Chronology of the three-body dissociation of 8 He

The space and time configurations of the dissociation of He into He+n+n, on C and Pb targets, have been explored simultaneously for the first time. The final-state interactions in the n-n and He-n channels are successfully described within a model that considers independent emission of neutrons from a Gaussian volume with a given lifetime. The dissociation on C target exhibits a dominant sequential decay through the ground state of He, consistent with neutrons being emitted from a Gaussian volume of fm with a n-n delay in the sequential channel of 1400 400 fm/c, in agreement with the lifetime of He. The lower-statistics data on Pb target correspond mainly to direct breakup, and are well described using the n-n volume measured, without any n -n delay. The validity of the phenomenological model used is discussed.