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Item Lead contamination of traditional hand-dug wells in parts of Kwale County, Kenya(International Journal of Physical Sciences, 201-05) Chege, M. W.; Hashim, N. O.; Merenga, Abdullah S.; Tschiersch, J.People’s health, behavior and intellect to a certain extent are a function of their environment. One important component of this environment is water, a basic necessity that human beings cannot do without. The water available, however, is not always safe for human consumption due to contamination. Among the many water contaminants, lead (Pb) is one of the most dangerous due to its bio-accumulative nature and high toxicity. Children are most vulnerable to Pb toxicity as their main organs such as the brain and the nervous system are still developing. For them, exposure to even relatively low concentrations can result in reduced intelligence quotient (IQ), learning disabilities and attention deficit disorders, behavioral problems such as delinquency and drug abuse, and such conditions as impaired hearing and mental retardation. The main aim of the present study was to determine Pb concentration in water from hand-dug wells in Kwale County in the coastal region of Kenya. Kwale County hosts a number of schools for mentally disabled and hearing impaired persons, while the coastal region in general has among the lowest literacy levels in the country and the highest incidences of drug abuse. Thirty-seven water samples were randomly collected from selected villages and analyzed for Pb content using a flame atomic absorption spectrophotometer (FAAS). Also analyzed were the water pH and conductivity using portable digital meters. Pb concentration in the water samples ranged from below detection limit to 1.397 mg/L with a mean concentration of 0.330 mg/L. Over 62% of the samples had Pb levels above WHO recommended value of 0.01 mg/L. The pH ranged from 5.8 to 10.4 with a mean value of 9.2 while conductivity ranged from 85.3 to 6060 μs/cm with a mean value of 930.351 μs/cm. Key words: Lead, well water, intelligence quotient (IQ), mental retardation, deafness, KwaleItem Emergence of Advance Waves in a Steady-state Universe(Kluwer Academic Publishers, 1979) Hobart, R. H.In standard Wheeler-Feynman electrodynamics advanced waves from any source are absolutely canceled by the advanced waves from the absorber responding to that source. The present work shows this cancellation fails over cosmic distances in a steady-state universe. A test of the view proposed earlier, in a paper which assumed failure of cancellation and hoc, that zero-point fluctuations of the electromagnetic field are such emergent advanced waves, is posed. The view entails anomalous slowing of spontaneous transition rates at longer emission wavelengths; available data go against this, furnishing additional argument against the suspect assumption that the universe is steady-state.Item Solar wind controlled pulsations: A review(Reviews of Geophysics, 1986-02) Odera, T. J.Abstract Studies of the solar wind controlled Pc 3, 4 pulsations by early and recent researchers are highlighted. The review focuses on the recent observations, which cover the time during the International Magnetospheric Study (IMS). Results from early and recent observations agree on one point, that is, that the Pc 3, 4 pulsations are influenced by three main solar wind parameters, namely, the solar wind velocity Vsw, the IMF orientation θxB, and magnitude B. The results can be interpreted, preferably, in terms of an external origin for Pc 3, 4 pulsations. This implies, essentially, the signal model, which means that the pulsations originate in the upstream waves (in the interplanetary medium) and are transported by convection to the magnetopause, where they couple to oscillations of the magnetospheric field lines.Item PC 3,4 magnetic pulsations observed simultaneously in the magnetosphere and at multiple ground stations(Reviews of Geophysics, 1991-09) Odera, T.J.; Swol, D.V.; Russell, C.T.; Green, C.A.Periods of magnetic conjugacy between ISEE and the magnetometer array of the Institute for Geological Sciences have been examined to search for the simultaneous occurrence of Pc 3,4 magnetic pulsations. When compressional waves are seen in space, waves are also observed on the ground at the same frequency and with similar waveforms. The wave amplitude on the ground at midlatitudes is similar to that in space at ISEE but at high latitudes the amplitudes are larger than in space. The one occurrence of a purely transverse signal at ISEE was not observed on the ground. These results confirm that Pc 3,4. wave energy is most readily transported through the magnetosphere by compressional fluctuations.Item The Hall effect and electronic conduction in amorphous Si(Taylor &Francis, 1995-02-17) Okumu, John; Holender, J. M.; Morgan, G. J.We show that the random-phase model for the electronic wavefunctions gives very reasonable agreement with recent computations of the conductivity for amorphous