_ 5.
UCRL- 6252
Radiochemical Studies of Neutron-Induced Fission of ue?
and ue
38
and the Two- Mode Fission Hypothesis
H,. B. Levy, H. G. Hicks, W. E. Nervik, P. CGC. Stevenson,
J. B. Niday, and J. C. Armstrong, Jr.
Lawrence Radiation Laboratory, University of California
Livermore, California
I.
INTRODUCTION
In past years many experiments have been performed in which the mass
distributions of fission products have been measured for a number of different
nuclides undergoing fission at various excitation energies.
No completely
satisfactory explanation has been found for the changes that occur in the mass
distribution of fission products when the excitation energy is varied in a particular nuclide about to undergo.fission.
However, recourse has frequently
been made to the suggestion of Turkevich and Niday? that there are two fundamentally different modes by which fission may proceed.
Both modes are
‘possible but one generally predominates at low excitation energies and the
other at high energies, the relative proportions of the two modes changing
with excitation energy.
These modes lead to distinctly different mass dis-
tributions of products, the observed mass distribution being the resultant of
the contributions of both modes.
The mass distributions attributed to the two modes are generally con-
sidered to be the two-humped "asymmetric'! and the one-humped "symmetric"!
mass distributions.
There has been some study on the competition between
symmetric and asymmetric fission; however, these studies have considered —
principally the gross features of the mass distribution.
9001901
Recently, Ford has