Pre-neutron-emission Mass Distributions For Neutron-induced Actinide Fission

Though the nuclear fission mechanism has been studied since it was discovered more than70years, there are not any theories or models which can well reproduce or predict the experimental data. Due to the actinide is the most important nucleus in the nuclear energy applications, the research of the neutron induced actinide nucleus fission is the most detailed. The pre-neutron-emission fragments mass distribution of the nuclear fission is one of the most important fission characteristics, which closely relative with the fission mechanism, the product yields, the neutron multiplicity, and the average total kinetic energy of the fragments. Therefore, this article focuses on studying the pre-neutron-emission fragments mass distributions of neutron induced actinide nucleus fission, which can lay foundation of the fission mechanism and the fission phenomenon.From the investigation of the neutron induced actinide nucleus fission can be found that, the pre-neutron-emission fragments mass distributions have double-peaks characteristics, and the positions and heights of the distributions dependent on the incident neutron energies. Therefore, based on the experimental data and the advantages of the available theory models, this paper proposed a new phenomenological fission potential theory for calculating the pre-neutron-emission fragments mass distributions of the middle-low energies neutron induced actinides fission. In this phenomenological fission potential theory, there are two important physical quantities, the positions and the heights of the peaks and the valley of the mass distributions. All of the coefficients of our theory can be written as the function of the positions and heights.(1) For the low energies (En≦7MeV) neutron induced actinide fission, the positions of the double-peaks can be calculated by the nucleus-nucleus potential which based on the the Shyme energy-density functional together with the extended Thomas-Fermi approximation. The positions of the heavy peak of the mass distributions A2≈140, which is good agreement with the measured result and the calculative result of the P. Moller. The heights of the peak and valley of the pre-neutron-emission fragments mass distributions are linear with the incident neutron energies in this paper.(2) Compared with low-energy fission, the intermediate energies (7MeV≦En≦60MeV, based on the new measured data) neutron induced actinide fission is more complicated. In this process, it must be considered the effect of neutron evaporation before the compound nuclear fission. For the neutron induced238U fission with incident energy less than60MeV, the investigation of the new experimental data found that the position of the light peak of the mass distributions A1=99, and the position of the heavy peak is closely relative with the multichannel. The heights of the double-peaks are index with the incident neutron energy. Based on the above conclusion, the pre-neutron-emission fragments mass distributions of different energies neutron induced actinide fission are calculated. The results of the calculations shown that, the phenomenological fission potential theory can not only well reproduce the measured data, but also can reasonably predict the unmeasured incident energies. The phenomenological fission potential theory is good for further study the fission phenomenon.