In the last few years, many experiments have been undertaken to
determine the number of the pre- and post-fission neutrons by measuring
evaporated neutrons in coincidence with fission fragments for different
angles. From these data, the corresponding multiplicities were
determined. In addition the emitted protons, alpha-particles and
-rays have been measured in various experiments. Due to the
Coulomb barrier the charged particles are emitted more seldom than neutrons.
It is important to study the influence of damping on decay of highly
excited compound nuclei when the fission barrier is relatively small.
Since, in this case, fission and particle evaporation compete with one
another and, consequently, particle emission depends on the
dissipation. On the average, fission fragments have enough excitation
energy to emit particles themselves. Since the directions of these
particles are correlated with those of the emitting fragments, they can
be separated, experimentally, from those particles evaporated from the
compound nucleus.
Our present efforts are dedicated to a stepwise improvement of
this general theory in order to reach a quantitatively reliable description
of the decay of the compound nuclei. The principle of our approach is to
describe the particle emission as a purely statistical process, i.e. by
the Weißkopf theory and the nuclear fission as a transport process
governed by the Langevin equation. We would like to perform a more detailed
calculations of the competing processes of
particle evaporation and the fission of compound nuclei. A set of
coupled differential equations formed by a Fokker and Planck
describing fission and master equations for calculating particle
evaporation was used. From these equations, one is able to determine
multiplicities of pre-fission neutrons, protons and alpha-particles
and their energy spectra.
We would like to study more carefully how the evaporation
probabilities depend on the deformation of the nucleus and on its
collective rotation. In the standard form of the evaporation theory,
the deformation of the decaying nucleus enters only through the level
densities of the initial and final nucleus. We take into account that
the transmission coefficient also depends on the deformation and, to
a smaller degree, on the collective rotation of the nucleus.
The properties of excited nuclei in thermal equilibrium are of great physical interest. We consider excitation energies below some (400-500) MeV, where we may still describe the nucleus as a system of neutrons and protons which interact by effective forces. All the information on the physical state of hot nuclei is then to be obtained from a careful study of their decay by emission of neutrons, protons, and alpha-particles and their decay by fission.