Time Relationships between Direct Particle Emission and Fragmentation: A Probe for Nuclear Expansion Prior to Fragment Freeze-Out

Abstract

A previously unexploited experimental observable is used to explore emission times for intermediate mass fragments relative to directly emitted ${}^{2}\mathrm{H}$ and ${}^{3}\mathrm{H}$ particles. Small-angle correlations are reported in central collisions for $34A\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$ ${}^{40}\mathrm{Ar}{+}^{\mathrm{nat}}\mathrm{Ag}$. High-velocity ${}^{3}\mathrm{H}$ and ${}^{2}\mathrm{H}$ particles follow a direct emission scenario with mean lifetime $\ensuremath{\tau}\ensuremath{\sim}30\ensuremath{-}60\phantom{\rule{0ex}{0ex}}\mathrm{fm}/c$. Fragmentation to Li is characterized by $\ensuremath{\tau}\ensuremath{\sim}120\mathrm{fm}/c$. Current model calculations suggest a delay time of $\ensuremath{\sim}100\ensuremath{-}200\phantom{\rule{0ex}{0ex}}\mathrm{fm}/c$ for expansion of the central collision zone prior to the onset of ``freeze-out'' into fragments. But the observed velocity difference spectra limit the delay time to $\ensuremath{\lesssim}50\mathrm{fm}/c$ for expansion between direct emission and fragmentation.