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

Abstract

We report on the first observation of ${\hat{I}}^3$ rays emitted from an $sd$-shell hypernucleus, ${}_{\hat{\mathrm{I}}}^{19}›\mathrm{F}$. The energy spacing between the ground state doublet, $1/2^{+}$ and $3/2^{+}$ states, of ${}_{\hat{\mathrm{I}}}^{19}›\mathrm{F}$ is determined to be $315.5\pm 0.4(\mathrm{stat}{)}_{-0.5}^{+0.6}(\mathrm{syst})\mathrm{keV}$ by measuring the ${\hat{I}}^3$-ray energy from the $M1(3/2^{+}\to 1/2^{+})$ transition. In addition, three ${\hat{I}}^3$-ray peaks were observed and assigned as $E2(5/2^{+}\to 1/2^{+})$, $E1(1/2^{-}\to 1/2^{+})$, and $E1(1/2^{-}\to 3/2^{+})$ transitions. The excitation energies of the $5/2^{+}$ and $1/2^{-}$ states are determined to be $895.2\pm 0.3(\mathrm{stat})\pm 0.5(\mathrm{syst})\mathrm{keV}$ and $1265.6\pm 1.2(\mathrm{stat}{)}_{-0.5}^{+0.7}(\mathrm{syst})\mathrm{keV}$, 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.