Identification of all intrinsic excitations below 2 MeV in168Er

Abstract

The levels and gamma decay in¹⁶⁸Er have been investigated following thermal neutron capture on enriched targets of ¹⁶⁷Er₂O₃. Precision measurements of the gamma spectrum with curved-crystal spectrometers revealed over 700 transitions up to an energy of 2.5 MeV, with intensities spanning five orders of magnitude. A list of primary gamma transitions populating levels in ¹⁶⁸Er up to an excitation energy of 3.1 MeV resulted from measurements with the pair spectrometer. Information on the gamma spectrum in the energy region 1400<E<2600 keV was obtained using a Ge(Li) spectrometer. The conversion electron spectrum up to 2.2 MeV electron energy was measured with a beta spectrometer, whence multipolarities of many of the transitions were derived. Average resonance capture measurements at neutron energies of 2 keV and 24 keV were made which ensured that the set of spin 2-5 levels below about 2.2 MeV was complete. From all these data, employing the Ritz combination principle, a level scheme was developed and the levels were arranged into a complete set of 20 low-K rotational bands. This is the most complete and extensive test of the limits of applicability of models for low-lying collective excitations.