${\mathrm{Er}}^{172}$ and ${\mathrm{Dy}}^{166}$, with half-lives of 48.7±0.5 hr and 81.8±0.2 hr, respectively, were produced by double neutron capture from ${\mathrm{Er}}^{170}$ and ${\mathrm{Dy}}^{164}$. Gamma rays of the following energies were found in the decay of ${\mathrm{Er}}^{172}$: two coincident 50-kev radiations, 72, 128, 170, 200, 410, 440, and 610 kev. The proposed energy levels for ${\mathrm{Tm}}^{172}$ consistent with the data are as follows: 410, 440, 482, 538, and 610 kev. $\mathrm{Log}\mathrm{ft}$ values of beta transitions indicate that the 538- and 610-kev levels have spin assignments of 0 or 1 with positive parity. The following gamma transitions were found in the decay of ${\mathrm{Dy}}^{166}$: 84, 291, 343, 373, and 427 kev. In addition to showing conversion electrons of the 84-kev transition, permanent-magnet spectrometer studies revealed the presence of two additional transitions of 30 and 54 kev which were not observed in scintillation studies. The multipolarity of the low-energy transitions are as follows: 30 kev-$M1\mathrm{}$, 54 kev-$E2\mathrm{}$, and 84 kev-$M1\mathrm{}$. The proposed energy levels for ${\mathrm{Ho}}^{166}$ are at 54, 84, 375, and 427 kev.

• Received 10 November 1961

DOI:https://doi.org/10.1103/PhysRev.126.642