ^-2. For the newly identified octupole correlated band-4 and -3, the \deltaE(I)=E(I) ^\mp-1/2[E(I+1) ^\pm+E(I-1) ^\pm] is found to be 38 keV at spin of 19/2, 1.7 keV at 23/2, -8 keV at 27/2 and -4.1 keV at 31/2. Thus, the energy levels approach the mid-way of the corresponding neighboring levels of the partner band, as expected for octupole correlations. The behavior of the band structure, especially of the opposite parity doublets in ^141Ba, resembles the neighboring ^143Ba, which supports the parity assignment and the identification of the octupole correlation. Work at Vanderbilt University supported by DOE grant DE-FG-05-88ER40407.

[DG.008] B(E2) Values for Low-Lying States in ^144,146Nd Using Coulomb Excitation on a Thick Target

B(E2) values have been determined for excited states of ^144,146Nd using Coulomb excitation on a thick target. The experiment was conducted at the Lawrence Berkeley National Laboratory, using the 8\pi detector array, consisting of 20 Compton-suppressed Ge detectors. 500 MeV ^144,146Nd beams were directed onto thick ^natNi and ^natPb targets. Gamma yields for several states, relative to the 2^+ \rightarrow 0^+(g.s.) transition (with a known B(E2)), were used to obtain the B(E2) values for these states, and relative yields in gated spectra were used to resolve the overlapping 3^- \rightarrow 2^+ and 6^+ \rightarrow 4^+ transitions. E1 and E2 Matrix elements were obtained by comparing yields with results from a Winther-De Boer code analysis. Results will be presented. Supported in part by the National Science Foundation grant PHY97-22692.

[DG.009] Coupled-channels treatment of deformed proton emitters

The structure of deformed proton emitters is studied in a coupled-channels approach. Decaying states are determined as standing waves, i.e., for real energy eigenvalues. The influence of the long-ranged Coulomb couplings on the decay rate is calculated to first order in the distorted wave Green's function method. The results are compared to predictions made in the adiabatic limit and to measurements. It is shown that the decay of the ^131Eu(3/2^+) ground state to the ground state of ^130Sm is quite sensitive to deformation in the spin-orbit force. It is found that the ground state decay of ^141Ho(7/2^-) is poorly described in the coupled-channels approach. This is ascribed to Coriolis mixing, which is too strong in the absence of pairing.

[DG.010] Decays of the New Isobars ^178Pb and ^178Tl

A bombardment of a 1.05 mg/cm^2 ~90target by 340 MeV ^78Kr ions was performed at Lawrence Berkeley National Laboratory’s 88-Inch Cyclotron. Reaction products were separated from the beam by the Berkeley Gas-Filled Separator (BGS) and implanted into a position-sensitive silicon strip detector. Specific isotopes were identified via observation of their alpha-decay chains. Two ~7.61 MeV alpha decays with decay times of ~0.2 ms, observed correlated with the known decay chain of ^174Hg, are assigned to the decay of ^178Pb. Approximately 1200 alpha decays with energies between 6.5 and 6.9 MeV, correlated to the decay of ^174Au, are assigned to decays of ^178Tl. The observed half-life was of order 200 ms. The results of this measurement will be discussed.

[DG.011] Level Lifetime Measurements in ^150Sm

Shape/phase coexistence and the evolution of structure in the region around ^152Sm have recently been of great interest. Experiments performed at WNSL, Yale University, measured the lifetime of low spin states in a target of ^150Sm with the recoil distance method (RDM) and the Doppler-shift attenuation method (DSAM). The low spin states, both yrast and non-yrast, were populated via Coulomb excitation with a beam of ^16O. The experiments were performed with the NYPD plunger in conjunction with the SPEEDY \gamma-ray array. The SCARY array of solar cells was used to detect backward scattered projectiles, selecting forward flying Coulomb excited target nuclei. The measured lifetimes yield, for example, B(E2) values for transitions such as the 2^+_2 \rightarrow 2^+_1 and the 2^+_3 \rightarrow 0^+_1. Data from the RDM measurment and the DSAM experiment will be presented. This work was supported by the US DOE under grants DE-FG02-91ER-40609 and DE-FG02-88ER-40417.

[DG.012] New Developments in N=88 Gd-152

Gammasphere, in combination with Lawrence Berkeley National Laboratory's 88" Cyclotron, was employed to study Dy-156 using the reaction Sn-124(S-36,4n) at 165 MeV. Although the alpha 4n channel was very weak (~1enough to significantly extend the level scheme of Gd-152. Previously observed bands were extended from spin ~20h to 40h and two new sequences were discovered. Evidence was found for a drastic shift in structure most likely associated with a prolate to oblate shape change. Detailed comparisons with other N=88 nuclei are ongoing.

[DG.013] Shape/phase coexistence in ^156Dy

Recent studies show that in the transitional nucleus ^152Sm_90 a spherical excited configuration coexists with a deformed ground state configuration. Detailed knowledge of other N=90 nuclei is important in studying the evolution of the shape/phase coexistence picture along this isotonic chain. The low-lying non-yrast states of ^156Dy_90 were populated in \beta decay and studied through \gamma-ray spectroscopy at the Yale Moving Tape Collector. The ^156Er parent nuclei were produced through heavy-ion fusion-evaporation, embedded into a Kapton tape, and transported out-of-beam for low-background spectroscopy of the daughter. \gamma-\gamma coincidence measurements were performed using a high-efficiency array of three Compton-suppressed Clover detectors. An upper limit is placed, using a known lifetime value, upon the 2^+_2 \rightarrow 0^+_2 absolute electromagnetic transition strength in ^156Dy. This transition strength is important in the interpretation of the structure of ^156Dy within the context of the shape/phase coexistence observed at N=90. Supported by US DOE grants DE-FG02-91ER-40609 and DE-FG02-88ER-40417.

[DG.014] Identification of Shape Coexistence in the N=90 Isotones: Gd-154 and Dy-156

We have studied the excited states of ^154Gd and ^156Dy following the radioactive decays of ^154Eu, ^154g,m1,m2Tb and ^156Ho using the 8\Pi spectrometer at the LBNL 88-inch cyclotron. Based on gamma-ray coincidence data, we have established band structure built on 0^+ states in ^154Gd and ^156Dy at 1182 and 1148 keV, respectively. These structures are either quasi-rotational or vibrational. There appears to be an analogous structure in ^152Sm built on a 0^+ state at 1083 keV. These results indicate that ^152Sm may possess an even more complex collective structure than that suggested recently^1.

^1 Phys. Rev. C 60,054312

Work supported in part by the US DOE under contracts: DE-FG02-96ER40958 (Georgia Institute of Technology), DE-FG03-98ER41060 (Oregon State University), and DE-AC03-76SF00098 (Lawrence Berkeley National Laboratory)

Part D of program listing