Oblate L=1 bands in 194,196–201Pb and 193Hg

Abstract

Reports of recent experiments have included observations of regular and irregular bands in neutron deficient Pb isotopes with A=194, 196–201. The bands are populated strongly in (HI, xn) reactions. The shared characteristics of the bands include: 1) bandhead energies of a few MeV; 2) high bandhead spin; 3) large alignments; 4) small dynamic moments of inertia, and 5) strong L=1 transitions and weaker L=2 crossover transitions, with B( M1) B( E2) ≈ 20 μ 2 e 2 b 2 . Lifetimes of band members in the 198Pb regular band are B(M1) ≈ 1 W.u. and B(E2) ≈ 12 W.u. (with large errors). These observations are consistent with an interpretation of the regular structures as collective oblate bands with both proton and neutron excitations involved; the closed proton shell at Z=82 is broken, and coupled to v( i 13 2 ) −n excitations. The irregular structures may correspond to triaxial shapes, with similar orbits involved. A similar structure has been also found in 193Hg.