Correlations between emission timescale of fragments and isospin dynamics in $^{124}$Sn+$^{64}$Ni and $^{112}$Sn+$^{58}$Ni reactions at 35 AMeV

TL;DR

This study introduces a new experimental approach to link the isotopic composition of fragments with their emission timescale in nuclear collisions, providing insights into the symmetry energy's density dependence.

Contribution

The paper presents a novel method to correlate isotope ratios of emitted fragments with emission timing, offering constraints on the nuclear symmetry energy at subsaturation densities.

Findings

Early emitted fragments have higher N/Z asymmetry and angular anisotropy.

Data suggests a linear density dependence of the symmetry energy.

Method constrains the symmetry energy term in the nuclear equation of state.

Abstract

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of $<$N/Z$>$ isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio $<$N/Z$>$ distributions than those produced in sequential statistical emission. All these effects support the concept of isospin "migration", that is sensitive to the density gradient between participant and quasi-spectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a Stochastic Mean Field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.