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Item 2d Geometry of Quadrapole Magnetic field Lines from Neutron Star and Associated Radiation Pressure(Addis Ababa University, 2011-06) Girma, Melkameshet; Wetro, Legesse (PhD)In this thesis we have derived the vector potential to determine the analytic expressions for the magnetic eld of dipole and quadrupole components of neutron star, we develop the eld line equations and the magnetic eld line geometry of both dipole and quadrupole terms. The magnetic eld varies with time and as well as the induced electric eld. As a result the neutron star can generate electromagnetic radiation. This electromagnetic radiation has pressure. We have derived the radiation pressure at any distance r out side of the surface of the neutron starItem A 2D Monte Carlo Investigation of Static and Dynamic Properties of Ring polymer(Addis Ababa University, 2015-02) Admasu, Ashenafi; Yergou, Tatek (PhD)Monte Carlo simulation technique has been used to investigate the static properties and translocation characteristics of ring polymers in two dimensions. Each chain consists of exible linkage between monomers forming a closed loop. We consider the unbiased translocation process of the ring where the polymer translocates purely due to thermal uctuations. The average escape time has been analyzed as a function of chain length N. And we found that, the polymer escape time of the ring polymers having a length N < 100, also has a scaling behavior N where is more closer to 1 + 2 in which is the Flory exponent of value 3/4 in 2D and 3/5 in 3D. Such scaling is also the scaling behavior of the unbiased long linear chains. We study the static property of the small cyclical polymer chains by calculating the mean-squared radius of gyration Rg. Bond uctuation method (BFM) is implemented to study both characteristics of the particular ring polymersItem 2D Spin-Dependent Electron Scattering by Nanomagnets(Addis Ababa University, 2012-02) Senbeta, TeshomeThe 2D scattering problem of an electron by a magnetized nanoparticle is solved in the Born approximation with account of the dipole - dipole interaction of the magnetic moments of electron and nanomagnet. The scattering amplitudes in this problem are the two-component spinors. They are obtained as functions of the electron spin orientation, the electron energy and show anisotropy in scattering angle. The initially polarized beam of electrons scattered by nanomagnet consists of electrons with no spin flipped and spin flipped. The majority of electrons with no spin flipped are scattered by small angles. This can be used as one method of controlling spin currents. 2D spin-dependent scattering of slow unpolarized beams of electrons by charged nanomagnets is analyzed in the Born approximation. The obtained scattering lengths are larger than those from the neutral nanomagnets approximately by one order. It is shown that for particular parameters of the system it is possible to polarize completely the scattered electrons in a narrow range of scattering angles. The most suitable system for realization of these effects is 2D Si electron gas with immersed nanomagnets. The 2D spin-dependent electron scattering by the linear chain of periodic nanomagnets with account of the diffraction effects was studied. This effect takes place in 2D electron gas with immersed nanomagnets. By tuning a distance between nanomagnets, it is possible to obtain diffraction maximum of the scattered electrons at scattering angle, which corresponds to complete spin polarization of electrons. The total diffraction scattering lengths are proportional to N2 (N is a number of nanomagnets). The proposed system can be an efficient separator of spin polarized currentsItem AB Initio calculation of Optical Absorption and Optical Gap of Silicon Nanocrystallites(Addis Ababa University, 2007-07) Gugsa, DereseThe electronic and optical properties of hydrogenated silicon nanocrystals have been investigated both in the ground- and in an excited-state configuration, through different ab-initio techniques. The presence of an electron-hole pair leads to a strong interplay between the structural and optical properties of the system. The aim of this work was to investigate the optical properties of hydrogenated Si nanoclusters (H-Si-nc) that has recent experimental interest for photonic applications. The optical absorption spectra of small SinHm nanoclusters are computed using a linear response theory within the time-dependent local density approximation (TDLDA). The TDLDA formalism allows the electronic screening and correlation effects, which determine exciton binding energies, to be naturally incorporated within an ab initio framework. We examined that the calculated excitation energies and optical absorption gaps to be in good agreement with experimentItem Absorption Coefficient