Structure of Superdeformed Bands Near the N=80 Shell Gap

Abstract

<p>As part of a systematic survey of superdeformation in nuclei in the vicinity of ¹⁴³Eu, six superdeformed bands in ¹⁴²Sm, ¹⁴²Eu and ¹⁴⁴Eu have been studied by their Υ-ray decay following heavy-ion fusion-evaporation reactions. All of these bands are based on a "second minimum" shell gap associated with the superdeformed (2:1 prolate ellipsoid of rotation) nuclear shape at neutron number N=80 and proton number Z=63. The first band in ¹⁴²Sm and the band in ¹⁴²Eu have been studied with the 8π spectrometer, while the bands in ¹⁴⁴Eu and the excited band in ¹⁴²Sm have been discovered with the GAMMASPHERE array. The Υ-ray decays of these superdeformed structures are compared to collective rotational models, and specifically to state-of-the-art Cranked Shell Model (CSM) calculations for Nilsson, Woods-Saxon, and Hartree-Fock models of the nuclear potential. The predicted roles of specific orbitals in the A~140 mass region are compared to quantities derived from the experiments and where possible, configurations with respect to an ¹⁴³Eu core are proposed.</p> <p>The first ¹⁴²Sm band consists of 19 Υ-ray transitions with an intensity of 0.5% of the reaction channel, and its behaviour is best described in the calculations as an ¹⁴³Eu superdeformed core with a hole in an Nosc=5 proton orbital. The second excited band consists of 15 transitions with an intensity ~ 20% that of the first band, of which the transitions above ~ 1 MeV are identical in energy to those in a ¹⁴⁶Gd band. This new band, which represents the second example of a two-proton, two-neutron identical bands pair, is most likely a ¹⁴⁶Gd superdeformed core with four holes in low Nosc orbitals. Structures based on simple excitations of the proton hole of the first band likely comprise a quasi-continuum ridge structure.</p> <p>The ¹⁴²Eu band has 15 transitions and an intensity of 1.2%. The detailed behavior of this band suggests that its configuration is an Nosc=5 neutron hole in the ¹⁴³Eu core. Such a configuration is not predicted to be energetically favourable.</p> <p>The three ¹⁴⁴Eu bands all have population intensities of < 0.2 % relative to the reaction cross-section. Two of the ¹⁴⁴Eu bands, with 17 and 18 transitions respectively, are quite likely Nosc=6 neutron particle states coupled to the ¹⁴³Eu core. The behavior of the third band indicates occupation of an Nosc=7 neutron intruder which does not influence the shape of the ¹⁴³Eu core.</p>