By using a new reaction model for light nuclei, the
double-differential cross section of n + 14N reactions at En = 14.2 MeV has
been analyzed. In the case of n+14N reactions, the reaction mechanism
is very complex, there are over one hundred opened partial reaction channels
even at incident energy En = 14.2 MeV. In this paper the opened reaction
channels are listed in detail. With LUNF code the model calculation is
performed to analyze the double-differential cross sections of total
outgoing neutron. The calculated results agree fairly with the experimental
data. The results indicate that the pre-equilibrium mechanism dominates the
whole reaction processes, and the recoil effect in light nuclear reactions
is essentially important. 5He emission has been considered, but it is
only a small contribution to the double-differential cross section at incident
energy En = 14.2 MeV.
The pre-neutron-emission mass distributions for reaction 232 Th(n, f) up to 60 MeV are systematically studied with an empirical fission potential model. The energy dependences of the peaks and valleys of the preneutron-emission mass distributions are described by the exponential expressions based on the newly measured data. The energy dependence of evaporation neutrons before scission, which plays a crucial role for the reasonable description of the mass distribution, is also considered. Both the double-humped and triple-humped shape of the measured preneutron-emission mass distributions for reaction 232 Th(n, f) are reasonably well reproduced at incident energies up to 60 MeV. The mass distributions at unmeasured energies and the critical energies at which the humped pre-neutron-emission mass distributions are transformed into each other are also predicted.
Non-equilibrium fission has been described by diffusion
model. In order to describe the diffusion process analytically,
the analytical solution of Smoluchowski equation in harmonic
oscillator potential is obtained. This analytical solution is
able to describe the probability distribution and the diffusive
current with the variable x and t. The results indicate that the
probability distribution and the diffusive current are relevant
to the initial distribution shape, initial position, and the
nuclear temperature T; the time to reach the quasi-stationary
state is proportional to friction coefficient β, but is independent
of the initial distribution status and the nuclear temperature T.
The prerequisites of negative diffusive current are justified.
This method provides an approach to describe the diffusion process
for fissile process in complicated potentials analytically.
By using a new reaction model for light nuclei, the
double-differential cross section of total outgoing neutron with LUNF code
for n + 16O reactions at En = 14.1 MeV and 18 MeV have been
calculated and analyzed. In this paper the opened reaction channels,
which have contribution to emitting the neutrons, are listed in detail.
To improve the fitting results the direct inelastic scattering mechanism
is involved. The calculating results agree fairly well with the
experimental data at En = 14.1 MeV and the deviation from calculated
results and experimental data in low energy region at En = 18 MeV has
been analyzed. Since the possibility of 5He has been affirmed
theoretically [J.S. Zhang, Sci. Chin. G 47 (2004) 137],
so 5He emission from n + 16O reaction is taken into account,
which plays an important role at the region of the outgoing neutron
energy εn < 3 MeV in total outgoing neutron energy-angular
spectrum. The calculated results indicate
that the pre-equilibrium mechanism dominates the whole reaction processes,
and the recoil effect in light nuclear reactions is essentially important.
Based on the light nucleus reaction model (Nucl. Sci. Eng. 133 (1999) 218), four aspects (neutron incident energy region, reaction channel analysis, the renewed level schemes and the optical model parameters) of n+ 12 C reaction are improved to calculate total outgoing neutron double-differential cross sections with modified LUNF code below 30 MeV. The calculated results agree fairly well with the experimental data at En = 14.1 MeV and 18 MeV. The analysis shows that the pre-equilibrium mechanism, which is exactly considered the conservation of energy, momentum and parity, dominates the whole reaction process. The contribution of the neutron emission from 5 He to total energyangular spectra is also considered properly. This modified LUNF code will be a useful tool to set up the file of neutron double-differential cross sections below 30 MeV in the neutron evaluation nuclear data library.
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