Evidence for a change of structure in the heavy mercury isotopes around 200Hg

Abstract

The (p, t) reaction has been performed on natural Hg and separated 196,198–202,204Hg targets, using a 25 MeV proton beam from the Orsay MP tandem accelerator and a split-pole spectrometer. In an excitation energy region below 2 MeV, new excited 0 + levels have been found. The enhancement factors of the populated 0 + states have been extracted. The resulting set of data exhibits striking discontinuities. In particular, both the energy and the (p, t) cross section of the 0 2 + level show a pronounced extremum in 200Hg. This observation, together with the previous experimental evidence, suggests a quite dramatic change in intrinsic structure of the Hg-isotopes at A=200. Microscopic calculations based on the deformed-shell-model approach suggest an explanation in terms of the Subshell closures at Z=80 and N=120 at weakly deformed oblate shapes, and a possible oblate-to-spherical shape transition. Qualitative theoretical arguments are confronted with the properties of pairing excitations calculated within a variant of the one broken-pair method. Within this approach the lowest excited 0 + states are interpreted as pairing vibrations. Energy positions of 0 + states and the pair transfer matrix elements are compared with the available experimental data.