Iranian Journal of Physics Research
Year:2010 | Volume:10 | Issue:3|Papers Count:14

Energetic secondary particles in gamma and hadron initiated showers can generate Cherenkov photons. Using CORSIKA code and simulation data, different characteristics of lateral distribution of Cherenkov photon density are studied. In particular, physical origin of these differences and also dependence of this distribution on primary energy and altitude of observation level are discussed. It is shown that the existence of a hump in lateral distribution of Cherenkov photon density in gamma initiated showers is a useful tool for separating gamma-hadron and rejecting hadron background in gamma ray astronomy.

In nuclear medicine, studies of important tissues such as cardiac, the emitted photons from the cardiac before reaching the gamma detectors are attenuated and scattered by other tissues inside the thorax. Therefore, the quality and contrast of the image will be reduced. In this research, to improve the quality of cardiac images by SPECT system, the most convenient algorithms for attenuation correction were studied and assessed in the first step. Then the best method using the line source in Transmission Attenuation Correction (TAC) method was modified and the experimental data wase obtained by using this new and modified method, cardiac phantom, Dual Head SPECT system and a line source of 201Tl with the activity of about 0.5 mCi. The data was collected and obtained in two steps: (1) Scanning the cardiac phantom and line source which was beside the cardiac phantom this step involves using emission and transmission simultaneously. (2) Scanning the cardiac phantom in the absence of line source which means using emission data. Next, the suggested attenuation correction formula was used and the calculated attenuation coefficient for each pixel was calculated and applied to each pixel. Our results showed a nice improvement in contrast and visibility of the images by this simple and in imporoved expensive method. The advantages of this method include simplicity, the available radionuclide, improved accuracy, quality and contrast of the final image, and finally, cost – effectiveness. These advantages may help the nuclear medicine centers to improve their ability to detect the physiological and functional defects of the cardiac, especially in the elder and women patients.

Electronic and optical properties of calcium, strontium and barium chalcogenide compounds in NaCl structure are studied using the band structure results obtained through the full potential linearized augmented palne wave method. Different linear relationships are observed between theoretical band gap and 1/a2 (where a is lattice constant) for calcium, strontium and barium chalcogenide compounds with and without oxygen, respectively. An abnormal behavior of electronic and optical properties are found for compounds containing oxygen. These effects are ascribed to the special properties of Ca-O, Sr-O and Ba-O bonds, which are different from chemical bonds between Ca, Sr and Ba and other chalcogen atoms.

In this paper, the enhanced continuum x-ray emission from nano-targets irradiated by long laser pulses is investigated numerically. The calculations are performed under different prepulse conditions, and the x-ray emissions from solid and nano-structured target are compared. The results show that x-ray yield can be enhanced significantly using a nano-layer with appropriate density. In addition, x-ray enhancement can be improved using a laser pre-pulse with appropriate intensity and delay time. The results also show that the enhancement can be reduced for prepulse intensities greater than the specific value (i. e. 1013 W/cm2). The effects of delay time between the main pulse and prepulse are also investigated.

Different thicknesses of 99.97% Cu are deposited on glass substrate by thermal evaporation method at the rate of 2A˚/sec. Kramers-Kronig method is used for the analysis of the reflectivity constant in the range of 200nm<λ<3000nm, and the reults are compared with the those of bulk sample. For E>2eV, by increasing the thickness, the imaginary part of refraction index, k, increases and real part, n, decreases. At higher energies, both constants reach the asymptotic value of 1. Also, for more thickness of the film, ε1, the real part of dielectric constant becomes more negative, and ε2, its imaginary part, decreases. For E<2eV, there are some oscillations on thin films curves. This effect occurs due to the void, grain boundaries, and size effects, which are not the case for bulk copper. The plasma frequency shows thickness dependence, which is similar to that for bulk sample in thickness of 40nm.

In this project, the Faddeev-Watson-Lovelace (FWL) formalism is generalized to large scattering angles. The angular range includes 0-180 degrees. Using this method, the charge transfer differential cross-sections are calculated, in a second-order approximation, for collision of energetic positrons and electrons with neutral positronium atoms. In this approximation, the rearrangement amplitude contains two first-order and three second-order partial amplitudes. The first first-order term is the Born amplitude in a first-order approximation. The second one corresponds to capturing the transferred particle without perturbing the state of this particle. This term, in fact, describes a knock-on process. Since the masses of the particles and the absolute values of their charges are equal, one expects that the second-order terms be similar in magnitude. This aspect causes the instructive interference of the partial amplitudes in some angles and destructive interference in some others. However, it is predicted that these amplitudes have local maxima in direction of the recoiling of the projectile. In order to investigate this situation, the second-order partial amplitudes are calculated and their relations with the parity of the initial and final states of the scattering system are analyzed. In particular, the role of dynamical interference of these partial amplitudes in creation of the kinematical peak and the peak corresponding to the knock-on scattering in angular distribution of the differential cross sections is investigated.

