Radioactive Decay ofLu171and Level Schemes forYb171andYb169

Abstract

Ytterbium oxide enriched to 93.8% in the mass number 171 was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of (8.28±0.04) days was produced and its assignment to ${\mathrm{Lu}}^{171}$ confirmed by the identification of the ytterbium $K$ x ray and by comparison with the activities produced by similar proton irradiations of each of the other enriched isotopes of ytterbium. The radiations observed in this activity were the $L$ and $K$ x rays of ytterbium and gamma rays with energies of 72-76, 668±7, 740±3, and 841±8 kev. Because neither particle nor annihilation radiation exists in this activity, the mode of decay is solely by electron capture to ${\mathrm{Yb}}^{171}$. Gamma-gamma coincidence measurements were performed for the observed radiations. An energy level scheme has been constructed for the decay of ${\mathrm{Lu}}^{171}$ using the results of the gamma-gamma coincidence measurements, an energy analysis of the conversion electron data, and calculations of transitions intensities for various trial multipole orders. Levels in ${\mathrm{Yb}}^{171}$ at 0 (½- [521]), 66.7 ($\frac{3}{2}-\frac{1}{2}$), 75.9 ($\frac{5}{2}-\frac{1}{2}$), 95.1 ($\frac{7}{2}+\left[633\right]\mathrm{} \tau >{10}^{-5\mathrm{}}$ sec), 122.4 ($\frac{5}{2}-\left[512\right]\mathrm{}$), 167.3 ($\frac{9}{2}+\frac{7}{2}$), 208.1 ($\frac{7}{2}-\frac{5}{2}$), 230.5 ($\frac{7}{2}-\frac{1}{2}$), 246.5 ($\frac{9}{2}-\frac{1}{2}$), 317.3 ($\frac{9}{2}-\frac{5}{2}$), (350), 835 ($\frac{7}{2}-\left[514\right]\mathrm{}$), 862 (possibly $\frac{5}{2}+\left[642\right]\mathrm{}$), 935 ($\frac{9}{2}+\left[624\right]\mathrm{}$), and 949 ($\frac{7}{2}$) kev account for thirty of thirty-one transitions reported for this activity. Transition probabilities and branching ratios for the electron capture decay have been calculated. Eighty-four percent of the disintegrations of the $\frac{7}{2}+\left[404\right]\mathrm{}$ ground state of ${\mathrm{Lu}}^{171}$ occur to the $\frac{7}{2}-\left[514\right]\mathrm{}$ state at 835 kev in ${\mathrm{Yb}}^{171}$. The previously reported 600-day activity of ${\mathrm{Lu}}^{171}$ was not found. An energy level scheme for the decay of ${\mathrm{Lu}}^{169}$ (1.5 days) as reported by other workers is proposed with levels in ${\mathrm{Yb}}^{169}$ at 0 ($\frac{7}{2}+\left[633\right]\mathrm{}$), 24.2 (½- [521]), 70.9 ($\frac{9}{2}+\frac{7}{2}$), 87.0 ($\frac{3}{2}-\frac{1}{2}$), 99.3 ($\frac{5}{2}-\frac{1}{2}$), 161.7 ($\frac{11}{2}+\frac{7}{2}$), 191.4 ($\frac{5}{2}-\left[512\right]\mathrm{}$), 244.0 ($\frac{7}{2}-\frac{1}{2}$), 264.5 ($\frac{9}{2}-\frac{1}{2}$), 278.7 ($\frac{7}{2}-\frac{5}{2}$), 389.7 ($\frac{9}{2}-\frac{5}{2}$), 523.1 ($\frac{11}{2}-\frac{5}{2}$), 570.5 ($\frac{5}{2}+\left[642\right]\mathrm{}$), 648.2 ($\frac{7}{2}-\left[514\right]\mathrm{}$), 962.2, 1451.6 ($\frac{9}{2}+\left[624\right]\mathrm{}$), 1456 ($\frac{9}{2}$), and 1465 ($\frac{9}{2}$) kev and with less certainity, at 488.9 ($\frac{11}{2}-\frac{1}{2}$), 798, 870, 953, and 973 kev. This scheme accounts for 53 observed transitions following the decay of ${\mathrm{Lu}}^{169}$. The ground state of ${\mathrm{Lu}}^{169}$ is probably the $\frac{9}{2}$ [514] orbital also assigned to the ground state of ${\mathrm{Lu}}^{173}$. This scheme is based upon an energy analysis of the reported transitions, intensity calculations for various trial multipole orders, and analogy with the scheme proposed for ${\mathrm{Yb}}^{171}$.