γ-ray spectroscopy of Λ16O and Λ15N hypernuclei via the O16(K-,π-γ) reaction

The bound-state level structures of the O-16(Lambda) and N-15(Lambda) hypernuclei were studied by gamma-ray spectroscopy using a germanium detector array (Hyperball) via the O-16 (K-, pi(-) gamma) reaction. A level scheme for O-16(Lambda) was determined from the observation of three gamma-ray transitions from the doublet of states (2(-), 1(-)) at similar to 6.7 MeV to the ground-state doublet (1(-), 0(-)). The N-15(Lambda) hypernuclei were produced via proton emission from unbound states in O-16(Lambda). Three gamma rays were observed, and the lifetime of the 1/2(+); 1 state in N-15(Lambda) was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the Lambda N interaction is discussed. In particular, the measured O-16(Lambda) ground-state doublet spacing of 26.4 +/- 1.6 +/- 0.5 keV A determines a small but nonzero strength of the Lambda N tensor interaction.