Low lying particle-hole states in 206Pb and 208Pb and the shell model

The nucleus 206 Pb differs from the doubly magic nucleus 208 Pb by two missing neutrons. In 208 Pb most states at E x < 7.4 MeV are described by one-particle one-hole configurations. The lowest configurations with a g 9 / 2 particle and dominant p 1 / 2 , f 5 / 2 , p 3 / 2 holes and admixtures from f 7 / 2 and a few more configurations build an ensemble of two dozen states at 3.2 < E x < 4.9 MeV. They are described by rather complete orthonormal transformation matrices of two dozen states with spins from 2 - to 8 - to configurations. In 206 Pb a similar ensemble of states is deduced from the analysis of angular distributions measured in 1969 for the 206 Pb ( p , p ′ ) reaction via the g 9 / 2 IAR in 207 Bi . An equivalent 208 Pb ( p , p ′ ) experiment was performed in 1968 at the Max–Planck–Institut für Kernphysik at Heidelberg (Germany). New spins are determined for 32 states and 22 levels in 206 Pb . The comparison to corresponding states in 208 Pb studied especially in 1982 yields both remarkable similarities and clear differences. Sizeable g 9 / 2 p 1 / 2 strength found in 4 - and 5 - states is interpreted as admixtures of p 3 / 2 - 2 and f 5 / 2 - 2 components to the ground state of 206 Pb with dominant p 1 / 2 - 2 character. The description of nuclear states by shell model particle-hole configurations in the lead region needs the inclusion of collective excitations at already very low excitation energies. For the two isotopes 206 Pb and 208 Pb a rather good agreement of excitation energies and configuration mixing is observed for states at 3.7 < E x < 4.7 MeV.