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Register a new userNeutron Energy Dependence of Delayed Neutron Yields and its Assessments
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Futoshi Minato
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Incident neutron energy dependence of delayed neutron yields of uranium and plutonium isotopes is investigated. A summation calculation of decay and fission yield data is employed, and the energy dependence of the latter part is considered in a phenomenological way. Our calculation systematically reproduces the energy dependence of delayed neutron yields by introducing an energy dependence of the most probable charge and the odd-even effect. The calculated fission yields are assessed by comparison with JENDL/FPY-2011, delayed neutron activities, and decay heats. Although the fission yields in this work are optimized to delayed neutron yields, the calculated decay heats are in good agreement with the experimental data. Comparison of the fission yields calculated in this work and JENDL/FPY-2011 gave an important insight for the evaluation of the next JENDL nuclear data.
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A systematic study of the total $beta$-decay half-lives and $beta$-delayed neutron emission probabilities is performed. The $beta$-strength function is treated within the self-consistent density-functional + continuum-QRPA framework including the Gamow-Teller and first-forbidden transitions. The experimental total $beta$-decay half-lives for the Ni isotopes with $Aleq$76 are described satisfactorily. The half-lives predicted from $A$=70 up to $A$=86 reveal fairly regular $A$-behaviour which results from simultaneous account for the Gamow-Teller and first-forbidden transitions. For $Zapprox$ 28 nuclei, a suppression of the delayed neutron emission probability is found when the $N$=50 neutron closed shell is crossed. The effect originates from the high-energy first-forbidden transitions to the states outside the $Q_{beta} - S_n$-window in the daughter nuclei. PACS numbers: 23.40.Bw,21.60.Jz,25.30.Pt,26.30.+k
A role of beta-delayed neutron emission and fission in r-process nucleosynthesis attracts a high interest. Although the number of study on them covering r-process nuclei is increasing recently, uncertainties of beta-delayed neutron and fission are still large for r-process simulations. Our purpose is to make a new database on beta-delayed neutron emission and fission rates. To this end, the data that are not investigated experimentally have to be predicted. Microscopic theoretical approaches based on a nuclear energy density functional and statistical models are one of the competent tools for the prediction. To obtain beta strength function, p-n relativistic QRPA is adopted. Particle evaporations and fission from nuclear highly excited states are estimated by the Hauser-Feshbach statistical model. beta-delayed neutron branching ratios (P_n) are calculated and compared with experimental data. beta-delayed fission branching ratio (P_f) are also assessed by using four different fission barrier data. Calculated P_n values are in a good agreement with experimental data. It is found that energy withdrawal by beta-delayed neutron emission sensitively varies P_n values for nuclei near the neutron drip line. P_f are sensitively dependent on fission barrier data. Newly calculated data on beta-delayed neutron emission and fission are summarized as a table in supplement material. They are provided for studies of r-process as well as other fields such as nuclear engineering.
Starting from a Skyrme interaction with tensor terms, the $beta$-decay rates of $^{52}$Ca have been studied within a microscopic model including the $2p-2h$ configuration effects. We observe a redistribution of the strength of Gamow-Teller transitions due to the $2p-2h$ fragmentation. Taking into account this effect results in a satisfactory description of the neutron emission probability of the $beta$-decay in $^{52}$Ca.
The symmetry energy obtained with the effective Skyrme energy density functional is related to the values of isoscalar effective mass and isovector effective mass, which is also indirectly related to the incompressibility of symmetric nuclear matter. In this work, we analyze the values of symmetry energy and its related nuclear matter parameters in five-dimensional parameter space by describing the heavy ion collision data, such as isospin diffusion data at 35 MeV/u and 50 MeV/u, neutron skin of $^{208}$Pb, and tidal deformability and maximum mass of neutron star. We obtain the parameter sets which can describe the isospin diffusion, neutron skin, tidal deformability and maximum mass of neutron star, and give the incompressibility $K_0$=250.23$pm$20.16 MeV, symmetry energy coefficient $S_0$=31.35$pm$2.08 MeV, the slope of symmetry energy $L$=59.57$pm$10.06 MeV, isoscalar effective mass $m_s^*/m$=0.75$pm$0.05 and quantity related to effective mass splitting $f_I$=0.005$pm$0.170. At two times normal density, the symmetry energy we obtained is in 35-55 MeV. To reduce the large uncertainties of $f_I$, more critical works in heavy ion collisions at different beam energies are needed.
A comparative study of fission of actinides specially $^{238}$U, by proton and bremsstrahlung photon is performed. Relative mass distribution of $^{238}$U fission fragments have been explored theoretically for both proton and photon induced fission. The integrated yield along with charge distribution of the products are calculated to find out the neutron richness in comparison to the nuclei produced by r-process in nucleosynthesis. Some r-process nuclei in intermediate mass range for symmetric fission mode are found to be produced almost two order of magnitude more for proton induced fission than photofission, although rest of the neutron rich nuclei in the asymmetric mode are produced in comparable proportion for both the processes.
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