Direct evidence of “washing out” of nuclear shell effects

Constraining the excitation energy at which the nuclear shell effect washes out has important implications on the production of superheavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in $\ensuremath{\alpha}$ induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that the nuclear shell effect washes out at excitation energy $\ensuremath{\sim}40$ MeV. The calculation shows that the second peak of the fission barrier also vanishes around similar excitation energy.