7ΛLi ground-state spin determined by the yield of γ-rays subsequent to weak decay

Abstract Using a large-acceptance germanium detector array (Hyperball), we measured a γ -ray spectrum in coincidence with production of 7 Λ Li bound states in the 7 Li( π + , K + ) reaction. We observed a γ -ray peak at 429 keV due to the transition from the first excited state of 7 Be produced by the weak decay of 7 Λ Li, 7 Λ Li → 7 Be ∗ (1/2 − ,429  keV )+π − . From the yield of this γ -ray, the branching ratio of the 7 Λ Li weak decay to 7 Be ∗ (429) was obtained to be (6.0 +1.3 −1.6 )×10 −2 . Compared with theoretically calculated values of the partial decay rates, the measurement indicates that the ground-state spin of 7 Λ Li is 1/2 but not 3/2.