  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 $R({\rm t/^3He})$ 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 $R({\rm t/^3He})$ 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.
