Evidence for weak deformation in the 88-neutron nucleus Tb153

The decay of 6.3-h153Dy to levels in Tb153 has been studied with high-resolution detectors. Sources were prepared by the Gd154(He3,4n) reaction and chemically purified by ion exchange. A total of 329 γ rays were assigned to Dy153 decay; another 71 γ rays may be associated with Dy153 decay. The proposed decay scheme of 54 levels includes 319 of the 400 γ rays. The Q value for β decay was measured to be 2250 ± 50 keV. Eleven low-lying positive-parity levels were interpreted in the Coriolis-mixed, particle-plus-rotor model as Nilsson states and rotational band members. Nilsson orbitals so interpreted include 52+[402], ground state; 32+[411], 147.4 keV; 52+[413], 218.6 keV; and 72+[404], 254.2 keV. The order and spacing of the negative-parity levels are consistent with rotation-alignment model predictions. For both positive- and negative-parity levels a deformation β of about 0.15 is required to fit the data. RADIOACTIVITY Dy153 [from Gd154(He3,4n)]; measured Eγ, Iγ, Ice, Qβ, γ-γ, and β+-γ coin; deduced αK, logft. Tb153 deduced levels, J, π. Enriched target, Ge(Li) and Si(Li) detectors.