Nuclear structure of ¹⁷⁶Lu and its astrophysical consequences : I. Level scheme of ¹⁷⁶Lu

Excited states of the deformed odd-odd nucleus Lu-176 have been investigated by the following experiments: measurement of the Lu-175(n,gamma)Lu-176 reaction with high resolution crystal spectrometers and of the Lu-175(n,e-)Lu-176 reaction with a double focusing magnet-spectrometer. In total, 509 gamma transitions could be identified in Lu-176, and multipolarities were determined for 228 of these transitions. Additionally, a measurement of gamma-gamma coincidences after neutron capture and an investigation of the Lu-175(d,p)Lu-176 transfer reaction were also performed. Information on the lifetimes of relevant levels was obtained by the technique of delayed coincidences, and, in one case, by the Doppler shift attenuation method. With these results, a level scheme was established, comprising 97 energy levels connected by 270 gamma transitions. About 30 Nilsson configurations and corresponding rotational bands were identified. The comparison with model calculations indicates that the level scheme comprises all excited states with spins 1 > I > 8 up to 900 keV. In particular, this scheme contains transitions that connect the I-pi = 7- ground state with the 1- isomer via mediating levels at higher excitation energy. From this coupling, the excitation energy of the isomer was precisely defined to 122.855 +/- 0.009 keV. Accordingly, the neutron separation energy of Lu-176 could be revised to 6287.91 +/- 0.15 keV. Based on the fact that more than 90% of the observed intensities could be placed in the level scheme, an isomeric ratio, sigma-p(i)/sigma-tot = 0.870 +/- 0.025, was deduced for the fractional population of the isomer by thermal neutron captures.