and Dielectric function of direct Band Gap silicon Nanocrystallites(Addis Ababa University, 2007-07) Gebrehiwet, HagosSilicon is an ubiquitous electronic material and the discovery of strong room temperature luminescence from porous silicon in 1990 raised hopes it may find a new lease of life in the emerging field of optoelectronics. First, the luminescence was shown to be emitted from nanostructures in a porous silicon network. Later the same emission was seen from silicon nanocrystallites and the concept of a silicon quantum dot emerged. A number of different models have been proposed for the origin of the light emission. Some involve interface states between a silicon nanocrystallite and a surrounding shell, while others consider the effect of quantum confinement in an indirect bandgap semiconductor. In order to clarify the influence of morphological properties, such as size or shape, of a silicon nanocrystallite on its optical properties, calculations of optical absorption coefficient and dielectric function of silicon nanocrystallites as a function of effective silicon nanocluster size (diameter) and photon energy were attempted using k.p method. To conclude, the work presented in this thesis gives support to the quantum confinement effect in explaining the optical properties of band gap nano-sized silicon below 10nm, as well as highlighting the importance of calculating optical parameters of silicon nanocrystallites to understand optical properties in the luminescence processItem Accretion Discs around magnetised Stars, in Particular Neutron Stars(Addis Ababa University, 2010-02) Belay, SolomonWe develop a self-consistent theoretical model for the steady-state of an axisymmetric thin ac- cretion disc with an internal dynamo around a magnetised stars. Starting from the vertically integrated equations of magnetohydrodynamics we derive a single ordinary di®erential equation for a thin accretion disc around a massive magnetic dipole and based on the analytical formu- lation we integrate this equation numerically from the outside inwards. Our numerical solution shows that the torque between the star and the accretion disc is dominated by the contribution from the dynamo in the disc. We extend this model for weak magnetic ¯elds and we present millisecond x-ray pulsars have weak magnetic dipole moments of » 1016 Tm3 compared to ordi- nary X-ray pulsars with dipole moments of 1020 Tm3. For this reason a surrounding accretion disc can extend closer to the neutron star, and thus reach a higher temperature, at which the opacity is dominated by electron scattering and radiation pressure is strong. We compute the self-similar structure of such a geometrically thin axisymmetric accretion disc with an internal dynamo for the three regions of the disc. For the outer disc region which corresponds to the gas pressure and Kramer's opacity dominated, in the middle region the gas pressure and electron scattering dominated , in the inner region the radiation pressure and electron scattering dom- inated accretion disc. Our numerical solution shows that the torque between the star and the accretion disc is dominated by the coupling between the stellar magnetic ¯eld and the dynamo in the disc. Finally, we have developed that the time-dependent equations for an accretion disc and linear stability analysis of steady-state disc solutions in the presence of a strong external magnetic ¯eld. The analytical and numerical analysis of the solutions to the stability properties and time evolution will tell us the observed behaviours of the torque between the disc and the star. Key words: accretion, accretion discs - magnetohydrodynamics (MHD) - magnetic ¯elds - stars: neutron X-rays: binaries- pulsars: generalItem Action of a Relativitic Charged Point Particle on Itself and other Charges(Addis Ababa University, 1992-06) Tirfessa, NegussieThe different expressions for the Green function of electromagnetic field of a point charge have been considered with the special attention to the advanced contribution that is shown to be of importance for the self-action of particle responsible for the radiation reaction. Simultaneous expansions for electromagnetic fields of a point charge are obtained in two different ways, by means of Lagrange expansions of the fields and the Taylor expansion of the Green function. The Lagrangian description for a system of charged particles and their electromagnetic field is given in terms of particle variables only, with the field expressed through the higher derivatives of the particles velocities. It is shown that the self-actions of particles may be included in their equations of motion with all divergent terms cancelled. An intrinisic relation between the retarded and advanced fields of charged particles has been also obtained. The results are important for an exact description of