Investigation of shell effects for heavy fissionable nuclei by the blocking technique

The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the Si-28 + Pt-nat reaction. Experimental results range from 10(-17) to 10(-18) s at bombarding energie s from 140 to 170 MeV. It is shown that fission barriers keep their double- humped structure for temperature up to 1.7-1.8 MeV, and that the lifetimes of excited states in the second potential well contribute essentially to th e observed delay times in the fission channel. (C) 2000 Elsevier Science B. V. All rights reserved.