VOLUME 76, NUMBER 20 PHYSICAL REVIEW LETTERS 13 MAY 1996 Energy Staggering in Superdeformed Bands in 131 Ce, 132 Ce, and 133 Ce A. T. Semple, 1 P. J. Nolan, 1 C. W. Beausang, 1 S. A. Forbes, 1 E. S. Paul, 1 J. N. Wilson, 1, * R. Wadsworth, 2 K. Hauschild, 2 I. M. Hibbert, 2 R. M. Clark, 2, † J. Gizon, 3 A. Gizon, 3 D. Santos, 3 and J. Simpson 4 1 Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 3BX, United Kingdom 2 Department of Physics, University of York, Heslington, York, YO1 5DD, United Kingdom 3 Institut des Sciences Nucléaires, IN2P3-CNRS/Université Joseph Fourier, 53 Avenue des Martyrs, 38026 Grenoble, France 4 CCL Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, United Kingdom (Received 8 November 1995) Superdeformed bands observed in 131 Ce, 132 Ce, and 133 Ce have sequences of g-ray transition energies that exhibit a DI  2 staggering. This staggering has different characteristics to that seen in previously known cases in other mass regions. The energy staggering starts at low rotational frequency ( ¯ hv  3 MeV for 131 Ce) at a magnitude of 60.3 keV, dies away to zero at intermediate frequency ( ¯ hv  0.6 2 0.7 MeV), and reappears at higher frequencies ( ¯ hv  0.7 MeV). [S0031-9007(96)00181-0] PACS numbers: 21.10.Re, 23.20.Lv, 27.60.+j The discovery of energy staggering reported recently in the superdeformed bands in 149 Gd [1] and 194 Hg [2] has caused much excitement. This had led to an increased ex- perimental effort to find more examples and many theo- retical papers attempting to explain the phenomenon. The effect is best seen in long rotational sequences, where the expected regular behavior of the energy levels with respect to spin (I) is perturbed. The result is that the rotational sequence is split into two parts with states separated by DI  4 shifting up in energy and the intermediate states shifting down in energy. The size of the shift is typically 50 eV. The staggering has been interpreted in a variety of ways. Hamamoto and Mottelson [3] have suggested that it is possible evidence for a new symmetry in the nuclear Hamiltonian, in which it is invariant under a rotation of 90 ± about the rotation axis, as opposed to 180 ± which results in the normal DI  2 sequence. Pavlichenkov and Flibotte [4] suggest that the staggering is associated with the align- ment of the total nuclear angular momentum along the axis perpendicular to the long deformation axis of the prolate nucleus. Macchiavelli et al. [5] interpret the staggering as due to the mixing of a series of rotational bands that differ by DK  4. All these suggestions lead to a mechanism from which the staggering could arise. However, none ex- plain why certain nuclei should show the effect. Recently, Sun, Zhang, and Guidry [6] have shown that a DI  4 effect comparable with the staggering pattern observed in the superdeformed (SD) bands reported in this paper arises from the projected shell model when there are two bands mixing near the yrast line, and have performed calculations on 132 Ce band 1 which predict the effect [7]. In this Letter we report the observation of DI  2 energy staggering in the superdeformed bands in the nuclei 131,132,133 Ce. This is the first example of the effect in neighboring odd and even nuclei and in a pair of identical bands ( 132 Ce band 1 and 133 Ce band 1). In order to investigate superdeformation in 131,132 Ce and 133 Ce, experiments [8,9] were performed using the Eu- rogam (phase I) [10,11] and Gammasphere [11,12] g-ray spectrometers. The nucleus 132 Ce was the first discovered where a discrete rotational band exhibiting superdeformed characteristics at high spin [13,14] was observed, but, sur- prisingly, prior to the experiments reported as a result of this work, there were no known excited SD bands in this or in any nucleus in the A  130 region. The nuclei 131 Ce and 132 Ce each contained a single SD band [13,15], while none were known to exist in 133 Ce. However, the Eurogam experiment has revealed one new excited SD band in 131 Ce [16] and two new excited SD bands in 132 Ce [8]. In addi- tion, the Gammasphere experiment has revealed three SD bands in 133 Ce [9]. Quadrupole deformations for the yrast SD bands in 131 Ce and 132 Ce have been determined from mean lifetime measurements to be Q 0  6.0 6 0.6 [17] and 8.8 6 0.8 e b [14], respectively. This difference in deformation can be interpreted as arising from the occu- pancy of a single i 132 neutron orbital for 131 Ce with a high N particle configuration of p 5 4 n6 1 for the SD band, and the occupancy of two i 132 neutron orbitals for 132 Ce with a high N particle configuration of p 5 4 n6 2 . Lifetime mea- surements of the 133 Ce SD bands show that bands 1 and 2 have quadrupole moments of Q 0  7.5 6 0.8 e b [18]. In addition to this, 132 Ce band 1 and 133 Ce band 1 are ob- served to be directly degenerate identical bands with tran- sition energies that are within 3 keV in the frequency range ¯ hv  0.6 1.0 MeV. The yrast SD bands in 131,132 Ce each have a population intensity of 5% with respect to their reaction channel. Bands 1 and 2 in 133 Ce have population intensities of 3% and 1.5%, respectively, with respect to the population of 133 Ce. The large statistics and increased resolving power obtained in the two experiments have allowed precise measurements of transition energies for these SD bands to be made. The g-ray spectra were calibrated using 152 Eu source data obtained before the start of the experiment. The energies were determined in the conventional way by fitting Gaussian line shapes to the SD g-ray peaks and are present in Table I. It should be noted that these energies differ from those previously published in Ref. [8]. This 0031-9007 96 76(20) 3671(4)$10.00 © 1996 The American Physical Society 3671