Inelastic proton scattering from 198Hg, 200Hg, 202Hg and 204Hg at Ermp=28MeV

Abstract

Results are presented of a high-resolution inelastic proton scattering experiment on the even Hg isotopes 198Hg, 200Hg, 202Hg and 204Hg performed at E p=28MeV. Highly enriched targets were used. A special target preparation technique has been developed in order to prepare thin (≈160 μg/cm 2) homogeneous targets, which resulted in a high (≈17 keV) energy resolution. Several previously unknown levels were observed. Angular distributions for levels up to between 2.6 and 4.4 MeV (depending on the isotope) have been obtained. The study of these four isotopes made it possible to compare analogous levels in these isotopes. A macroscopic CC analysis, using a symmetric rotational model, was performed for the ground, 2 + 1 and 4 + 1 states, which were assumed to be members of a ground-state band. For the 2 + 2 states a phenomenological CC analysis was performed, yielding B p(E2)-values that are about a factor of two larger than the electromagnetic B(E2)-values. This is assumed to be due to a large neutron contribution to the excitation of these states. For the analysis of the remaining levels a quasi-DWBA scheme was used, in which the coupling of the 2 + 1 state with the ground state was included, but no further couplings were taken into account. This allowed making J π assignments for some levels. In 200Hg a low-lying 4 + state is observed, which was previously incorrectly given a 3 + assignment, while also in 198Hg and 202Hg low-lying 4 + 2 states are observed. The summed excitation strength of the 4 + 1 and 4 + 2 states appears to be approximately constant in the studied isotopes. From our analysis of the 4 + states there is, in agreement with results from other experiments, evidence that a shape transition may occur between 200Hg and 202Hg. It is not clear, however, within which nuclear model this shape transition should be described.