Iranian Journal of Physics Research
Year:2002 | Volume:3 | Issue:1|Papers Count:8

One of the most interesting application of the many-body methods to the nuclear physics is the calculation of the properties of the nuclear matter, especially its binding energy. In this paper, we have studied the influence of Δ-isobar on the properties of the symmetrical nuclear matter using the lowest order constrained variational (LOCV) method with the V28 potential. It is shown that the inclusion of Δ-isobar substantially affect the saturation properties of the symmetrical nuclear matter. We have shown that, at low (high) densities, the saturation curve of nuclear matter is shifted downward (upward). This is due to the fact that the repulsive effect of the V28 potential increases by increasing density. It is seen that the equation of state of nuclear matter with the V28 potential is much harder than those with the AV14 potential.

In this research, the magnetic phase transition and anisotropic magnetoelastic effects of a polycrystalline sample of Ndl4Dy2Fe78sB6 were studied by susceptibility and magnetostriction measurements. The composition was prepared by casting method. SEM pictures and XRD analysis showed that it was composed of polycrystalline magnetic phases with the main phase of 2-14-1.Results of ac susceptibility measurements show that the sample undergoes a magnetic phase transition at about 130 K, which is due to a gradual spin reorientation accompanied with crystal structure distortion. Magnetostriction measurements in three orthogonal directions of the main body of the sample were performed in the range of 77 K to 300 K and if the field up to 1.5 T. At temperatures below 125 K, the magnetostriction values in three orthogonal directions differ by a factor of two to three. This anisotropy in magnetostriction indicates preferential orientation of the crystalline grains in the volume of the sample. In addition, magnetostriction curves show minimum values near the phase transition temperature with different values in the different directions. Minimum values of magnetostriction can be explained by the critical behavior of the orbital magnetic moments of the Fe atoms at the phase transition temperature.

The fission fragment angular distributions have been measured for proton-induced fission of 2o9Bi and 197Au nuclei using surface barrier detectors at several energies between 25 MeV and 30 MeV. The experimental anisotropies are found to be in agreement with the predictions of the Standard Saddle-Point Statistical Model (SSPSM). The fission cross sections of 209Bi and197Au nuclei were also measured and compared with the previous works.

Exposure buildup factors for water and lead have been calculated by the Monte-Carlo method for an isotropic point source in an infinite homogeneous medium, using the latest cross sections available on the Internet. The types of interactions considered are: photoelectric effect, incoherent (or bound-electron Compton) scattering, coherent (or Rayleigh) scattering and pair production. Fluorescence radiations have also been taken into account for lead. For each material, calculations were made at 10 gamma ray energies in the 40 keV to 10MeV range and up to penetration depths of 10 mean free paths at each energy point. The results presented in this paper can be considered as modified gamma ray exposure buildup factors and be used in radiation shielding designs.

The granular Gd 1-x--z Pr x CazBa2Cu3O7-δ-(GdPrCa-123) high-temperature cuprate samples with 0.0≤x≤0.3 and 0.0≤z≤0.35 are prepared by standard solid state reaction and characterized by XRD and SEM techniques. The BaCu02 and impurity phases are less than 1% in the samples having high levels of Pr and Ca concentrations. The electrical resistivity measurements show a nonlinear reduction in Tc(x,z=etc) versus x. Moreover, the Tc(z) curve with x=0 is nonlinear and a plateau appears at z≈0.05-0.015. For Pr-Ca-doped samples with a constant concentration of Pr, Tc increases with the increase of Ca up to an optimum value of Ca doping and then in decreases. Based on these observations, we suggest that hole filling and hole localization the main effects of Pr ion substitution. The magnetic measurements indicate that the value of HCI is in the order of 10 mT. The mangetoresistance measurements have been measured and analyzed. The superconducting transition region is broadened by the application of magnetic field. The experimental data near the onsen of superconductivity are fitted with the Ambegakor and Halperin (AB) phase-slip model. We observe that the AH parameter, y(H), depends not only on the temperature and the magnetic field, but also on the Pr and Ca ions concentrations. It is observed that the critical current density increases with Ca substitution and decreases with Pr substitution in Gd-123 system. We suggest that the Pr ion substitution probably enhances the weak link, but the Ca ion acts as flux pinning center in the GdPrCa-123 system.

Logarithmic conformal field theory can be obtained using nilpotent weights. Using such scale transformations various properties of the theory were derived. The derivation of four point function needs a knowledge of singular vectors which is derived by including nilpotent variables into the Kac determinant. This leads to inhomogeneous hypergeometric functions. Finally we consider the theory near a boundary and also introduce the concept of superfields where a multiplet of conformal fields are dealt with together. This leads to the OPE of superfields and a logarithmic partner for the energy momentum tensor.

The electronic properties of YBa2Cu3O7-δ have been investigated by the energy-pseudopotential method within the local density approximation (LDA) with and without including generalized gradient corrections (GGC). The band structure, density of states and charge density of YBa2CU3O7-δ have been calculated. The results are compared with other approaches such as LAPW, LCAO and LMTO for this system. The overall shape of the band structure, density of states and charge density are in agreement with other works. However, in details, like other approaches, there are some differences. Considering our accuracy, the differences in results for LDA and GGC approaches are small. Also, the electronic and structural properties for this system have been investigated by applying pressure within LDA. We have calculated band structure, density of states, charge density, and length of bonds for high pressures, and the changes in hole concentration in this system with respect to pressure. Our results show the increase of hole in both CuO2planes and Cu-O chains under high pressures. Although this result is in agreement with the pressure-induced charge transfer (PICT) model, it is in contrast with the definition of this model which believes that hole increases in the CuO2 planes come from the Cu-O chains. Bulk modules and equilibrium volume have been also calculated to be equal 184 GPa and  174.89 A°3 , respectively. The results of these calculations have been compared with the experimental and theoretical reports on this system.