Time-dependent isospin composition of particles emitted in fission events followingAr40+Au197at 35 MeV/u

Abstract

Fission fragments resulting from the fission of target-like nuclei produced in the $^{40}\mathrm{Ar}+^{197}\mathrm{Au}$ reaction at 35 MeV/u are measured in coincidence with the emitted light charged particles (LCPs). Comparison of the $N/Z$ composition of the LCPs at middle and large angles in the laboratory frame shows that particles emitted at smaller angles, which contain a larger contribution from dynamical emission, are more neutron rich. A moving-source model is used to fit the energy spectra of the hydrogen isotopes. A hierarchy from proton to deuteron and triton is observed in the multiplicity ratio between the intermediate velocity source and the compound nucleus source. This ratio is sensitive to the dynamical emission at early stages of the reaction and to statistical emission lasting up to the scission point. Calculations with the improved quantum molecular dynamics (ImQMD) transport-model qualitatively support the picture that more free and bound neutrons are emitted during the early stage, showing a clear dependence of $N/Z$ on the parametrization of the symmetry energy. The time-dependent isospin composition of the emitted particles thus may be used to probe the symmetry energy at subsaturation densities.