High-spin neutron excitations in neutron-deficient even-mass Pb isotopes

Abstract

High-spin states in 194, 196pb have been investigated using the 188Os( 12C, χn) 194, 196Pb and the 198Hg(α, 6n) 196Pb reactions. The experiments included γ-ray excitation function, γ-γ coincidence, γ-∑γ coincidence, γ-ray angular distribution, lifetime and conversion electron measurements. The level schemes of 194Pb and 196Pb have been extended up to states with J π = 15 − ( E = 3955 keV) and J π = 20 + ( E = 5444 keV), respectively. All levels except two new isomers which have most likely spin and parity J π = 10 − ( 194 Pb) and J π = 11 − ( 196 Pb) can be interpreted as neutron excitations. The excitation energies of levels with spin J ⩽ 12 are well accounted for by the one-broken-pair model including 1p1h excitations. The B(E2, 12 + −10 +) values reflect the systematic trend for a pure ( i 13 2 ) 2 configuration. Levels with spin 12 < J < 20 can be described as two-broken-pair model configurations. The excitation energies of these states can be reproduced quite well in a phenomenological approach in which two i 13 2 neutron holes are coupled to core excitations in the A + 2 nucleus and also by calculations performed within the two-broken-pair model. Within this framework the J π = 20 + state observed in 196Pb has a pure ( i 13 2 ) 4 configuration. The two new isomers have most likely a proton 2p2h structure. The excitation energy of the J π = 11 − isomer in 196Pb agrees with that predicted for a deformed 1 2 + [404] − 29 2 − [514] 13 2 + [606] 11 − state.