Revisit to the yield ratio of triton and ${}^3$He as an indicator of neutron-rich neck emission

Abstract

The neutron rich neck zone created in heavy ion reaction is experimentally probed by the production of the $A=3$ isobars. The energy spectra and angular distributions of triton and ${}^3$He are measured with the CSHINE detector in ${}^{86}$Kr +${}^{208}$Pb reactions at 25 MeV/u. While the energy spectrum of ${}^3$He is harder than that of triton, known as "${}^3$He-puzzle", the yield ratio $R(\mathrm{t}{/}^3\mathrm{He})$ presents a robust rising trend with the polar angle in laboratory. Using the fission fragments to reconstruct the fission plane, the enhancement of out-plane $R(\mathrm{t}{/}^3\mathrm{He})$ is confirmed in comparison to the in-plane ratios. Transport model simulations reproduce qualitatively the experimental trends, but the quantitative agreement is not achieved. The results demonstrate that a neutron rich neck zone is formed in the reactions. Further studies are called for to understand the clustering and the isospin dynamics related to neck formation.