JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
nucleus[5]. Therefore, the conditions 2) and 3) of SC and Iwamoto paper explained
above are shown to be satisfied within this model. Such a feature is in sharp constrast
to what we expect for light-ion induced surrogate reactions which exhibit typical
quantum-mechanical diffraction patterns (see Fig. 7), which makes the spin distribution
very sensitive to the detection angle, Q-value and target mass. The quantal model will
be used to investigate physics occuring in the initial stage of the reaction.
It was shown above by a quantum-mechanical model that the reaction 18O+ 238U proceeds in a semi-classical manner. Therefore, it makes sense that we construct a
semi-classical model which can describe the whole process of surrogate reactions. Such
a model is implemented[6] based on the unified model of Zagrevaev and Greiner[7]. In
this model, the reaction is assumed to go through initially on a diabatic potential energy
surface of the total composite system, 256Fm, and nucleon transfer is described by an
inertialess change of asymmetry parameter. Then, we switch to the potential enregy
surface of residues, e.g., 240U, and decay of it is considered (Fig. 8). Time evolution of the
whole process is described by a dissiption-fluctuation theorem in terms of a set of coupled
Langevin equations. The potential energy surfaces are calculated by a folding model for
initial diabatic phase and by the 2-center shell model otherwise.
This semi-classical model is powerful enough so it can predict almost all of the
observables of the surrogate reactions, namely, the angular and energy distribution of the
ejectile, the mass, energy and angular distribution of the fission fragments, and angular
and energy distribution of emitted neutrons via evaporation, pre-scission emission and
emission from fission fragments. These imformation are vital to assess the spin
distribution of populated compound nuclei.
An example of the predicted fission fragment mass distribuions from 256Fm and 240U are compared in Fig. 9 with experimental data obtained in the test experiment (still
preliminary). These data can be obtained cimultaneously in the 18O+ 238U reaction
system. We notice that the present model can describe both the single-peaked
distribution from a highly-excitged 256Fm nucleus and the double-peaked asymmetric
Fig.8 Schematic picture of the semi-classical model developed to desribe whole process of
the surrogate reaction[6]. This fugure corresponds to 238U( 18O, 16O) 240U reaction