100 Years of Nuclear Isomers

Half-life of short-lived bare isomer $^{94m}_{44}$Ru$^{44+}$ has been studied for the first time at the HIRFL-CSR facility in Lanzhou by employing the Isochronous Mass Spectrometry (IMS) method. It was populated in projectile fragmentation of a $^{112}$Sn beam on a $^{9}$Be target. Fragmentation products in the region of interest were passed through the radioactive ion beam line and injected into the experimental ring. Isochronous mass spectrometry was performed normally 33 microsecond after fragmentation were injected into the experimental ring by using a pulsed high-voltage power supply. The transition of 8$^+$ isomer $^{94m}_{44}$Ru$^{44+}$ via $\gamma$ cascades to a final 0+ ground state was directly observed when it was being stored in the ring. The half-life of $^{94m}_{44}$Ru$^{44+}$ obtained from 39 decay cases and 29 no decay cases is considerably longer than that of its neutral atom due to the hindrance of internal conversion in an bare ion. The isomeric half-life was deduced to be 102(17) $\mu$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, $^{94m}$Ru$^{44+}$ represents the shortest-lived nuclear state whose mass has ever been measured directly.