Humidity sensors are fabricated based on nanoporous alumina, using hard anodization technique. In order to investigate the effect of the anion incorporated in the alumina template during anodization, two different kinds of sensors, wall and barrier layer sensor, with various current densities at 38 V and 44 V anodization voltages are fabricated. The effect of frequency measurement, ranging from 3 to 40 kHz, different applied voltages and relative humidity amplitudes (40% to 90%) are investigated using impedance spectrometry at room temperature. Higher sensitivity is seen for the barrier layer sensors as well as for sensors made at higher anodization current density. It is also found that the sensitivity is inversely proportional to the frequency, and also variation of the applied voltage has no effect on the sensitivity.

In this paper, the director distribution is calculated for a nematic liquid crystal, in the cell with different surface anchoring conditions and external fields. The effects of finite and infinite surface anchoring on molecular orientations for one dimensional geometry are discussed. In these situations, the planar alignment is considered. Then, in a two dimensional geometry the planar and homotropic anchoring conditions are assumed for wall- interfaces of confined nematic, and the reorientation of liquid crystal molecules is determined for two dimensional deformations. Dependence of the threshold field on deformation, geometry of the cell, and magnetic field strengths is investigated.

This research aims to study the solidification processes of Ag-X%Au alloy by molecular dynamics simulation technique at the NPT ensemble. The quantum Sutton-Chen many-body interatomic potential is used to calculate the energy and forces experienced by the particles. The coupled differential equations of motion of the particles are solved using Velocity Verlet algorithm. The solidification temperature, the cohesive energy and the solidification latent heat of Au, Ag pure metals as well as Ag-X%Au alloys with various concentrations of Au are determined. Furthermore, the solidification process of alloy is studied at different cooling rates. The molecular dynamics simulation results show glass structure is achieved at fast cooling rates while crystallization occurs at slow cooling rates. Also, this cooling rate is different for various concentrations of Au.

Fast ignition is a new method for inertial confinement fusion (ICF) in which the compression and ignition steps are separated. In the first stage, fuel is compressed by laser or ion beams. In the second phase, relativistic electrons are generated by pettawat laser in the fuel. Also, in the second phase 5-35 MeV protons can be generated in the fuel. Electrons or protons can penetrate in to the ultra-dense fuel and deposit their energy in the fuel. More recently, cylindrical rather than spherical fuel chambers with magnetic control in the plasma domain have been also considered. This is called magnetized target fusion (MTF). Magnetic field has effects on relativistic electrons energy deposition rate in fuel. In this work, fast ignition method in cylindrical fuel chambers is investigated and transportation of the relativistic electrons and protons is calculated using MCNPX and FLUKA codes with 0.25 and 0.5 tesla magnetic field in single and dual hot spot. Furthermore, the transfer rate of relativistic electrons and high energy protons to the fuel and fusion gain are calculated. The results show that the presence of external magnetic field guarantees higher fusion gain, and relativistic electrons are much more appropriate objects for ignition. MTF in dual hot spot can be considered as an appropriate substitution for the current ICF techniques.

To obtain the maximum output power of a fast-axial-flow CW-CO2 laser, gas flow velocity can be optimized by using genetic algorithms. Our theoretical approach shows that the gas flow velocity after optimization increases the laser output power substantially from 500W, obtained in our present system, to 2203W.

Two stream instability in plasma is simulated by PIC method. The execution time of the sequential and parallizable sections of the program is measured. The sequential program is parallelized with the help of the MPI functions. Then, the execution time of the sequential program versus the number of the grid points and the execution time of the parallel program on 3 and 5 processors versus the number of the grid points are obtained. Finally, by using these results the Speedup of the parallel program versus the number of the grid points is derived.

In this work, the Isfahan Miniature Neutron Source Reactor (MNSR) is first simulated using the WIMSD code, and its fuel burn-up after 7 years of operation (when the reactor was revived by adding a 1.5 mm thick beryllium shim plate to the top of its core) and also after 14 years of operation (total operation time of the reactor) is calculated. The reactor is then simulated using the MCNP code, and its reactivity variation due to adding a 1.5 mm thick beryllium shim plate to the top of the reactor core, after 7 years of operation, is calculated. The results show good agreement with the available data collected at the revival time. Exess reactivity of the reactor at present time (after 14 years of operation and after 7 years of the the reactor revival time) is also determined both experimentally and by calculation, which show good agreement, and indicate that at the present time there is no need to add any further beryllium shim plate to the top of the reactor core. Furthermore, by adding more beryllium layers with various thicknesses to the top of the reactor core, in the input program of the MCNP program, reactivity value of these layers is calculated. From these results, one can predict the necessary beryllium thickness needed to reach a desired reactivity in the MNSR reactor.