G-factors of high-spin states in 154Dy

Nuclear Physics A$S3 (1993) 527c-530c North-Holland, Amsterdam

NUCLEAR PHYSICS A

G-Fact0rs of High-SpinStates in lf Dy U. Birkental a, A.P. Byrne a . S. Heppner a, H. Hiibela, W. Schmitz a, P. Fallonb, P.D. Forsyth b, J.W. Roberts b, H. Kluge c, E. Lubkiewiczd and G. Goldringe a b c d e

Institut f'fir Strahlen- u. Kemphysik, Nussallee 14-16, D-5300 Bonn, Germany Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 3BX, U.K. Hahn-Meitner-lnstitut, Glienicker Str. 100, D-1000 Berlin, Germany Gesellschaft for Schwerionenforschung, D-6100 Darmstadt, Germany Weizmann Institute of Science, Rehovot 76100, Israel

Absm G-factors of excited states in lS4Dy were measured up to high spins by a recoil-distance transient field technique. The results can be explained by i13/2 neutron alignment at the first band-crossing and cc1£irm the alignment of protons in the spin 30 region.

1. INTRODUCTION Electromagnetic moments of nuclear states contain detailed information on the wave function and can be used to test nuclear structure models. In the case of collective states the intrinsic spin contributions from the nucleons largely cancel at low spin and the g-factors arise principally from the proton current distribution~ With increasing angular momentum, however, Coriolis forces tend to align the nucleon spins with the axis of rotation so that a fraction of the total nuclear spin is carried by individual nucleons. Thus, g-factors can be a sensitive probe of the interplay between collective rotation and single-particle motion. In this work we report on a measurement of g-factors of excited states in lS4Dy using the recoil-distance transient field (RDTF) method 1,2) in coincidence mode. 154Dy is a transitional nucleus which is soft to deformation changes induced by the nuclear rotation or by the alignment of particles. The energy level spectrum 3) of 154Dy shows rotational bands up to spin I ~. 30, but at higher spin these sequences stop and the level pattern becomes irregular. This "band termination" behaviour can be explained by a transition from prolate to oblate shape. Indeed, lifetime measurements 2,a) show a drastic loss of collectivity along the yrast line with increasing spin that is - at least qualitatively - in agreement with the band termination picture. 0375-9474/93/$06.00 © 1993 - ElscvicrScicncc PublishersB.V. All rights rcscrvcd.

528c

U. Birkenml et al. ! G-factors of high-spin states in tSJDy

2. EXPERIMENTAL PROCEDURE AND DATA ANALYSIS Excited states in tS4Dywere populated in the reaction ll°pd(48Ca,4n)154Dy at a beam energy of 210 MeV. The beam was provided by the Tandem Van de Graaff accelerator at Daresbury. Gamma-ray coincidences were measured with the TESSA3 spectrometer array which comprises 16 Compton-suppressed Ge detectors and an inner ball of 50 BGO counters. The TESSA3 detector geometr3, is well suited for a g-factor measurement, since with the magnetic field direction perpendicular to the reaction plane, the detector arrangement is symmetric for field up and field down5). The lifetimes of the high-spin states in I54Dy lie in the range of a fraction of a picosecond to several picoseconds 2,4) and the large transient magnetic field6) for Dy in ferromagnetic Gd was used in order to induce measurable Larmor precessions. A Gd foil of 5.2 mg/cm 2 was rolled onto an Ag foil (the Gd layer facing the beam). This stopper foil was stretched and cooled to liquid Nitrogen temperature. It was polarized perpendicular to the reaction plane by a NdFeB permanent magnet which provided a field of 0.18 T. A stretched self-supporting metallic l l°pd foil of 1 mg/cm 2 thickness was used as a target. A fixed distance of d = 105 ~m was used for most of the time ot the experiment. This distance, which corresponds to a delay time of 13.5 ps between the time of the reaction and the entrance of the recoiling 154Dy ions into the ferromagnetic Gd foil, was monitored during the experiment using the intensity ratios of the shifted and stopped components of the high-spin ,r-ray transitions. For both directions of the polarizing field the r-ray coincidences were sorted into E~,I-E~-2 matrices with the events from the groups of detectors positioned at the different angles to the beam on the two axes. Gates were set on the shifted and unshifted peaks to obtain one-dimensional coincidence spectra. The total projection spectra were also analyzed. They contain more ,t-ray lines (including contaminations) than the coincidence spectra, but clean spectra could be obtained with appropriate gates on the fold and sum-energy measured in the BGO ball. Examples of the perturbed angular distributions are displayed in Fig. 1. The experimental points from the detectors below the reaction plane (~ = -19°) are plotted between o -- 0° and 180° and those from detectors above that plane (~ -- 19°) between 180° and 360 °, The curves represent the angular distribution function W(o-+Ao,¢) which was obtained in a Monte-Carlo-type calculation of a system of energy levels and lifetimes, sidefeeding intensities and sidefeeding times to extract the populations of all states and the coefficients for unfolding the nuclear precessions. The computer code is based on programs which were developed Lubkiewicz et al. 1). These simulations follow the decay of the excited nuclei and trace the spin alignment from the nuclear reaction and the hyperfine

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deorientation in vacuum, it includes also the effects from unobserved transitions and solid angles of the detectors. The g-factors determined in this way from the precession angles r,o are shown in Fig. 2.

3. DISCUSSION Bengtsson and Aberg 7) have calculated g-factors for the ground band of 154Dy in a diabatic approach. Within this model, which neglects the ground and s-band mixing, the g-factors rise from 0.35 for the ground state to 0.43 for the 10÷ state. Clearly, our data do not show such an increase. Rather, the experimental values decrease significantly above the 4+ state. Therefore, we have to assume that the mixing of the ground band with the neutron it3/2 band (s-band) plays an important role. The ii3/a neutrons have a small negative g-factor 8) and their contribution to the wave function causes a decrease from the rotational value. The simplest approach to judge this effect is to assume a sudden gain in

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Spin Figure 2. G-factors for the positive-parity yrast sequence. alignment of 9 h between the 12 + and 16÷ states, which is compatible with the energy level spectrum. The resulting calculated9) g-factors are shown as dashed curve in Fig. 2. Two significant deviations of the experimental data from this curve are observed: (i) Around spin 12 the experimental g-factors lie below the simplified calcuIation, indicating mixing of the ground and i~/2 band wave functions. (ii) At the highest spins the experimental data lie above the calculation. This is probably due to proton alignment which has been predicted 31 to occur in this spin range where the valence nucleons align and the nucleus is driven to oblate shape. Thus, the result of the g-factor measurement is consistent with the "band termination" model suggested to e.'q~lain the observed behaviour of the energy levels. This work was supported by the Bundesminister for Forschung u. Technologie BRD

1) E. Lubkiewicz et al., Z. Phys. ~ (1990) 369 2) H. Htibel et al., Prog. Part. Nucl. Phys. 28 (1992) 295 3) H.W. Cranmer-Gordon et a!., Nuc!. Phys..A_.~_5 (!987) 505 4) W.C. Ma et al., Phys. Rev. Lett. 61 (1988) 46 5) P.J. NoIan et al., Proc. Int. Nucl. Phys. Conf., Harrogate, Inst. Phys. Conf. Set. 86 (1986) 155 6) O. H~iusser et at., Nucl. Phys. A412 (1984) 141 7) R. Bengtsson and S. Aberg, Phys. [.ett. B172 (1986) 277 8) H. Htibel, Fortschr. Phys. 25 (1977) 327 9) S. Frauendorf, Phys. Lett. B | ~ (1981~ 219