First Determination of the Level Structure of an Sd-Shell Hypernucleus, F-19(Lambda)

We report on the first observation of gamma rays emitted from an sd-shell hypernucleus, F-19(Lambda). The energy spacing between the ground state doublet, 1/2(+) and 3/2(+) states, of F-19(Lambda) is determined to be 315.5 +/- 0.4(stat)(-0.5)(+0.6) (syst) keV by measuring the gamma-ray energy of the M1(3/2(+) -> 1/2(+)) transition. In addition, three gamma-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.