Half-life measurement of short-lived ${}_{44}{}^{94m}\mathbf{Ru}^{44+}$ using isochronous mass spectrometry

Decay of the $8^{+}$ isomer in fully stripped ions ${}^{94}\mathrm{Ru}^{44+}$ is observed during its circulation in the experimental Cooler Storage Ring (CSRe) at the Heavy Ion Research Facility in Lanzhou (HIRFL). The ${}^{94}\mathrm{Ru}^{44+}$ ions were produced via projectile fragmentation and stored in CSRe tuned into the isochronous ion-optical mode. The timing signals of the ions, passing through a time-of-flight detector, were consecutively registered and used to determine the variation of revolution time as a function of revolution number. A sudden change of the revolution time at a specific revolution was identified as a fingerprint of the ${}^{94}\mathrm{Ru}^{44+}$ isomer decay. The isomeric half-life was deduced to be 102(17) $\mu \mathrm{s}$, which agrees well with the theoretical expectation by blocking the internal conversion decay of the isomer. Our work proved the feasibility of studying decays of short-lived isomers in high atomic charge states using the isochronous mass spectrometry. In addition, ${}^{94m}\mathrm{Ru}^{44+}$ represents the shortest-lived nuclear state whose mass has ever been measured directly.

Figure 1

An example of timing signals from the MCP detector recorded by the oscilloscope. The detector was switched on $33\mu \mathrm{s}$ after the injection of ions into CSRe. The entire measurement of $200\mu \mathrm{s}$ and a part of the spectrum zoomed on the first $3\mu \mathrm{s}$ are illustrated in panels (a) and (b), respectively.

Figure 2

The revolution time spectrum enlarged in a narrow time window of 669.0 ns $\le$ t $\le 676.5$ ns. The inset shows the well-resolved peaks of the ground (g) and isomeric (m) states of ${}^{94}\mathrm{Ru}$. Identified decay events of ${}^{94m}\mathrm{Ru}$ were excluded from this spectrum.

Figure 3

The revolution time as a function of revolution number for no-decay (a) and decay (b) events. The residuals of the linear fit of the data for these two events are shown in panels (c) and (d), respectively. The vertical lines in panels (b) and (d) mark the decay time. Please note the significantly different scales in the upper and lower panels. The oscillation-like behavior seen in the upper panels may be due to the finite emittance of the beam [41, 42].