Levels and transitions in Pb204 and the four valence neutron-hole configurations

Levels of the nucleus Pb204 have been investigated using the (n,n) reaction, and rays from low-spin excited levels have been observed. Forty-three low-spin levels connected by 78 rays are found below 2.9 MeV, whereas only about 28 levels had previously been known. The levels below 2 MeV excitation energy are expected to be dominated by the p1/2, f5/2, and p3/2 valence neutron hole excitations, and 0+ levels at 0, 1730, and 2433.1 keV are associated primarily with these configurations. These states are at almost the same excitation energies as parent 0+ excitations in Pb206. Approximately six unnatural-parity levels are identified; this is close to the number predicted in six orbit valence-space shell model calculations. The number of natural-parity levels found, however, is almost twice that calculated with the shell model. Levels and transitions below 2 MeV excitation energy are consistent with expectations basing Pb204 states on correlated two-hole excitations dominant in Pb206.