Session B15 - Nuclear Structure I.
ORAL session, Saturday morning, April 28
Room 15, Renaissance Hotel$

[B15.001] Coulomb Excitation of ^124,126Xe Studied with Segmented Germanium Detectors

An experiment on the Coulomb excitation of ^124Xe and ^126Xe beams was recently performed at the ATLAS facility at Argonne National Laboratory. The Xenon isotopes have been interpreted as a realization of the O(6) symmetry of the IBA model, which corresponds to the geometrical picture of a \gamma-soft rotor. Although the level structure and decay branching ratios seem to fit this picture rather well, the most crucial experimental obserables missing in many of these studies are high-precision lifetimes of yrast and non-yrast low-spin states; a topic which we now address. To perform this experiment six 32-fold segmented germanium detectors of the Michigan State University germanium array were utilized. The counters were placed around the target to yield an \approx6% photopeak efficiency at 1.3~MeV and have a segment opening angle of \approx10^\circ. Scattered Xe nuclei were detected in a large-area four-quadrant PPAC. This work is supported by the Department of Energy under grant W-31-109-ENG38 and the National Science Foundation under grants PHY-9528844, PHY-9724299, and PHY-9875122.

[B15.002] First identification of excited states in ^195At

Excited states in proton-rich ^195At, populated via the ^142Nd(^56Fe, p2n) reaction, have been observed for the first time using the recoil-decay-tagging technique at the University of Jyväskylä, Finland. Prompt \gamma rays were detected using the \sc jurosphere ii Ge array and recoiling fusion-evaporation products were separated from the primary beam and fission products using the \sc ritu recoil separator. Two new states have been identified, based on the 9/2^- isomeric state (\alpha-decay work has revealed that the 1/2^+ intruder state becomes the ground state in ^195At, the 9/2^- level being the ground state in heavier At nuclei). The new levels are tentatively assigned spins and parities of (13/2^-) and (17/2^-), based on level-energy systematics of heavier At isotopes. The energies of the new levels suggest that ^195At continues the trend of increasing collectivity moving away from the Z = 82 and N = 126 shell closures, in agreement with theoretical predictions.

[B15.003] Lifetimes Measurements in ^84Zr

Using the reaction ^58Ni(^32S,\alpha\,2p), high angular momentum states in ^84Zr were populated. A Ta backing was used as the stopping material for the recoiling nuclei. \gamma-\gamma coincidences and evaporated charged particles were detected using the arrays GAMMASPHERE and MICROBALL. Lifetimes and sidefeeding times of more than 20 levels were measured using the Doppler Shift Attenuattion method. The deduced quadrupole moment for the yrast band is quite constant around a value of 2.2 \,eb up to a spin of I=28 \hbar . It is the first time that quadrupole moments have been determined up to so a high spin of a ND band in the A \approx 80 region.

[B15.004] Pairing correlations near the drip-lines: HFB on a 2-D lattice

Near the neutron or proton drip lines, large pairing correlations are expected. The outermost nucleons are weakly bound and are strongly coupled to the particle continuum. These features represent major challenges for the mean field theories. We solve the HFB equations for deformed, axially symmetric even-even nuclei on a two-dimensional lattice. High accuracy is achieved by a Basis-Spline representation of operators and wavefunctions; a combination of the Galerkin and collocation methods is utilized [1]. This work represents a natural extension of the 1-D calculations for spherical nuclei by Dobaczewski et al. We diagonalize the 2-D HFB lattice Hamiltonian using LAPACK yielding a quasiparticle energy spectrum up to 2000 MeV. In calculating observables, we cut off this spectrum at an equivalent s.p. energy of about 60 MeV. Calculations with Skyrme mean field interactions and density-dependent delta pairing forces are currently underway.

[1] V.E. Oberacker and A.S. Umar, in ``Perspectives in Nuclear Physics'', World Scientific Publ. Co. (1999), p. 255-266; nucl-th/9905010.

[B15.005] Transition probability from the ground to the first-excited 2^+ state of even-even nuclides

The deformation of the nuclear ground state is a fundamental property that can be deduced in even-even nuclei from the B(E2) value for the first 2^+ state (2_1^+). In 1987, we published a compilation of such B(E2;0_1^+ \rightarrow 2_1^+) values for \sim 280 even-even nuclei(S. Raman, C. H. Malarkey, W. T. Milner, Jr., and P. H. Stelson, At. Data Nucl. Data Tables 36, 1 (1987).). We have recently updated the 1987 compilation. The current compilation(S. Raman, C. W. Nestor, Jr., and P. Tikkanen, At. Data Nucl. Data Tables, to be published.) contains adopted B(E2;0_1^+\rightarrow 2_1^+) values for 328 nuclides. We have also compared the adopted B(E2;0_1^+\rightarrow 2_1^+) values with those given by various theoretical models (Single-Shell Asymptotic Nilsson Model, Finite Range Droplet Model, Woods-Saxon Model, Relativistic Mean-Field Calculations, Extended Thomas-Fermi Strutinski-Integral Method, Hartree-Fock Calculations, and Dynamical Microscopic Model).

