Fast decision in favor of the slow fission process.

ABSTRACT

The fission probability P(f) of highly excited targetlike nuclei produced in reactions of 2.5 GeV protons on Au, Bi, and U was studied as a function of excitation energy E* whereby E* is deduced eventwise from the multiplicity of evaporated light particles. At the highest E* of 1000 MeV P(f) amounts to approximately 30% with all 3 target nuclei irrespective of the initial fissility. Statistical-model calculations satisfactorily reproduce the observed evolution of P(f) with E*--provided that no extra transient delay is introduced. Fission thus is decided upon very fast and early in the long deexcitation chain towards scission which comprises as much as approximately 80% of all evaporated alpha particles.