Si. Furthermore the model also yields the double-sign anomaly in the Hall coefficient observed experimentally, which we have also confirmed by computer simulation. In addition we provide a simple explanation of a computer experiment performed by Weaire and Hobbs in 1993 in which amorphous Si behaves very much like crystalline Si. We argue that this result is not in contradiction with the Hall anomaly.Item A Preliminary Study of Relative Permeability in Geothermal Rocks Authors(Geothermal Resources Council, Davis, CA (United States), 1995-12-31) Ambusso, W.; Satik, C.; Castanier, L.M.; Horne, R.N.This paper reports preliminary experimental and numerical efforts towards obtaining steam-water relative, permeability and capillary pressure functions under steady-state and adiabatic conditions. In the experimental direction, steady-state nitrogen-water relative permeability experiments were conducted in a Berea sandstone core as a first step. Results obtained from this type of experiment will be compared to those from steam-water relative permeability experiments in order to explore the importance of phase change and heat transfer. Using a high resolution X-ray computer tomography (CT) equipment, saturation distributions along the core were obtained and relative permeabilities for both nitrogen and water were calculated. Preliminary results showed strong end effects for the core length and total flow rate used in the experiment, which therefore suggested either to use of a longer core or to work at a higher total flow rate. Along with the experiment, numerical simulations of simultaneous injection of steam and water into a core were also carried out by using a commercial thermal simulator. At steady-state flow conditions, effects of steam quality and total injection rate on saturation profiles were investigated. Numerical simulation results suggested a core length of 38.10 cm for a flat saturation profile region to exist under typical experimental conditions.Item Determination of Relative Permeability for Steam-Water Flow in Porous Media(Society of Petroleum Engineers, 1996-01-01) Ambusso, W.; Satik, C.; Horne, R.N.A set of relative permeability relations for simultaneous flow of steam and water in porous media were obtained from a steady state experiment. The experiment was conducted under conditions that minimize most of the difficulties encountered in the past for similar experiments. A high resolution X-ray CT scanner was used to obtain three-dimensional porosity and saturation distributions during the experiment. Pressure and temperature data were collected with the use of several pressure transducers and thermocouples located at various locations along the core. Relative permeability values were calculated over the core lengths identified as having flat saturation profiles. These aspects constitute a major improvement m the experimental method compared to those used in the past. The experimental results showed the relative permeability to both steam and water vary linearly with saturation, in contrast to those obtained by Corey (1954) for gas/water flow experiments. Comparison of the saturation profiles measured by the X-ray CT scanner during the experiments shows good agreement with those predicted by numerical simulation. To obtain results that are applicable to general flow of steam and water in porous media, similar experiments should be done at higher temperature and with porous rocks with different wetting characteristics and porosity distribution. Introduction In general, the concept of relative permeability is an attempt to modify Darcy's law for single-phase flow in porous media in order to account for simultaneous flow of multi phases. In this description the flow of each phase is governed by the individual microscopic pressure gradients and by the fraction of the overall permeability that is associated with it. This fraction, normally expressed as a fraction of the medium's permeability to a single-phase fluid, is called the relative permeability. Relative permeability relations have been traditionally expressed as a function of saturation principally because it was believed that they depended on the pore volume occupied by the fluids (Hassler, 1944). While a great number of experiments have shown this to be true, a number of other experiments have shown that relative permeability depends also on several other parameters such as interfacial tension, wetting characteristics and viscosity ratios of the flowing fluids etc. (Fulcher et al., 1983; Osoba et al., 1951). In addition it is necessary to define residual saturations which normally indicate the lowest saturation value for a given phase to become mobile. The curves and the residual saturations together define the relative permeability relations (Corey, 1954; Brooks and Corey, 1964). For most applications in petroleum engineering involving the flow of oil and water as in water flooding and the flow of oil and gas as in gas injection, the relative permeability relations are well known