  Mass distributions of fission fragments arising from the slow quasi-fission
process have been derived by comparing the measured distributions with the
theoretical distributions based on compound nuclear fission model for several
reactions. The mass-distributions corresponding to quasi-fission events for all
the systems show the following common features: (1) they are double peaked with
fixed peak-centroids and nearly same width at different incident energies, (2)
the yield of quasi-fission events decreases with the increasing projectile
energy, and (3) peak corresponding to lighter fragment is observed at A $\sim$
96 for all the systems, whereas the peak of heavier fragment increases linearly
with the mass of the di-nuclear system. All the above observations are quite
similar to the ones observed in well known asymmetric fission of actinides,
thus providing clear evidences of shell effect in slow quasi-fission where the
lighter fragment is possibly nuclei around $^{96}$Zr, a new doubly magic
nucleus. This finding has great implications in the study of nuclear reactions,
structure and particularly in super-heavy element synthesis where quasi-fission
is synonymous.
