Using a germanium detector array, Hyperball, we have measured \gamma rays from several \Lambda hypernuclei. In the previous experiments, spin-dependence of \Lambda N effective interaction was investigated. In the latest experiment, we have studied ¹¹_\Lambda B in which six \gamma transitions have been observed. The result of this experiment suggests that the level structure is quite different from a theoretical prediction based on the \Lambda N spin-dependent effective interaction parameters determined in the previous experiments.

PACS numbers: 21.80.+a, 21.10.-k, 23.20.-g, 27.20.+n