$K^{\pi }=0^{+}$ 2.29 s isomer in neutron-rich ${}^{174}\mathrm{Tm}$

Gamma-ray and conversion-electron spectroscopy have established the existence of a 2.29(1) s, $K^{\pi }=0^{+}$, isomeric state in neutron-rich ${}^{174}\mathrm{Tm}$. The isomer deexcites via 100- and 152-keV electromagnetic transitions. First results from a newly commissioned Si(Li) detector array have established their $M1$ and $E3$ multipolarities, respectively. The single-particle configurations of the excited states suggest that the $E3$ transition originates from a $\pi h_{11/2}^{-1}\to \pi d_{3/2}$ configuration change, whereas the $M1$ transition occurs between members of a Gallaghar-Moszkowski doublet. From the measured half-life, the deduced $B(E3)$ value of 0.024(2) W.u. is highly hindered. The reported measurements resolve ambiguities in the previously proposed β decay scheme of ${}^{174}\mathrm{Er}$ to ${}^{174}\mathrm{Tm}$.

Figure 1

The short-lived singles γ-ray spectrum showing prominently the Tm K x rays and the 100- and 152-keV γ-ray transitions. The 5.4-m ${}^{174}\mathrm{Tm}$ ground-state β-decay component has been subtracted. The inset shows the growth and decay of the isomer gated by the 100-keV γ-ray transition.

Figure 2

The singles conversion-electron spectrum obtained by subtracting the 5.4-m ${}^{174}\mathrm{Tm}$ ground-state β-decay component. $K_{\gamma }$ denotes the conversion-electron corresponding to the γ-ray transition, whereas the Tm x rays are labeled $K_{\alpha }/K_{\beta }$.

Figure 3

The conversion-electron spectrum gated by the 100-keV γ-ray transition. The earlier [15, 17], incorrect, $M1$-multipole assignment for the 152-keV transition would have given rise to strong K conversion and very little L and M conversion. The inset depicts the isomer decay scheme as a result of the present work.