Properties of 192, 190Pb: Behaviour of the very neutron-deficient even lead isotopes

The two very neutron-deficient isotopes of lead A = 190, 192 have been produced and studied through the reaction 156, 154Gd(40Ar, 4nγ)192, 190Pb. The identification of those nuclei was also checked by cross bombardment using the two additional reactions 182W(16O, 6γ)192Pb and 156Gd(40Ar, 6nγ)190Pb. The pulsed 40Ar beam available at ALICE (Orsay) enabled us to record singles γ-spectra as well as four-dimensional (energy-time) spectra corresponding to the correlation of two Ge(Li) detectors for different time windows. Tentative level schemes are proposed. No significant deviation from the systematic behaviour previously reported for the heavier isotopes is found. The data are consistently reproduced within a two-quasiparticle formalism using a surface δ-interaction. However, a slow decrease of the 21+ level energy with decreasing A is observed, which could be an indication of a weakly increasing collective trend when the number of neutron holes increases. These experimental conclusions are in fair agreement with the theoretical results obtained by Dickman and Dietrich who predict a spherical shape for the ground states of the lead isotopes with A ≧ 184 .