First Determination of the Level Structure of an sd -Shell Hypernucleus, F Λ19

We report on the first observation of γ rays emitted from an sd-shell hypernucleus, FΛ19. The energy spacing between the ground state doublet, 1/2+ and 3/2+ states, of FΛ19 is determined to be 315.5±0.4(stat)-0.5+0.6(syst) keV by measuring the γ-ray energy of the M1(3/2+→1/2+) transition. In addition, three γ-ray peaks are observed and assigned as E2(5/2+→1/2+), E1(1/2-→1/2+), and E1(1/2-→3/2+) transitions. The excitation energies of the 5/2+ and 1/2- states are determined to be 895.2±0.3(stat)±0.5(syst) and 1265.6±1.2(stat)-0.5+0.7(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.