Nuclear Data Sheets for A = 155

The present work constitutes an updating of the previous A = 155 data compilation, which appeared in the Nuclear Data Sheets in November 1963. All data available prior to August 1974 were considered for this compilation. Data are given for 10 nuclides, 155Sm(Z = 62) through 155Lu(Z = 71). The ground-state spins are now well established for the Z = 62 through Z = 67 members of this A-chain. More recent data on atomic masses have resulted in a change in the Q value for the epsilon decay of 155Tb from 1 MeV in the earlier compilation to 817 keV. The bulk of the new data has come from single-nucleon transfer reactions and from semiconductor-detector studies of γ rays from radioactive decay and charged particle-induced reactions. The results have given a much more complete picture of the energy-level structure of these nuclei. 155Sm, 155Eu, 155Gd, and 155Tb are classified as deformed nuclei since they all exhibit well-developed rotational-band structure. The energy-level systematics of these nuclei are treated in terms of the Nilsson model including various well-established couplings. The N = 89 nuclide, 155Dy, also exhibits rotational-band structure. However, since it is situated in a mass region where a transition from a deformed to a spherical equilibrium shape is found, the configuration assignments for the observed energy levels are somewhat tentative. Absolute β--, 0000e)-, and epsilon(+β+)-branching ratios are given on the decay-scheme drawings. In all cases these quantities are based on a (γ+ce) intensity balance normalized to 100 percent decay to the ground state of the daughter nucleus. Specific details concerning the methods used to estimate the β- or epsilon-branching to the ground state of the daughter nucleus are discussed in connection with the individual nuclides. (U.S.)