Identification of a $6.6\mu \mathrm{s}$ isomeric state in ${}^{175}\mathrm{Ir}$

An experiment has been performed to study excited states in the neutron-deficient nucleus ${}^{175}\mathrm{Ir}$ via the use of the JUROGAM II high-purity germanium detector array and the RITU gas-filled separator at JYFL, Jyväskylä. By using isomer tagging, an isomeric state with a half-life of 6.58(15) $\mu \mathrm{s}$ has been observed in ${}^{175}\mathrm{Ir}$ for the first time. It has been established that the isomer decays via a 45.2 ($E1$)–26.1 ($M1$) keV cascade to new states below the previously reported ground state in ${}^{175}\mathrm{Ir}$ with $I^{\pi }=(5/2^{-})$. We now reassign this ($5/2^{-}$) state to the isomeric state discovered in this study.

Figure 1

(a) Recoil-gated isomeric PGD $\gamma$-ray energy spectrum observed within $3–20\mu \mathrm{s}$ of the implantation of an ER in the DSSD (black). In red are the time-random events observed within a time interval of $-5$ to $0\mu \mathrm{s}$ before the recoil implantation and normalized with respect to the black spectrum, based on the ratio of widths of the time intervals. (b) Background subtracted PGD energy spectrum of $\gamma$ rays observed within $3–20\mu \mathrm{s}$ of the implantation of an ER. The peak at 147.4 keV is a known transition in ${}^{177}\mathrm{Pt}$. The inset shows the decay curve of the 45.2 keV transition together with the fit (in red) which consists of a exponential (in blue) plus a constant background (dashed black).

Figure 2

(a) Recoil-gated singles JGII $\gamma$-ray spectrum tagged with the 45.2 keV isomeric transition detected in the PGD. A timing condition of $\mathrm{\Delta }T(\text{ER}-45.2\text{keV})=3–20\mu \mathrm{s}$ was also applied for the PGD. Transitions in the $(1/2^{+})\left[660\right]$ and $(1/2^{-})\left[541\right]$ bands are marked with a dot and open box respectively. (b) Projection of a recoil-gated $\gamma -\gamma$ matrix for events detected in JGII with a gate on the 372.4 keV transition. A contaminant transition is seen at 444 keV and is marked as *. (c) Same as (b), but tagged on the 45.2 keV transition in the PGD. The transitions used to calculate ${\alpha }_{\mathrm{tot},\exp }$(45.2) are labeled in the panels (b) and (c).

Figure 3

Simplified partial level scheme of ${}^{175}\mathrm{Ir}$ showing states relevant to this work, based on the new data from this study and on the previous decay schemes (cf. Fig. 4 in Ref. [4] and Fig. 1 in Ref. [3]). For convenience we use the decay energies, to the nearest keV, as deduced by Dracoulis et al. [4], for which our values are in good agreement. All states and transitions above the ($5/2^{-}$) state, known from the previous studies, are shown in black; the bands are labeled as in the previous works: $K^{\pi }[N,n_z,\mathrm{\Lambda }]$ in Ref. [4] and (1–6) in Ref. [3]. The $(9/2^{-})\to (5/2^{-})$ transition is unobserved in this work and is shown as a dashed transition. The states and transitions in red show the newly observed 45.2–26.1 keV cascade from the $6.6\mu \mathrm{s}$ isomeric state. The dashed 145 keV transition from the ($13/2^{+}$) state shows schematically the decay to the ($5/2^{+}$)[402] band which is unobserved in the IDT spectra. The decay scheme also includes the ($9/2^{-}$)[514] band which is unobserved in the IDT spectra.

Figure 4

Background subtracted PGD energy spectrum for $\gamma$ rays in prompt coincidence ($\mathrm{\Delta }t(\gamma -\gamma )\le 50$ ns) with the 45.2 keV transition. All events must occur within $3–20\mu \mathrm{s}$ of an ER implantation in DSSD.

Figure 5

A comparison of the decay of the $6.6\mu \mathrm{s}$ isomer observed in ${}^{175}\mathrm{Ir}$ with the decay of a 326 ns isomer in ${}^{179}\mathrm{Au}$ [19], showing the similar decay paths.