a system of relativistic charged particlesItem Age Plastion Polaritons and Determinatio X Dielectric Permittivity and Thickness Thin Metal (Semiconductor) Films(Addis Ababa University, 1989-06) Ahmed, Abdurahman; Letov, D.A. ( PhD)Su}~face pla817i\To~~'pol~n~itoriH (SP1P) whIch can be excited on dielectric/met~l (dielc(·trlc/semicouductor) interface can be used as highly sensitive non distructive probe for th" determination of tJ!e dielectric constant £ (H) = (' (,,) + iE" (w) and thickness h of a metal semiconductor) fil,n without [l preset expression for (w). The theoretical background of the method [3] is based on the solution of the HaJmell's equatioIls which provides the disperssion equatIon of SrlP at the surface of a thin metal (semiconductor) film and its interpretation in terms of attenuated total reflection (ATR) characteristics. The physical sense of t,he solution is demonstrated through the power flows and the field distributions of SPIP and other poss~ble excitations. It is sho'"" that for strong absorbing metals (semiconductors) low n"jiative SPIP caa exist at frequencies higher than the plasma frequency H • , The analysts of the approximate solution of the approximate • p sollltion of dispersion equation for thin metal (semiconductor) fillil [3] established a I,ell-defined limited frequency range of its applicability. It was found that the upper frequency limit depends upon the thickness of the film and falls dOHl1 from W / ~2 to '0.35 l~ ss h+O. ,.~ . P lL P A computer model simulating realistic ,experimental conditions of measurement ATR characteristics is discussed. The accuracy of (I(W), E"(W) and h determination is establishedItem The Analysis and interpretatlon of the Anomalies Transition impurity diffusion in aluminium(Addis Ababa University, 1985-06) Mammo, YewondwossenAnalysis of the existing interpretations on the anomalous of transition imnurity dlffusion At is made, A synthes.izing simple theoretical model is developed, assuming dislocations decor a tee, by segregation phases, which made it possible to interpret theItem Analysis of Interference Phenomena Observed at Ethio-Finno Observatory (EFO)(Addis Ababa University, 2006-07) Tadesse, Desalegn; Damte, Baylie (PhD)The pulsation magnetometer at Ethio-Finno Observatory (EFO) has been operational since June 2004. The major objectives of this instrument is to characteristically investigate the ionospheric phenomena of the upper polar atmosphere. Results from the data analysis in time and frequency domain depicts existence of strong interferences which are not observed in other similar observatories. In this thesis we shall investigate the disturbance characteristics and delve out the cause of peaks by relating with solar activities, geomagnetic activities and local effects such as ferrous object, and ground loop. This is done by carefully investigating the data collected during some days and by conducting controlled experimentItem Analysis of Radon Product Activities from Filtered Air Samples(Addis Ababa University, 2021-10-07) Alayu, Daniel; Tesfaye, Tilahun (PhD)Radon is a colorless, odorless, tasteless and radioactive gas. It forms naturally from the decay of radioactive elements such as uranium. It is a natural radioactive gas that occurs ubiquitously throughout the world. In this work, analysis of radon product activities from filtered air samples taken place based on garb sampling of known volume of air on a glass fiber filter with high retention power followed by gross beta counting of the filtered air sample counted over large number of successive time intervals using an end window Geiger - M¨uller ( GM ) counter were reviewed. This method results determination of concentrations of radon and thoron daughter products at a time. The gross beta count is determined using a mathematical model based on Bateman differential equations. Theoretically the radionuclides 218 Po, 214 Pb, 214 Bi, which are progenies of radon ( 222 Rn ) and 212 Pb, 212 Bi, 208 Tl, which are progenies of thoron ( 220 Rn ) have significant contributions to the total beta counting rate. To determine their individual contributions to the total count rate, the Weighted Least Square Method ( WLSM ) or the Steady State Jaccobi’s Model ( SSJM ) can be applied. The review of this paper shown that using the Weighted Least Square Method ( WLSM ) results high random errors than that of the Steady State Jaccobi’s Model ( SSJM ). In both methods accurate results can be obtained for 214 Pb, 214 Bi, 212 Pb, EEDC 222 and EEDC 220. However, the results for 218 Po and 212 Bi are much more inaccurate and the concentration of 208 Tl is difficult to determine. The Steady State Jaccobi’s Model ( SSJM ) takes in to consideration air ventilation rate as a base line to determine radon progenies in a confined system. It gives better detection efficiency and