Decay of Long-LivedHo166

Abstract

The gamma radiation following the decay of the long-lived ${\mathrm{Ho}}^{166}$ activity has been studied using lithium-ion-drifted germanium counters and NaI(Tl) scintillation detectors. In addition to providing more precise energy measurements for many of the transitions than was possible in our earlier study, the higher resolution of the germanium detector revealed that two of the previously reported high-energy gamma rays are doublets. Thus, the level previously reported at 1680 keV has been revealed as two levels, separated by 26 keV. Gamma-gamma coincidence studies have revealed the existence of transitions having energies of 96, 122, 137, and 163 keV which excite these two levels. Gamma-gamma directional correlation measurements have been carried out on prominent cascades. The results of the correlations confirm the spins previously reported. The existence of levels in ${\mathrm{Er}}^{166}$ having the following energies in keV and spin and parity assignments is proposed: 0,0+; 90,2+; 265,4+; 544,6+; 787,2+; 861,3+; 910,8+; 957,4+; 1074,5+; 1215,6+; 1374,7+; 1662,5-; 1688,5-; 1784,6-; and 1825,6-. The gamma-ray relative intensities indicate that there is no appreciable beta feeding of the two 5- states. The mixing-ratio parameters δ for the interband transitions of 529, 830, and 810 keV are all positive and large. For the first two of these, the M1 admixture is less than 0.1%, while for the last, it is about 0.1%. An interpretation of the properties of the positive-parity states is given in terms of the strong-coupling nuclear model with band mixing and in terms of a generalized asymmetric-rotator model. Possible interpretations of the nature of the negative-parity states are presented.