Theoretical study on fusion dynamics and evaporation residue cross sections for superheavy elements

The nuclear dynamical deformation, the fusion probability and the evaporation residue （ER） cross sections for the synthesis of superheavy nuclei are studied with the di-nuclear system model and the related dynamical potential energy surface. The intrinsic energy and the maximum dynamical deformations for 48Ca＋248Cm are calculated. The effect of dynamical deformation on the potential energy surface and fusion is investigated. It is found that the dynamical deformation influences the potential energy surface and fusion probability significantly. The dependence of the fusion probability on the angular momentum is investigated. The ER cross sections for some superheavy nuclei in 48Ca induced reactions are calculated and it is found that the theoretical results are in good agreement with the experimental results.     

Porosity effects on the neutron total cross section of graphite

For subthermal neutron energies, polycrystalline graphite shows a larger total cross section than predicted by existing theoretical models. In order to investigate the origin of this discrepancy we measured the total cross section of graphite samples of three different origins, in the energy range from 0.001 to 10 eV. Different experimental arrangements and sample treatments were explored, to identify the effect of various experimental parameters on the total cross section measurement. The experiments showed that the increase in total cross section is due to neutrons scattered around the forward direction. We associate these small angle scattered neutrons (SANS) to the porous structure of graphite, and formulate a very simple model to compute its contribution to the total cross section of the material. This results in an analytic expression that explicitly depends on the density and mean size of the pores, which can be easily incorporated in nuclear library codes.     

Investigation of decay modes of superheavy nuclei

A detailed investigation of different decay modes,namely alpha decay,beta decay,cluster decay,including heavy particle emission(Zc＞28),and sponta-neous fission,was carried out,leading to the identification of new cluster and beta-plus emitters in superheavy nuclei with 104≤Z≤126.For the first time,we identified around 20 beta-plus emitters in superheavy nuclei.Heavy-particle radioactivity was observed in superheavy elements of atomic number in the range 116≤Z≤126.292-293 Og were identified as 86Kr emitters,and 298122 and 300122 were identified as 94Zr emitters,whereas heavy-particle radioactivity from 91Y was also observed in 299123.Fur-thermore,the nuclei 300124 and 306126 exhibit 96Mo radioactivity.The reported regions of beta-plus and heavy-particle radioactivity for superheavy nuclei are stronger than those for alpha decay.The identified decay modes for superheavy nuclei are presented in a chart.This study is intended to serve as a reference for identifying possible decay modes in the superheavy region.     

Influence of screening length modification on the scattering cross section in LEIS

Scattering cross sections for He⁺ ions in the energy range of 100 eV to 100 keV and for Al, Cu and Au target atoms were calculated. Employing the Thomas-Fermi-Molière model the potential strength was tuned by variation of the screening length. The resulting change in scattering cross section was analyzed and the absolute value is compared to cross sections obtained from potentials commonly employed in the medium-energy ion scattering (MEIS) regime. A large influence on the scattering cross section is observed for targets with large atomic number in the very low energy range. For instance, the scattering cross section for 100 eV He⁺-ions scattered from Au by 129° changes by a factor of 2.5 between different potential strengths claimed in the literature to be suitable for low-energy ion scattering (LEIS) energies. An experiment to determine electronic energy loss of very slow ions in metals is presented. It shows how uncertainties in the scattering potential strength can lead to systematically wrong results, although perfect agreement between experimental data and simulations is found. The impact of these results on quantitative surface structure and composition analysis is discussed.     

