The neutron richness of the light charged particles emitted out of the fission plane in heavy ion reactions has been experimentally investigated via the production of \(A=3\) mirror nuclei in \(^{86}\) Kr + \(^\textrm{nat}\) Pb reactions at 25 MeV/u. The energy spectra and angular distributions of triton (t) and \(^3\) He in coincidence with two fission fragments are measured with the Compact Spectrometer for Heavy IoN Experiment (CSHINE). The energy spectrum of \(^{3}\) He is observed harder than that of triton in the fission events, in accordance with the phenomena reported as “ \(^{3}\) He-puzzle” in inclusive measurements. With a data-driven energy spectrum peak cut scenario, it is observed that the yield ratio \(R(\mathrm{t/^3He})\) increases with the angle to the fission plane, showing an enhancement of neutron-rich particle emission from out-of-fission-plane. A qualitative comparison with the transport model calculations suggests that this observation may serve as a new probe for the nuclear symmetry energy.
