Observation of a spin-flip M1 transition in Λ7Li

Using a large-acceptance germanium detector array (Hyperball),we have observed a spin-flip M1 gamma transition between the ground-state spin doublet of Li-7(Lambda) (3/2(+) --> 1/2(+)). The observed energy of 691.7 +/- 0.6(stat) +/- 1.0(syst) keV provides crucial information on the strength of the spin-spin interaction between a Lambda and a nucleon. This is the first observation of well-identified hypernuclear gamma transitions using germanium detectors.