lower the random error of radon daughter concentration.Item Analysis of Tec Variation During June 21, 2020 Annular Solar Eclipse Over Addis Ababa and Djibouti(Addis Ababa University, 2021-10-10) Sete, Shibre; Elfagd, Yitagesu (PhD)On June 21, 2020, an annular solar eclipse happened. The annular eclipse's central path passed through parts of Central and Eastern Africa. A partial solar eclipse was visible across much of Africa and it traverses Ethiopia and Djibouti. During a solar eclipse the Moon blocks a portion of the solar radiation from reaching the Earth. Following the eclipse a decrease in the ionization is expected due to the obscuration of solar radiation, and thus a decrease in the amount of ionospheric total electron content (TEC). The e_ect of this partial solar eclipse on the ionospheric TEC was studied using two Global Positioning System (GPS) stations found in Addis Ababa and Djibouti. The objective of this thesis is to study the e_ect of the June 21, 2020 annular eclipse on the ionospheric TEC. Because eclipse geometry varies from eclipse to eclipse, it's not always clear whether a di_erence in ionospheric reaction is due to a change in eclipse time of occurrence, obscuration level, geometry, or background conditions such as: di_erent solar and geomagnetic activities, seasons, latitudes and longitudes. Therefore, individual eclipse studies are signi_cant. TEC data taken from the two GPS stations were used to examine the ionospheric behavior during the eclipse. The partial solar eclipse was observed to cause a signi_cant TEC depletion during the eclipse period at both stations. Which indicates that the amount of TEC in the ionosphere was a_ected by the decrease in solar radiation during the eclipse. The TEC measurements during the eclipse period were compared to two days before, two days after the eclipse period and three years of June 21 to make sure that the cause of the TEC depletion was not due to a daily variation in the ionosphere. A maximum TEC reduction of up to 35% were observed compared to the days after and before the eclipse. A time lag of 45 and 15 minutes were found between the maximum obscuration and a maximum TEC depletion over ADIS and DJIG, respectively. After the eclipse ended, the TEC began to return to normal levels and recovered fully after 1 hr over DJIG and 1 hr and 18 minutes over ADIS stations. The TEC reduction shows the partial solar eclipse a_ects the ionospheric TEC and since the ionosphere is mostly governed by the photoionization, the decrease in solar radiation causes the density of the ionosphere to decrease. The eclipse occurred during a period of low solar and magnetic activity. The reference days TEC data and the magnetic activity indicated the eclipse was the cause of the TEC depletion. Further research is needed to study the e_ect of the eclipse in the low latitude region as it can be a_ected by the equatorial ionization anomaly.Item Analysis of the Factors Affecting Ionospheric Total Electron Content in East Africa and Middle East(Addis Ababa University, 2021-11-29) Eshetu, Mulu; Elfagd, Yitagesu (PhD)The low latitude and equatorial ionospheric total electron content (TEC) variations are investigated using dual frequency GPS TEC data taken from 7 GPS stations in which 6 are found in East Africa and one is found in Saudi Arabia in the year of 2015. This GPS TEC data was used to study the diurnal, seasonal, geomagnetic and latitudinal variations of ionospheric TEC at these GPS stations as insu_cient research results of ionospheric variations are available for these areas. The results of the analysis showed that the ionospheric TEC undergoes diurnal, seasonal, geomagnetic (quiet and disturbed day) and latitudinal variations. With regard to the diurnal variation, the TEC value shows minimum at predawn and gradual increase with time of the day attaining a maximum in the afternoon and a gradual decrease after sunset. This is the result of photo ionization of the atmosphere due to the increased intensity of solar radiation during day. On the other hand during night electron loss processes (ion-electron recombination and electron attachment) dominate and the TEC value is decreased. The TEC value in March shows a very rapid decrease after sunset due to the high recombination rate that is proportional to the square of the high electron density in March. Moreover, the night time TEC value shows a secondary minima maxima near dawn which may be due to the Counter Electrojet (CEJ). The TEC value in the solstice months of June and December don't show plateau and secondary maxima. It simply increase till noon and then decreases slowly during afternoon and during night. The seasonal variation is caused by the relative position of the Earth with the sun which changes the solar zenith angle. The greatest TEC value occurred in the equinoctial month of