and determined routinely from laboratory experiments (Osoba et al., 1951). However, for the flow of steam and water or for the general case of multi component multi phase flows these relations are not well known. A look at previous literature shows that there is a significant discrepancy and inconsistency in the results previously obtained (Verma, 1986; Sanchez, 1987; Clossman and Vinegar, 1988). The main difficulties encountered in the previous experiments, as we show later in this paper, have been mainly due to inaccurate measurements of fluid saturations and inappropriate assigninent of pressure gradients to the individual phases. Techniques involving analysis of enthalpy transients from producing geothermal fields have been proposed to infer relative permeability relations (Grant, 1977; Sorey et al., 1980; Horne and Ramey, 1978).Item Ohmic and Space-Charge-Limited Conduction in Doped Amorphous Arsenic Triselenide(Faculty of Science Kenyatta University, 1998) Wafula, A. B.The electrical characteristics of asernic triselenide have been studied. A number of parameters is evaluated on the basis of the theory of space-charge limited conduction and the following values were obtained: hole mobility Il == 2.68 x 1O-l3m2v-'s-1,room temperature hole concentration Po == 7.21xl023m-3, temperature parameter of trapping distribution T, == 790K; total trap concentration Nt == 3.49 x 1025m-3.Both deep and shallow traps were present. The latter extended to a depth of 0.27 ± 0.03 eV while the deep traps were at about the fermi level.Item A generalized Coulomb-projected Born Calculation using Screened Coulomb Potential(Faculty of Science Kenyatta University, 1998) Singh, C. S.The Coulomb-projected Born method proposed by Geltman has been generalized by using the screened Coulomb potential of the form in place of the nuclear potential experienced by the incident electron. The results obtained by using this generalized Coulomb projected Born method for electron impact excitation of 2s state of hydrogen atom have been presented.Item Analysis of Soil for Trace Elements along Two Highways of Kenya(Faculty of Science Kenyatta University, 1998) Hashim, N.O.; Kinyua, A. M.; Mangala, M. J.; Rathore, I. V. S.Energy dispersive X-ray fluorescence technique has been used to study the levels of lead and other toxic trace elements in the soil samples collected along two major highways (Mombasa and Thika) of Kenya at the various distances off the road. The soil samples from Thika road are found to have higher concentration values of the trace elements: Cr, Zr; Mn, Fe, Y; Ni, Cu, Zn, Br and Sr than those from Mombasa road. The samples from Thika and Mombasa roads have maximum concentrations of Lead, 56 ug/g and 90 ug/g respectively at the distance of 100m off the roads, which is below the admissible value of 250 Ug/g for agricultural practices.Item Optical studies of doped amorphous arsenic triselenide(Faculty of Science Kenyatta University, 2000) Wafula, A. B.; Dod, T. S.Photoconductivity of As2Se3:Alx and As2Se3:Cux was investigated using light of visible spectrum of wavelengths A. = 0.67 um & 0.47 urn. Two regimes were observed: regime 1, the high temperature region above maximum photoconductivity where photoconductivity (crph) is less than dark conductivity (crd), and regime 2, the intermediate temperature region below the maximum photoconductivity where crph > crd' Maximum crph for A. =0. 67 urn occurred at Tm= 313 K for all samples while for A. = 0.47 urn, it occurred at Tm= 323 K for pure As2Se3, 303 for As2Se3:Cux and at 318 K for As2Se3:A Ix. Activation energies of crph were lower than those from crd and those of doped samples were lower than those of pure sampleItem Calculation of linear and non-linear optical properties of amorphous Silicon(Faculty of Science Kenyatta University, 2001) Okumu, J.; Morgan, G.J.We have used the random phase model for the electronic wave functions to calculate the imaginary part of the dielectric function 82 (m) and the magnitude of the third harmonic generation, 11'3(OJ ~ using a model structure of a-Si containing 1728 atoms. We show that this simple calculation gives reasonable results for e(e) and furthermore the behaviour of .l. «(l). is similar to more detailed theoretical calculationsItem Mechanical stresses upon crystallization in phase change materials(American Institute of Physics, 2001) Njoroge, W. K.; Kalb, J.; Wamwangi, D.; Wuttig, M.; Spaepen, F.Crystallization processes in different Te alloys, employed in phase change materials for optical data storage, have been investigated by in situ mechanical stress measurements. Upon crystallization a considerable stress buildup is observed, which scales with the volume change upon crystallization. Nevertheless the observed stress change only corresponds to approximately 9% of the stress estimated for a purely elastic transformation. Further evidence of stress relief phenomena comes from the temperature dependence of the stress in the crystalline and amorphous states. Ultrathin