Decay ofTm173

The mass assignment of ${\mathrm{Tm}}^{173}$ has been confirmed by cross bombardments involving the reactions ${\mathrm{Er}}^{170}(\alpha , p){\mathrm{Tm}}^{173}$, ${\mathrm{Yb}}^{173}(n, p){\mathrm{Tm}}^{173}$, and ${\mathrm{Yb}}^{174}(\gamma , p){\mathrm{Tm}}^{173}$ with isotopically enriched targets, and its decay has been studied by $\beta$-ray and $\gamma$-ray scintillation spectrometry. The half-life of ${\mathrm{Tm}}^{173}$ is 8.24±0.08 h. Three gamma rays, with energies (and relative intensities) of 66 (0.01), 399 (1.0), and 465 (0.09) keV, were observed. Analysis of the $\beta$-ray spectrum indicates the presence of two major groups with end-point energies of 1.32±0.03 MeV (2%) and 0.89±0.04 MeV (98%). Gamma-gamma coincidence measurements and the $\gamma$-ray intensity data show that the 0.89-MeV $\beta$-ray group is composed of two components which differ in energy by 66 keV. The proposed decay scheme for ${\mathrm{Tm}}^{173}$ involves excited levels in ${\mathrm{Yb}}^{173}$ at 399 and 465 keV. The 399-keV level is an isomeric state with a 3.5±0.3 μsec half-life, and it de-excites by a pure $E2\mathrm{}$ transition which is hindered by a factor of ≈1.6×${10}^3$ compared to the single-particle estimate. This hindrance is attributed to violation of the asymptotic selection rules for deformed nuclei.