March and the least occurred during the winter solstice month of December and intermediate TEC value is recorded in summer solstice month of June. The e_ects of geomagnetic storms on TEC values have been also considered in this study. The result reveals that the storm day TEC value is greater than the quiet day TEC value. This is due to the fact that high amount of solar particles reach the earth and there is more ionization during the storm period. The TEC value also vary with latitude. The highest TEC value is obtained for stations that are closer to the geomagnetic equator (MOIU in Kenya) in the southern hemisphere. This may be the result of the occurrence of highly radiation of the sun and the DEBK station in Ethiopia located around EIA crest region is due to the fountain e_ect. The least value of TEC is observed for stations far from the geomagnetic equator (SOLA in Saudi Arabia) which is located beyond the EIA region. Moreover, TEC value of January 3, 2015 (at perihelion) is greater than that of the July 3, 2015 (aphelion), 2015. For the future more research need on the perihelion and aphelion cases of ionospheric TEC.Item Analysis of Thermoluminescence Glow Curves for First-Order Kinetics(Addis Ababa University, 2019-04-01) Alemayehu, Alemu; Mesfin, Belayneh (PhD)We have analyzed thermoluminescence glow curves the first-order kinetics using the method based on various heating rate. Employing this method, thermoluminescence intensity versus temperature of the glow curves of first order kinetics was plotted for the different values of heating rate, . by usingMATHEMATICA and SPSS softwares, we analyzed these glow curves and discussed the results through evaluating numerical and graphically the activation energies and the frequency factors from glow curves based on the method of various heating rates. For first-order peaks, the method of finding the glow parameters by measuring the shift of the maximum temperature with changing heating rate is studied. In addition, the instantaneous concentration of electrons in the electron traps and the conduction band are simulated.Item Analysis of two Channel Sequential Nitrogen Laser Circuit(Addis Ababa University, 2013-06) Zerihun, SolomonThe analysis of two channel sequential nitrogen laser circuit is developed and scaling parameters are identi ed with Fortran.f90 programing so as to enable a study of the laser over its range of discharge modes. Results are presented for critically damped laser gap case and for the more typical case of critically damping factor 0.4. The analysis shows in a typical nitrogen laser the rate of rise of the laser gap voltage dVg dt prior to laser gap breakdown is 0.5kv/ns and such a dVg dt is required for a uniform breakdown across the laser gap for a good lasing action by comparing two channels. For e ectively critically damping factor at effL = 0.4 the normalized power absorption is increased from 0.00005 to 0.00015 for the rst channel.For the second channel it increases from 0.005 to 0.017 at similar damping factorItem Analytical Representation of Numerical Self Consistent field (SCF) Hartree-Fock Radial Wave Functions for Ions(Addis Ababa University, 1996-06) Getachew, EliasNumerical values of waw functions. calculated from Hartree-fock method. are nowadays available for many atoms and ions. Analytical forms of these functions are important in some cases. By studying Pron~"s method of representing numerical results analytically into sums of exponentials. it is shown that it is possible to apply the technique for approximating the numerically given radial part of atomic \va\'e functions into sums of exponentials analytically. for each important step in Prony's method computer program is developed. The integrated program of the Prony's method is applied to the numerical radial self consistent field (SCf) wave functions of a closed-shell Cd++ ion and the exponents and coefficients for the exponentials of the approximated analytical functions are determined, Comparison between the analytical functions and the numerical functions show good agreement for \\'a\'e functions with smaller principal quantum number value, ,\ new algorithm for approximating tabulated numerical radial SCf atomic waw functions analytically into sums of exponentials is introduced, ,\ computer program is developed for the proposed new algorithm and the technique is applied to ,Id function of cadmium ion. Cd++. and to 2s and 2p functions of floride ion .f-. Exponents and coefficients of the exponentials of the approximating functions are determined. The approximated analytical forms of the numerical radial wave functions are used to reproduce numerical values of the function. The analytical and numerical functions are found to be in good agreement. These techniques can also be applied to analytically approximate nllll1erical SCF functions of other