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

We report on the first observation of $gamma$ rays emitted from an $sd$-shell hypernucleus, $rm ^{19}_{Lambda}F$. The energy spacing between the ground state doublet, $1/2^{+}$ and $3/2^{+}$ states, of $rm ^{19}_{Lambda}F$ is determined to be $rm 315.5 pm 0.4 (stat) ^{+0.6}_{-0.5} (syst)~keV$ by measuring the $gamma$-ray energy from the $M1(3/2^{+} rightarrow 1/2^{+})$ transition. In addition, three $gamma$-ray peaks were observed and assigned as $E2(5/2^{+} rightarrow 1/2^{+})$, $E1(1/2^{-} rightarrow 1/2^{+})$, and $E1(1/2^{-} rightarrow 3/2^{+})$ transitions. The excitation energies of the $5/2^{+}$ and $1/2^{-}$ 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.