At a certain finite neck radius during the descent of a fissioning nucleus
from the saddle to the scission point, the attractive nuclear forces can no
more withstand the repulsive Coulomb forces producing the neck rupture and the
sudden absorption of the neck stubs by the fragments. At that moment, the
neutrons, although still characterized by their pre-scission wave functions,
find themselves in the newly created potential of their interaction with the
separated fragments. Their wave functions become wave packets with components
in the continuum. The probability to populate such states gives evidently the
emission probability of neutrons at scission. In this way, we have studied
scission neutrons for the fissioning nucleus 236U, using two-dimensional
realistic nuclear shapes. Both the emission probability and the distribution of
the emission points relative to the fission fragments strongly depend on the
quantum numbers of the pre-scission state from which the neutron is emitted. In
particular it was found that states with Ωπ = 1/2+ dominate the
emission. Depending on the assumed pre- and post-scission configurations and on
the emission-barrier height, 30 to 50% of the total scission neutrons are
emitted from 1/2+ states. Their emission points are concentrated in the region
between the newly separated fragments. The upper limit for the total number of
neutrons per scission event is predicted to lie between 0.16 and 1.73
(depending on the computational assumptions).Comment: 31 pages, 16 figures, 2 table
