Prediction and possible observation of an oblate shape isomer in 190 W

The small number of oblate-shaped nuclei found in nature, compared with the many prolate nuclei, is a surprising feature of nuclear structure, which seems to be related to the strength of the nuclear spin-orbit interaction [1]. Furthermore, with increasing angular momentum, the collective rotation of an oblate shape, about an axis perpendicular to its axis of symmetry, is disadvantaged relative to prolate rotation, on account of the mass distribution leading to a larger moment of inertia for the latter. Therefore, the prediction that there would be “giant backbending” in the well deformed nuclide Hf at I ≈ 26h̄, made by Hilton and Mang in 1979 [2], was remarkable. They performed HFB calculations to show that collective oblate rotation, incorporating rotationaligned nucleons, could take place at a lower energy than prolate rotation. The transition from one shape to the other would represent a striking and sudden structural change, quite unlike anything yet observed. Nevertheless, despite experimental and theoretical advances [3–5], experimental evidence for the oblate mode in the mass180 region has been inconclusive. For example, it has not proved possible to reach high enough angular momentum in Hf [6], though some evidence for oblate rotation has been found in Hf at I ≈ 40h̄ [5]. However, it has been shown, on the basis of Total Routhian Surface (TRS) calculations [3], that the angular momentum at which oblate rotation becomes favoured decreases with increasing neutron number. Therefore, more detailed investigation of neutron-rich nuclei may give the best chance of finding the oblate rotational mode in this mass region. Indeed, it can be argued [3] that this region is optimal on account of reinforcing proton and neutron shell structures, with both Fermi levels being high (but not too high) in their respective shells. While a similar effect occurs for neutrons in the mass-130 region [7], with associated oblate states, the protons then favour prolate shapes at high angular momentum. The competing proton and neutron contributions lead to triaxiality, which itself has interesting consequences [8, 9].