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Register a new userPre-neutron-emission mass distributions for reaction $^{238}$U(n, f) up to 60 MeV
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The pre-neutron-emission mass distributions for reaction $^{238}$U(n, f) up to 60 MeV are systematically studied with an empirical fission potential model. The energy dependence of the peaks and valleys of the pre-neutron-emission mass distributions is described by an exponential form based on the newly measured data. The energy dependence of evaporation neutrons before scission is also considered, which plays a crucial role for the reasonable description of the mass distributions. The measured data for the pre-neutron-emission mass distributions for reaction $^{238}$U(n, f) are reasonably well reproduced up to 60 MeV. The mass distributions at unmeasured energies are also predicted with this approach.
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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 pre-neutron-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 pre-neutron-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.
According to the driving potential of a fissile system, we propose a phenomenological fission potential for a description of the pre-neutron emission mass distributions of neutron-induced actinide fission. Based on the nucleus-nucleus potential with the Skyrme energy-density functional, the driving potential of the fissile system is studied considering the deformations of nuclei. The energy dependence of the potential parameters is investigated based on the experimental data for the heights of the peak and valley of the mass distributions. The pre-neutron emission mass distributions for reactions 238U(n, f), 237Np(n, f), 235U(n, f), 232Th(n, f) and 239Pu(n, f) can be reasonably well reproduced. Some predictions for these reactions at unmeasured incident energies are also presented.
The normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber from the reaction threshold to $30$~MeV. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time-of-flight. The $^{238}$U(n,f)/$^{235}$U(n,f) ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}$U(n,f) cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$beta$5 at 14.5 MeV.
The production cross sections for primary and residual fragments with charge number from $Z$=70 to 120 produced in the collision of $^{238}$U+$^{238}$U at 7.0 MeV/nucleon are calculated by the improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model (HIVAP code). The calculation results predict that about sixty unknown neutron-rich isotopes from element Ra ($Z$=88) to Db ($Z$=105) can be produced with the production cross sections above the lower bound of $10^{-8}$ mb in this reaction. And almost all of unknown neutron-rich isotopes are emitted at the laboratory angles $theta_{lab}leq$ 60$^circ$. Two cases, i.e. the production of the unknown uranium isotopes with $Ageq$ 244 and that of rutherfordium with $Ageq$ 269 are investigated for understanding the production mechanism of unknown neutron-rich isotopes. It is found that for the former case the collision time between two uranium nuclei is shorter and the primary fragments producing the residues have smaller excitation energies of $leq$ 30 MeV and the outgoing angles of those residues cover a range of 30$^circ$-60$^circ$. For the later case, the longer collision time is needed for a large number of nucleons being transferred and thus it results in the higher excitation energies and smaller outgoing angles of primary fragments, and eventually results in a very small production cross section for the residues of Rf with $Ageq$ 269 which have a small interval of outgoing angles of $theta_{lab}$=40$^circ$-50$^circ$.
Quantal diffusion mechanism of nucleon exchange is studied in the central collisions of $^{238}$U + $^{238}$U in the framework of the stochastic mean-field (SMF) approach. For bombarding energies considered in this work, the di-nuclear structure is maintained during the collision. Hence, it is possible to describe nucleon exchange as a diffusion process for mass and charge asymmetry. Quantal neutron and proton diffusion coefficients, including memory effects, are extracted from the SMF approach and the primary fragment distributions are calculated.
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