Hypernuclear fine structure in 9ABe

With a germanium detector array (Hyperball), we observed two γ-ray peaks corresponding to the two transitions (5/2 + → ½ + and 3/2 + → ½ + ) in the 9 A Be hypernucleus which was produced by the 9 Be(K - , π - ) reaction.The energies of the y rays are 3029 ′ 2 ′ 1 keV and 3060 ′ 2 ′ 1 keV. The energy difference was measured to be 31.4 + 2 . 5 - 3 . 6 keV, which indicates a very small A-spin-dependent spin-orbit force between a A and a nucleon. This is the smallest level splitting by far ever measured in a hypernucleus.