Increased isomeric lifetime of hydrogen-like ${}^{192m}\mathrm{Os}$

An excited metastable nuclear state of ${}^{192}\mathrm{Os}$ in a hydrogen-like charge state has been studied for the first time. It was populated in projectile fragmentation of a ${}^{197}\mathrm{Au}$ beam on a ${}^9\mathrm{Be}$ target with the UNILAC-SIS accelerators at GSI. Fragmentation products in the region of interest were passed through the fragment separator and injected into the experimental storage ring (ESR). Cooling of the injected beam particles enabled Schottky mass spectrometry to be performed. Analysis shows the lifetime of the state to be considerably longer than that of the neutral ion $[{\tau }_{\mathrm{neut}}=8.5\left(14\right)\mathrm{s}]$; this change is attributed to hindrance of internal conversion in hydrogen-like ${}^{192}\mathrm{Os}$. Calculations have been performed to estimate the lifetime, and the result has been compared with that measured experimentally. There is good agreement between the expected $[{\tau }_{\mathrm{H}-\mathrm{like}}=13.0\left(24\right)\mathrm{s}]$ and measured lifetimes (${\tau }_{\mathrm{rest}}=15.1_{-1.3}^{+1.5}$ s) from the internal decay of ${}^{192m}\mathrm{Os}$. This provides a test for the reliability of the values obtained from internal conversion coefficient calculations in highly ionized systems and is the first measurement of its kind to be performed using the ESR setup.

Figure 1

Partial decay scheme of ${}^{192}\mathrm{Os}$, including the 8.5 s isomer, figure adapted from Ref. [19]. The square brackets denote implied transitions, and for clarity some $\gamma$-ray energies have been omitted.

Figure 2

A typical example of a Schottky frequency spectrum with 3.5 s time slices. Large amounts of background noise are observed before adequate time is allowed for cooling. The isomer ${}^{192}\mathrm{Os}$ is resolved from the ${}^{192\mathrm{g}}\mathrm{Os}$ state. The sudden absence of the previously observed isomer corresponds to the nuclear decay of the excited state. At 31.5 s after injection, in all analyzed injections the background noise was sufficiently low to use this as a start time for lifetime measurements.

Figure 3

Histogram showing the experimentally measured number of ions observed in each time bin. The points are the number of ions expected within each bin derived from a ${\chi }^2$ fit to the data, with an uncertainty evaluated from the error in the fit.