Experimental evidence of shell effects in the fission time of heavy nuclei

The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the Si-28 + (nat)pt reaction . Experimental results range from 10(-17) to 10(-18)s at bombarding energie s from 140 to 170MeV. It is shown that fission barriers keep their double-h umped structure for nuclear temperature up to 1.7-1.8 MeV, and that the lif etimes of excited states in the second potential well contribute essentiall y to the observed delay times in the fission channel.