مجله فیزیک کاربردی
سال یازدهم شماره 2 (پیاپی 25، تابستان 1400)

The use of metal matrix composites can provide a combination of desirable properties of metals as well as the special physical properties of neutron absorber reinforcing particles such as boron carbide, which alone may be brittle. Therefore, in the present study on neutron attenuation power of composite shielding, several Al-B4C composite samples with weight fractions of 5, 10 and 20% B4C have been used. In order to investigate the neutron absorption properties of the studied samples, the MCNP Monte Carlo code and the neutron source of the dry channel of the MNSR reactor with a flux of 2.13E+5 n.cm-2.s-1 have been used  ,which provided in nominal reactor power of 30 kW. The results show that the neutron flux in the presence of 5, 10 and 20% boron carbide samples is predicted to be 1.32E+05 n.cm-2.s-1,1.12E+05 n.cm-2.s-1 and 1.07E+05 n.cm-2.s-1, respectively. With this increase in the percentage of reinforcement phase, neutron flux is reduced down to 50%.

In this paper, the relativistic Dirac equation in one dimension is investigated for a particle in an external electromagnetic field, with the property of position-dependent effective mass approximation (PDEM), in the absence of vector potential. By removing the lower spinor component and combining the pair of equations, a Schrödinger-like equation is obtained for the upper spinor component. Using canonical transformations and introducing two first-order Hermitian and anti-Hermitian differential operators, a formalism for pseudo-hermitic Hamiltonians with parity-time reversal symmetry (PT) has been obtained. Comparing the equation derived from pseudo-Hermitian Hamiltonian with the non-relativistic Schrödinger equation leads to a general formalism for one-dimensional solvable imaginary non-Hermitian potentials with real energy spectra. Also, using this process, the complex potentials of Pöschl-Teller and Scarf II with real energy spectra in Dirac equation with PDEM approximationand PT symmetry have been investigated and their application has been expressed. For some particular parameters we will see the phenomenon of energy-levels crossing. In fact, it means that energy levels disappear from the spectrum. Also, for the mentioned examples, potential figures are drawn.

In this paper, the standing magnetohydrodynamic slow waves in the magnetic flux tubes are investigated under coronal conditions. The temperature and equilibrium plasma density of the tube is assumed to be homogeneous and constant and the compressive viscosity is considered as the damping mechanism. Also, the plasma flow is considered in the flux tube. Assuming that the damping rate to be much smaller than the oscillation frequency of the waves, the perturbation method is used to solve the problem. The oscillation frequency and the eigenfunctions are found from the first-order perturbation while the damping rate is determined from the second-order perturbation. To increase the flow speed makes the oscillation frequency decreases and the damping rate increases. Also, increasing  each of the flow speed and background temperature of the tube, decreases the ratio of the damping time to the oscillation period. But this ratio increases due to the increasing the length of the tube The results found here are consistent with the theoretical results obtained earlier. Also, the obtained results can justify some observational cases.

The unification between the theory of general relativity and the standard model of particle physics predicts the existence of a minimal measurable length on the order of the Planck length. Nowadays phenomenological studies of field theory in the presence of a minimal observable distance are extensively performed. The existence of a minimal measurable length leads to the generalized uncertainty principle (GUP). In this work, we obtain at first the angular wave frequency in the presence of a minimal observable length by considering the generalized uncertainty principle. Then, by expanding the generalized angular wave frequency, the nonlinear Schrodinger equation is found. Also, the soliton solution of the generalized nonlinear Schrodinger equation is obtained. In the limit, the soliton solution in generalized space becomes the same as usual soliton solution. The value of minimalobservable length is considered about.

The structural of MgB2 such as lattice constant, volume modulus, volume modulus derivative and compressibility of different degrees have been calculated. The calculations have been performed using a Full Potential-Linearized Augmented Plane Wave (FP-LAPW) method in the framework of Density Functional Theory (DFT) with various Approximation. The results show that the linear compressibility in direction of c is about twice the compressibility in direction of a. In addition, the electronic properties of the compound, including the structure of the energy bands, the density of the states, and the types of bonds in the MgB2 Superconductivity have been studied. The results of a double degeneracy above the Fermi level show that it plays an important role in the superconducting property. According to energy bands, it has been determined that the lattice parameters have a large effect on the s band of boron atoms. The results predict that the constant increase of the lattice along the c-axis has an increase in the density of states at the Fermi level. The results express this fact that the calculation using by GGA has the better agreements with the other theoretical and experimental approximations.

Using all optical devices for implementing all optical communication networks is very important. By using all optical frequency shift keying demodulators one can generate binary codes from the frequency of optical waves. In this paper a novel structure has been proposed for designing photonic crystal based all optical frequency shift keying demodulator. Three resonant cavities with different resonant modes were used for the frequency selecting section. In order to separate three different resonant modes, the radius of the defect rods used inside the cavities was chosen with different size. The simulation results show the proposed structure can generate four binary codes according to the frequency of the input optical waves. Therefore the final structure can work as an all optical frequency shift keying demodulator which can generate 00, 01, 10 and 11 codes at the output ports. The maximum delay time of the proposed structure is 4 ps.