Observation of hypernuclear fine structure in 16ΛO

We observed two γ-ray transitions of 16ΛO in the 16O(K−,π−γ) reaction using a germanium detector array, Hyperball. The γ rays are assigned as the M1 transitions from the 6 MeV excited state (1−) to the ground state doublet (1−,0−). The energies of the γ rays are 6534.1±1.5keV and 6560.2±1.3keV, and the energy spacing is 26.1±2.0keV, which corresponds to the ground state doublet spacing. From their yield ratio of 0.65±0.11(N(6534)/N(6560)), the spin order of the ground state doublet is determined. This result gives a small value for the ΛN tensor force strength.