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Description
The odd--odd iodine isotopes are known to exhibit a rich variety of nuclear structures, particularly high-$K$ isomeric states~\cite{ref1}. These isomers are generally associated with coupled configurations involving the proton intruder $g_{9/2}$ orbital and the neutron intruder $h_{11/2}$ orbital~\cite{ref2}. In particular, an isomeric state with spin--parity $I^{\pi} = 7^{-}$, originating from the coupling of a proton in the $g_{9/2}[404]9/2$ orbital with a neutron in the $h_{11/2}[532]5/2$ orbital, has been identified in the $^{116,118,120,122}\mathrm{I}$ isotopes~\cite{ref3}. More recently, Moon \textit{et al.} reported several high-$K$ isomeric states among the low-lying excited states of $^{124}\mathrm{I}$, connected through low-energy $\gamma$-ray transitions~\cite{ref4}. Motivated by these findings, the present work investigates the low-lying excited states of $^{118}\mathrm{I}$ with the aim of identifying isomeric structures.
The excited states of $^{118}\mathrm{I}$ were populated through the $^{109}\mathrm{Ag}(^{13}\mathrm{C}, 4n\gamma)^{118}\mathrm{I}$ reaction at a beam energy of $54$~MeV. A $^{13}\mathrm{C}$ beam, provided by the $14$UD Pelletron accelerator (TIFR), was incident on a $^{109}\mathrm{Ag}$ target of thickness $1.62$~mg/cm$^{2}$ backed with $10.5$~mg/cm$^{2}$ of Au. The emitted $\gamma$ rays were detected in the Indian National Gamma Array (INGA), which consisted of $18$ Compton-suppressed clover HPGe detectors. The two-fold and higher-fold data were stored in list mode using the PIXIE-16-based digital data acquisition system.
We determined the half-lives of the $8^{+}$ and $8^{-}$ excited states in $^{118}\mathrm{I}$ using the time-difference spectrum. These states were previously reported to arise from the configurations $\pi h_{11/2}[523]7/2 \otimes \nu h_{11/2}[514]9/2$ and $\pi g_{9/2}[404]9/2 \otimes \nu h_{11/2}[523]7/2$~\cite{ref5}. The time-difference spectrum was constructed by appropriately combining four conditional spectra: $\gamma_{1}$ (start), $\gamma_{2}$ (stop), and the respective background contributions associated with $\gamma_{1}$ and $\gamma_{2}$. A brief description of the procedure for generating the time-difference spectrum can be found in Ref.~\cite{ref6}. The half-life was then extracted by fitting the resulting time-difference spectra with a convolution of Gaussian and exponential functions, yielding values of $68(5)$~ns and $57(6)$~ns for the $8^{+}$ and $8^{-}$ states, respectively.
Since these states involve high-$\Omega$ orbitals for both the proton and neutron, they are interpreted as $K$-isomeric states. The observed half-life of the $8^{-}$ state is longer than that reported for $^{124}\mathrm{I}$, but shorter than that observed in $^{122}\mathrm{I}$. A shell-model calculation using the \textsc{ANTOINE} code is ongoing to determine the transition probabilities for these states.
In summary, the half-lives of the $8^{+}$ and $8^{-}$ states in $^{118}\mathrm{I}$ were determined by fitting the time-difference spectra using a convolution of Gaussian and exponential functions. These states have been identified as $K$-isomers associated with high-$\Omega$ orbitals. Shell-model calculations are currently in progress to evaluate the transition probabilities of these states.
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\bibitem{ref1} P. Walker and G. Dracoulis, Nature (London) 399, 35 (1999).
\bibitem{ref2} G. D. Dracoulis, Phys. Scr. T 88, 54 (2000).
\bibitem{ref3} E. S. Paul et al., J. Phys. G 22, 653 (1996)
\bibitem{ref4} C.-B. Moon, B. Moon, and J. Park Phys Rev C 103, 034318 (2021).
\bibitem{ref5} C.-B. Moon et al., Nuclear Physics A 728 (2003).
\bibitem{ref6} Md. S. R. Laskar et al., Phys. Rev. C 104, L011301 (2021).
\end{thebibliography}