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

7 pags., 5 figs., 1 app.
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.
The support provided by the technical staff of LPC and the LISE crew is gratefully acknowledged, as are the efforts of the GANIL cyclotron operation team for providing the primary beam. We also wish to express our appreciation for the vital contributions made by our late colleague and friend Jean-Marc Gautier to all the CHARISSA+DEMON experiments undertaken by our collaboration.