Two-quasiparticle states in 168Er

Abstract

Abstract The level structure of 168Er and the de-excitation mechanism in this nucleus have been studied by radiative capture of thermal neutrons in samples of natural erbium which give a cross-section contribution of 89.8±2.3 % for 167Er. Interference from the 9.1±2.1 % capture contribution of 166Er was well known from a separate study with enriched 166Er samples. Highresolution measurements of the γ-ray spectrum have been performed using the Karlsruhe Ge(Li) anti-Compton spectrometer below 2 MeV and a Ge(Li) double-escape spectrometer above 5.16 MeV. The high accuracy of the data allows the construction of a considerably extended transition diagram up to 2 MeV excitation energy. A large number of new levels has been assigned to specific configurations and their superimposed rotational bands. Conclusive evidence is given that the previously known states at 1094 keV and 1542 keV are the two-quasiparticle neutron levels [633 ↑ + 524 ↓]nn and [633↑−521↓]nn, respectively. Some aspects of the corresponding log ft values are discussed in detail. Rotational bands at 1354 keV, 1569 keV and 1542 keV have to be identified with the Kπ = 1−,2− and 3− octupole vibrational bands. The properties of the Kπ = 1− state are presumably close to those of the neutron level [512 ↑–633↓]nn. The 3− rotational member of the Kπ = 0− octupole band probably occurs at 1914 keV. The simple model of pairing plus state-independent octupole force provides a surprisingly good microscopic description of the octupole excitation energies. The first Kπ = 0+ band has been well established to occur at 1217 keV. Within the model of pairing plus quadrupole and spin-quadrupole force one obtains the interaction constants kq = 5.3 and kt = 8.8 (in units of A − 1 3 h ω 0 ). The neutron level [521↓ + 642 ↑]nn has been tentatively assigned to a rotational band at 1893 keV. The properties of some other more or less collective states are discussed. No positive evidence has been found for the occurrence of two-quasiparticle proton levels below 1820 keV excitation energy. The neutron separation energy was determined to be 7771.24 ±0.48 keV.