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The Sn isotopes have the largest number of stable isotopes of any element; this is largely due to their semi-magic nature (Z=50). They span two neutron shell closures (N=50 to N=82), with 10 stable isotopes between $^{112}$Sn and $^{124}$Sn. This makes them a great laboratory for testing theoretical models as well as investigating changes in nuclear properties with varying neutron number. Much can be said about positive-parity states, such as how shape coexistence in $^{118}$Sn and its neighbouring isotopes is manifested in the observation of ground state positive-parity bands in the vicinity of deformed rotational bands at low energies [1]. On the other hand, little is known about the negative-parity states in these semi-magic nuclei. It is likley that the negative-parity sates in $^{118}$Sn are due to a neutron in the negative-parity $h_{11/2}$ orbital coupling to another neutron in one of the positive-parity $s_{1/2}$, $d_{3/2}$, $d_{5/2}$, or $g_{7/2}$ orbitals [2]. This provides the motivation for an in depth spectroscopic study of the negative-parity structure in $^{118}$Sn.
The negative-parity states in $^{118}$Sn were populated by the $\beta$ decay of the 8.5 second 8$^-$isomer of $^{118}$In, produced in the ISAC facility at TRIUMF. The GRIFFIN [3] spectrometer was used to detect the resulting $\gamma$-rays; GRIFFIN is an incredibly powerful tool, consisting of 16 Compton-suppressed HPGe clover detectors [4] with a total of 64 crystals. GRIFFIN was coupled to the ancillary SCintillating Electron Positron Tagging ARray (SCEPTAR), comprised of 20 plastic scintillators for tagging $\beta$ particles [3] and was fixed with a 20 mm Delrin shield. A total of 6.7 $\times$10$^{7}$ events in addback singles and 2.5 $\times$10$^{7}$ $\gamma-\gamma$ addback events in a 350 ns coincidence window were detected during the experiment.
In the present work, the isomeric composition of the $^{118}$In radioactive ion beam (RIB) produced at the ISAC facility has been determined. Additionally, thirteen new transitions and four new states have been added to the level scheme of $^{118}$Sn. In the near future, angular correlations will be performed to confirm the spin-parities of these new excited states.
[1] K. Ortner $\textit{et al}$., Phys. Rev. C 102, 024323 (2020).
[2] S. Das, $\textit{et al}$. Nucl. Phys. A, 1006, 122079 (2021)
[3] A. Garnsworthy $\textit{et al}$., NIM. A 918, 9 (2019).
[4] U. Rizwan $\textit{et al}$., NIM A 820, 126 (2016).