First Determination of Level Structure of an $sd$-Shell Hypernucleus, ${}_{\mathrm{\Lambda }}^{19}\mathrm{F}$

We report on the first observation of $\gamma$ rays emitted from an $sd$-shell hypernucleus, ${}_{\mathrm{\Lambda }}^{19}\mathrm{F}$. The energy spacing between the ground state doublet, $1\mathrm{/}{2}^{+}$ and $3\mathrm{/}{2}^{+}$ states, of ${}_{\mathrm{\Lambda }}^{19}\mathrm{F}$ is determined to be $315.5\pm 0.4(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}{)}_{-0.5}^{+0.6}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{k}\mathrm{e}\mathrm{V}$ by measuring the $\gamma$-ray energy from the $M1(3\mathrm{/}{2}^{+}\to 1\mathrm{/}{2}^{+})$ transition. In addition, three $\gamma$-ray peaks were observed and assigned as $E2(5\mathrm{/}{2}^{+}\to 1\mathrm{/}{2}^{+})$, $E1(1\mathrm{/}{2}^{-}\to 1\mathrm{/}{2}^{+})$, and $E1(1\mathrm{/}{2}^{-}\to 3\mathrm{/}{2}^{+})$ transitions. The excitation energies of the $5\mathrm{/}{2}^{+}$ and $1\mathrm{/}{2}^{-}$ states are determined to be $\rm 895.2 \pm 0.3 (stat) \pm 0.5 (syst)~keV$and$\rm 1265.6 \pm 1.2 (stat) ^{+0.7}_{-0.5} (syst)~keV$, respectively. It is found that the ground state doublet spacing is well described by theoretical models based on existing $s$- and $p$-shell hypernuclear data.