[B15.006] High-spin States in ^174Pt

High-spin states in the neutron deficient ^174Pt have been studied using the reactions ^92Mo(^84Sr,2p2n) and ^94Mo(^84Sr,2p4\alphan) at beam energies of 390 MeV and 400 MeV, respectively. Due to the large gamma ray flux created by fissioning nuclei, the Fragment Mass Analyzer was used in conjunction with the Gammasphere spectrometer to associate emmited gamma rays with A=174 recoils. The ground-state band has been extended from 14^+ to 24^+ and side bands have been identified for the first time. Analysis of the rotational structures and possible configuration assignments will be addressed.

[B15.007] Gamma-ray spectra of individual components of decay series

In order to obtain high quality spectra to update the ``Heath Catalog" of spectra with Ge detectors, we (I and a group of Univ. of Michigan undergraduates*) have investigated the neutron activation and decay products related to natural thorium and natural uranium. We have used the UM Ford Nuclear Reactor for irradiations and the UM Phoenix Memorial Lab for the remaining work. Spectra have been obtained with a variety of detectors, some suitable for low energies (>5 keV) and others efficient (70%) for high energies. Sources have been obtained in a variety of forms, each of which has been characterized in comparison with a thin, point source. We will report on A=233 (Th, Pa, and U, and the Np-237 parent of Pa-233), A=234 (Th and the Pa isomers), and A=239 (U, Np, and the Am-243 parent of Np-239). *The 2000-01 undergrad research group consists of Jason Banker, Adam Berro, Adam Cole, Amelia Deschamps, Erik Epp, Ralph Pierre, and Emma Wong.

[B15.008] Precise Measurement of the Half-life of ^22Mg

As part of a program to test CKM unitarity via superallowed \beta-decay, we have measured the half-life of ^22Mg with high precision. The ^1H(^23Na, 2n)^22Mg reaction was used to produce a pure ^22Mg beam from the MARS recoil spectrograph. The beam was collected for 8 s on a fast tape-transport system, and the sample (>99.9% pure) moved into the center of a 4\pi proportional gas counter located in a low background region 1 m away. The counter signals were then multi-scaled for a period of 80 s, and then the whole cycle was repeated. A pulse synthesizer accurate to 5 ppm provided the time-stamp, and a gate generator with non-extendable dead time defined a dominant fixed dead time. A total of 30 million ^22Mg decays were recorded in more than 30 separate runs. To test for possible systematic errors, from one run to another we changed the critical settings of the acquisition chain: the discriminator threshold, the detector bias and the dominant dead time. Two fitting procedures were applied to extract the half-life: (i) a maximum-likelihood fit of the sum of all dead-time corrected decay spectra and (ii) a global fit of individual cycles, with common half-life but with amplitudes and dead-time correctly matched to each cycle. The second procedure gives the ultimate precision, as it contains no approximation. Our preliminary result is t_1/2 = 3.8782(9) s.

[B15.009] Possible Chiral-Twin Bands in ^136Pm

The possible chiral-twin band observed in ^136Pm has been extended to high spins [I=(21)] in the reaction ^105Pd(^35Cl,2p2n). The extension was made possible by the use of the Gammasphere \gamma-ray spectrometer and the Microball charged-particle detector array. A more-detailed analysis of the suggested chiral-twin band could be performed in order to determine the nucleons on which it is based. Rotational alignments and B(M1)/B(E2) ratios confirm that the this band likely has the same configuration as the yrast \pi h_11/2\nu h_11/2 band.

[B15.010] Competition between terminating and collective structures above spin 40 \hbar in ^154Dy

High spin states in ^154Dy were populated via the ^36S(^122Sn, 4n) reaction with a 165-MeV beam provided by the 88" cyclotron at LBNL. The target consisted of a stack of three 0.33 mg/cm^2 self-supporting foils. The decay \gamma rays were detected with Gammasphere spectrometer array, which consisted of 103 Compton-suppressed germanium detectors at the time of the experiment. A total of 1.5 x 10^9 events was collected, with a requirement of \geq 5 suppressed Ge detectors in prompt coincidence. Previously known bands were extended to higher spins, up to 51\hbar, and at least 4 new bands were found, including a pair of strongly coupled high-K bands. Several bands maintain a smooth rotational behavior up to the highest spins observed. Sharp band terminations were observed at high spins with positive and negative parities, in excellent agreement with theoretical calculations based on the configuration-dependent cranked Nilsson-Strutinsky approach.

Part B of program listing