Investigation of High-Spin States inEr166

Abstract

The decay of the long-lived isomer of ${\mathrm{Ho}}^{166}$ has been studied using gamma-ray scintillation spectrometers and silicon-diode detectors. The results of gamma-gamma coincidence experiments, together with least-squares analysis of the gamma-ray singles spectra, establish the presence of twenty-seven gamma-ray transitions in the decay of this isomer. The conversion-electron spectra provide relative intensities for several of the weak transitions and provided an upper limit for the endpoint energy of the main beta transition from ${\mathrm{Ho}}^{166}$. The data are consistent with the existence of levels whose energies (in keV) and assignments are 80 (2+), 265 (4+), 545 (6+), 787 (2+), 861 (3+), 910 (8+), 957 (4+), 1074 (5+), 1220 (6+), 1374 (7+), 1680 (5-), 1785 (6-), and 1840 (6-). The data indicate that the energy of the most intense beta transition from the long-lived ${\mathrm{Ho}}^{166}$ state must lie between ≅55 and ≅65 keV. Several possible nucleon configurations for the negative-parity states in ${\mathrm{Er}}^{166}$ observed in the decay of this activity are presented and discussed. A description of the observed positive-parity states in terms of an asymmetric-rotator model is given. The agreement between the energies and relative transition probabilities calculated using this model and those observed experimentally is excellent. Formulas for reduced transition probabilities for gamma-ray transitions between states having spins of 6, 7, and 8 are presented.