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

Abstract

The space and time configurations of the dissociation of ${}^8$He into ${}^6$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 ${}^6$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 ${}^7$He, consistent with neutrons being emitted from a Gaussian volume of $r_{nn}^{\mathrm{rms}}=7.3\pm 0.6$~fm with a $n$-$n$ delay in the sequential channel of $1400\pm 400$~fm/$c$, in agreement with the lifetime of ${}^7$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.

Accepted for publication in J. Phys. G, 11 pages, 5 figures