Ytterbium oxide enriched to 95.9% in mass number 172 was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of (6.70±0.04) days was produced and its assignment to ${\mathrm{Lu}}^{172}$ confirmed by the identification of the ytterbium $K$ x ray and by comparison with the activities produced by similar proton irradiations of the other enriched isotopes of ytterbium. The 4.0-hour positron activity previously assigned to ${\mathrm{Lu}}^{172}$ was not observed and is best attributed to an impurity. The observed activity of ${\mathrm{Lu}}^{172}$ consists of the $L$ and $K$ x rays of ytterbium and gamma rays with energies of 79, 91, 113, 182, 203, 270, 324, 373, 490, 527, 697, 809, 900, 912, 1093, 1402, and 1583 kev. Because no positron radiation exists in the activity of ${\mathrm{Lu}}^{172}$, the mode of decay is solely by electron capture to ${\mathrm{Yb}}^{172}$. Gamma-gamma coincidence measurements, energy considerations, and consideration of the relative numbers of the radiations observed in the activity of ${\mathrm{Lu}}^{172}$ have led to the assignment of energy levels at 530 (6+), 1172 (3+), 1263 (4+), 1375 (5+), 1662 (3-), (1699), and 2072 (4+) kev in ${\mathrm{Yb}}^{172}$ in addition to the previously known 78.7 (2+)- and 260.2 (4+)-kev levels. The positions of all of the observed radiations and some observed only in conversion electron measurements are shown in a proposed energy level scheme for the decay of ${\mathrm{Lu}}^{172}$. Approximate branching ratios for the electron capture disintegrations of ${\mathrm{Lu}}^{172}$ are also shown in the level scheme. Few, if any, electron capture transitions of ${\mathrm{Lu}}^{172}$ occur to the ground and first excited states of ${\mathrm{Yb}}^{172}$. Of the two predicted spins for the ground state of ${\mathrm{Lu}}^{172}$, 4- is more consistent with the proposed energy level scheme.

• Received 5 October 1959

DOI:https://doi.org/10.1103/PhysRev.118.1067