Chronology of the three-body dissociation of $^8$He

The space and time configurations of the dissociation of 8He into 6He+n+n, on C and Pb targets, have been explored simultaneously for the first time. The final-state interactions in the n-n and 6He-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 7He, 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 7He. 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.