Energy levels of 166Er excited in the decay of 166Tm

Abstract

The electron capture decay 166 Tm → 166 Er has been investigated in a six-gap β-ray spectrometer and with Ge(Li) γ-ray detectors. Conversion lines, single γ-ray spectra and e-γ coincidence relations have been extensively studied. The results with respect to the ground-state band and the gamma-vibrational band of 166Er are in good agreement with those of earlier investigations. Above the gamma-vibrational band, ten levels are definitely established, having the following energies (in keV), spin values and parities: 1460, (2) −; 1516, 3 −; 1574, (4) −; 1920, 3 −; 1941, (2 or 3) +; 1980, (2, 3 or 4) +; 2004, (4 −?); 2135, 3 +; 2162, 3 + and 2218, (1,2, or 3) −. Further three levels are suggested at 1530, 2023 and 2293 keV. The 1590 keV and 1687 keV (5 −) levels found in other investigations are probably also populated in the decay of 166Tm. The decay scheme accounts for 70 of the 90 γ-transitions observed in the present work. The structure of the levels of 166Er is discussed in terms of current nuclear models. In particular, the 1460 keV level is interpreted as an octupole-vibrational K = 2 − state, with the associated rotational band having parameters A = 9.5 keV and B = −10 eV. Evidence for a complicated structure of the 2135 keV 3 + state is presented.