High-spin structure in154Er

Abstract

High spin states in the nucleus lS4Er have been reinvestigated using the *23Sb(asc1, 4n) reaction and a variety of spectroscopic techniques including excitation functions, 7-7 coincidences, 7 angular distribution and linear polarization measurements. From the measured energies, relative intensities and transition multipolarities a new level scheme has been deduced up to an excitation energy of ~ 12 MeV and spin 36. An interpretation of the experimental results is given in terms of the deformed Woods-Saxon orbitals. Gigantic backbending (superdeformation) effect is studied theoretically within the cranking model.

Decay properties of very-high-spin states in transitional Er nuclei aroundA=154

“…The energies and the angular momenta of the observed levels are accounted for by considering the alignment of valence particles along the symmetry axis of the presumably oblate core. Indeed, a recent measurement of the signs of the quadrupole moments of isomers in 147Gd shows that the core is [2][3][4]. A rapid change of the structure is observed at spins I<30 h, from irregular in lS~ to rotational in lSSEr oblate at least at lower spins [10], so the presumption probably holds true generally.…”

“…It was observed that the two bumps at E~=0.65 and 1.3 MeV move in energy far less than expected for a rotational structure when the entrystate spin is increased [16]. The high-energy bump is to a large extent made up of stretched E2 transitions, while stretched dipole and quadrupole transitions, non stretched quadrupoles, as well as the discrete lines [3], are assumed to be mixed in the low-energy bump. At low energies, E~<0.5 MeV, evidence for the presence of transitions with M1 character at high spins has recently been reported [17].…”

“…This change can obviously be related to the addition of the eight neutrons. In a S4Er an intermediate structure is observed in the discrete cascade extending to I=36h [3]. However, the maximum angular momentum obtainable from the eight valence nucleons is 1=37h, 4 "3 as given by the ~hll/21~t13/2vh9/2 configuration.…”

“…In Fig. 1, the behaviour of the discrete spectra of three Er isotopes viz lS~ ~S4Er and lSSEr [2][3][4], is shown. These nuclides have four protons outside the Z=64 shell closure and 0,4 and 8 neutrons, respectively, outside the N=82 shell closure.…”

“…The structure of the yrast lines of Er Nuclei near N=82 (A=152-154) is known from discrete-line spectroscopy to high excitation energies, Eexc~8-12MeV, and high spins, I~36h, cf. [3,[5][6][7][8][9]. The energies and the angular momenta of the observed levels are accounted for by considering the alignment of valence particles along the symmetry axis of the presumably oblate core.…”