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.