电子阻止截面对平均投影射程计算结果的影响

以氦离子轰击碳靶和镍靶为例，通过五种半经验理论计算与实验结果的比较，指出基于ＢＫ模型的通用计算公式可以在很宽能区范围内得到与实验值非常接近的电子阻止截面。还用Ｂｉｅｒｓａｃｋ给出的一阶线性常微分方程的平均投影射程算法，研究了不同电子阻止截面对平均投影射程计算结果的影响。     

Influence of plasma induced by radionuclide layer on the radar cross section of spherical objects

The influence of theα-decay radionuclide layer(the energy ofα-particles are 5.45 Me V)on the radar cross section(RCS)of sphere objects was calculated under different radioactivities,frequencies,and sphere radii.When the sphere radius is smaller than 50 cm,the tendency of the electron densities of the plasma slab is to ascend first and then descend,and the typical maximum electron densities with a radioactivity of 10 Ci/cm2raises from 7.02×1010to 1.76×1011when the sphere radii increases from 10 to 300 cm.The average data of a normalized RCS of a sphere with radius of 12.5 cm,which is coated with a radionuclide layer with different radioactivities are-0.35,-0.50,-0.79 and-1.13 d B when the radioactivity is 1,2,5 and 10 Ci/cm2,respectively.     

Modeling the heavy ion upset cross section

The standard Rectangular Parallelepiped (RPP) construct is used to derive a closed form expression for, σ¯(&thetas;, φ, L) the directional-spectral heavy ion upset cross section. This is an expected value model obtained by integrating the point-value cross section model, σ(&thetas;,φ,L), also developed here, with the Weibull density function, f(E), assumed to govern the stochastic behavior of the upset threshold energy, E. A comparison of σ¯(&thetas;,φ,L) with experimental data show good agreement, lending strong credibility to the hypothesis that E-randomness is responsible for the shape of the upset cross section curve. The expected value model is used as the basis for a new, rigorous mathematical formulation of the effective cross section concept. The generalized formulation unifies previous corrections to the inverse cosine scaling, collapsing to Petersen's correction, [cos&thetas;-(h/l)sin&thetas;]^-1, near threshold and Sexton's, [cos&thetas;+(h/l) sin &thetas;]^-1, near saturation. The expected value cross section model therefore has useful applications in both upset rate prediction and test data analysis     

Screening corrections to the Rutherford cross section

Differential cross sections for elastic p-Au scattering were measured in the energy range between 0.2 and 0.8 MeV for scattering angles from 30° to 150° in order to determine corrections to the Rutherford cross section due to the screening of the nuclear charge by the atomic electrons. Furthermore, differential cross sections have been calculated in the weakly screening region using various screening functions. A simple analytical expression has been derived for the representation of both experimental and theoretical results.     

Studies of 2D reflector effects in cross section preparation for deep burn VHTRs

The prismatic VHTR neutronic simulation presents challenges due to thermalization of the neutrons in the graphite reflector which leads to a spectral change in the peripheral fuel blocks. Two calculation schemes were tested on a simple 2D core calculation: a single block path wherein a classical single block lattice calculation provides the homogenized cross-sections, and a supercell path where the homogenized cross-sections are generated using a lattice of a fuel block surrounded by some of its surroundings. In both paths, several group condensations were performed to assess the effect of increasing the number of groups in the core calculation from 2 to 295. Core and lattice calculations were validated with respect to MCNP. The study revealed that the supercells lead to improvement in the calculation of power shape over the single-block path. This improvement is rather pronounced with small numbers of energy groups. For larger numbers of energy groups, however, both solution methods appear to yield adequate accuracy and the improvement gained through supercells in these cases may not warrant the computational cost. Lattice depletion calculations also show that the presence of the reflector creates strong heterogeneities on isotopic densities after 1000 days of burning.     

Reaction mechanisms in massive nuclei collisions and perspectives for synthesis of heavier superheavy elements

We discuss a hardship in synthesis of heaviest super heavy elements in massive nuclei reactions due to the hindrance to complete fusion of reacting nuclei caused on the onset of quasifission process which strongly competes with complete fusion and due to the strong increase of fission yields along the de-excitation cascade of the compound nucleus in comparison with the evaporation residue formation. The hindrance to formation of compound nucleus and evaporation residue is determined by the characteristic of the entrance channel.