ionsItem The Anomalous Low Temperature Electrical Resistivity Properties of Dilute Magnetic Alloys(Addis Ababa University, 2007-07) Kitachew, Girma; Bedassa, Tesgera (PhD)The existence of localized magnetic moments in dilute magnetic alloys that couple to the conduction electrons has important consequences for the electrical resistivity. The magnetic impurities act as scattering centers, and if they are the predominant type of impurity then at low enough temperature the scattering causes the electrical resistivity. The exchange interaction between the conduction and the localized moments leads to scattering events in which the electron spin is flipped. Calculating the transition probabilities of the conduction electrons to the third order of interaction it is shown that the transition probability has a logarithmic anomaly at the Fermi-surface and that in the resistivity there appears a term proportional to lnT and diverges as the temperature approaches zero.Item Anomalous Skin Effect in Layered Conductors(Addis Ababa University, 2001-06) Gashe, Bekele; Bedassa, Tesgera (PhD)The theory of anomalous skin effect in layered conductors with quasi-twodimensional energy spectnlll1 under high frequency electromagnetic wave propagation along and perpendicular to the layers is studied in the account of Fermi-liquid effects in the absence of static magnetic field. The surface impedance, the conductivity and the skin depth are calculated explicitly for quadratic energy spectlUm and it is observed that the magnitudes differ in different orientations of the sample relative to the direction of propagation of the wave. The dependence of these quantities on the Landau's correlation function is analyzed and they are compared qualitatively with the results in gas approximation theoretical results under the studies of normal metalsItem Application of Electrical Resistivity Techniques in Building Foundation Assessment at Ududaytu Area in the Tandaho Graben of Afar, North Eastern Ethiopia(Addis Ababa University, 2020-07-07) Issa, Yayso; Fisseha, Shimeles (PhD)A geophysical survey involving the electrical resistivity method utilizing the Vertical Electrical Sounding (VES) and Electrical Resistivity Imaging Techniques were conducted around Ududaytu area in the Tandaho graben of Afar, North eastern Ethiopia with the aim to characterize the suitability of the Ududaytu area for building foundation to support engineering structures. A total of 16 VES stations and four Electrical Resistivity Imaging profiles were conducted. The VES stations were measured using Schlumberger Configuration with AB/2 varying from 150 m to 220 m. Whereas Electrical Resistivity Imaging profiles were acquired using Wenner-Schlumberger configuration with electrode spacing of 5m. Four to five main geoelectric sequences were delineated within the study area; these include the topsoil, the upper clay, fine to coarse sand and the bottom clayey sandy /clayey silty layer. A major NNW - SSE discontinuity (fracture zone) was identified. Therefore, this study has showed the usefulness of geophysical methods to delineate the lateral and vertical variations of the subsurface layers and its implication in engineering structures. From geotechnical point of view, the geoelectrical results has shown that the study area is underlain thick sequence of sediments that are not competent enough to bear loads from heavy engineering structures. The study area being within tectonically active region, with peak ground acceleration (PGA, g > 0.3), erecting multistory structures over such relatively weak formation require proper engineering measures during the design and construction stage. Specially, since the amplification potential of such soft formations in the site-soils, could be ominously larger, the design should address the entire relevant factor towards proper mitigation.Item Application of Electrical ResistivityMethodFor Ground "Vater ExplorationIn,South Western Butajira Area,Guraghe Zone(Addis Abeba university, 1997-05) Mebrahtu, BereketThe basic mathematical foundation for the vertical electrical sounding method has been presented by deriving the formula for the potential distribution at the surface of a horizontally stratified earth. The apparent resistivity and its relation with resistivity transform function has been shown particularly for . Sclumberger Configuration. The theoretical background for resistivity inversion with the ridge regression technique has been explained following the method of Inman. On the basis of the above theoretical consideration the electrical resistivity method has been applied for ~ound water expluratinn and possible site for borehole location in South Western Butajira Area, Southern Ethiopia. To do this survey along three profile lines were taken with a total of 20 YES points out of which 15 were analyzed in detail.