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

TL;DR

This study investigates the dissociation process of helium-8 into helium-6 and neutrons on different targets, revealing details about the decay mechanisms and neutron emission characteristics through a novel simultaneous analysis.

Contribution

It introduces a model that describes the space-time configurations and final-state interactions in helium-8 dissociation, including neutron emission timing and volume, for the first time.

Findings

Sequential decay observed on C target with neutron emission delay

Direct breakup dominates on Pb target without neutron delay

Neutron emission volume measured as approximately 7.3 fm

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 $r_{nn}^{\rm{rms}}=7.3\pm0.6$~fm with a $n$-$n$ delay in the sequential channel of $1400\pm400$~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.