dielectric layers have a profound influence on the crystallization process as evidenced by simultaneous optical reflectance and mechanical stress measurements. This observation can be explained by heterogeneous nucleation of crystallites at the interface between the dielectric layer and the phase change film.Item Crystallization kinetics of sputter-deposited amorphous AgInSbTe films(American Institute of Physics (AIP), 2001-10) Njoroge, Walter K.; Wuttiga, MatthiasAgInSbTe films have recently attracted considerable interest as advanced materials for phase change recording. For this application the determination of crystallization kinetics is of crucial importance. In this work the temperature dependence of structural and electrical properties of sputtered AgInSbTe films has been determined. Temperature dependent measurements of the electrical resistance have been employed to study the kinetics of structural changes of these films. Upon annealing a major resistivity drop is observed at around 160 °C which can be attributed to a structural change as corroborated by x-ray diffraction. X-ray diffraction shows an amorphous phase for as-deposited films, while crystalline films with hexagonal structure (a54283 Å, c516 995 Å! are obtained upon annealing above 160 °C. By applying Kissinger’s method, an activation energy of 3.0360.17 eV is obtained for the crystallization. X-ray reflection measurements reveal a density increase of 5.2%60.2% and a thickness decrease of 5.5%60.2% upon crystallization. © 2001 American Institute of Physics. @DOI: 10.1063/1.1405141#Item Density changes upon crystallization of Ge2Sb2.04Te4.74 films(2002-05) Njoroge, W. K.; Wöltgens, H. W.; Wuttig, M.The density of sputtered Ge2Sb2.04Te4.74 thin films upon annealing has been precisely determined by x-ray reflection and compared to the values determined from x-ray diffraction (XRD) data. The film density increases in two steps around 130 and 280 °C upon annealing up to 400 °C. These increases are consequences of phase transitions from amorphous to NaCl type and from NaCl type to hexagonal structure, respectively, as revealed by XRD. Average density values of 5.87±0.02, 6.27±0.02, and 6.39±0.02 g/cm3 were measured for the amorphous, NaCl-type, and hexagonal phases, respectively. This corresponds to density changes upon crystallization of 6.8±0.2% and 8.8±0.2% for NaCl-type and hexagonal phases, respectively. The accompanying film thickness reductions were determined to be 6.5±0.2% and 8.2±0.2%, which compares very well with the density changes. The corresponding XRD values are determined to be 6.43–6.48 and 6.48 g/cm3 for NaCl-type and the hexagonal phases, respectively. This shows that nearly void-free films are formed.Item In situ measurements of thickness changes and mechanical stress upon gasochromic switching of thin MoOx films(American Institute of Physics (AIP), 2004) Okumu, J.; Koerfer, F.; Salinga, C.; Wuttig, M.Thin films of MoOx coated with platinum as a catalyst were prepared by dc magnetron sputtering from a molybdenum target in an oxygen and argon atmosphere. The films were colored and bleached by exposure to diluted hydrogen gas and air, respectively. In situmeasurements of x-ray reflectance and substrate curvature were performed during switching cycles. Upon hydrogen exposure a remarkable thickness decrease of 11% and a density increase of 6.5% are observed by x-ray reflectance measurements. Corresponding changes in optical transmittance and mechanical stress up to several hundreds of MPa are also observed. These changes are not fully reversible as revealed by measurements of multiple switching cycles. Both the change of thickness and density, as well as the change of optical transmittance and mechanical stress can be explained by the existence of a mixed phase of edge and corner sharing MoO3 octahedra whereupon the first coloration the ratio between edge and corner sharing octahedra is changed.Item Current-voltage characterization of doped polystyrene thin films with view to gas sensing(Kenyatta University, 2004) Wafula, A. B.; Katana, G.Current-voltage measurements of iodine doped polystyrene thin films as a function of dopant concentration, temperature and radiation have been carried out. The thin films were prepared by solution casting method taking the constituents in right proportions. All devices showed memory arid threshold switching except for doped annealed samples that lost both memory threshold switching at a temperature of 90°(:. The thickness was varied from 3-7/lm while the concentration of iodine varied from 0.025 to 0.100 g. The threshold voltage was lower in pure samples than in doped samples indicating an increase in disorder in the arrangements of the atoms. Irradiated and annealed devices showed a marked increase in threshold voltage. Once the pure samples switched from OFF-state to ON-state they remained in the ON state but the doped samples returned to their original state after 33 minutes to 3 hours of switching depending on the concentrations. The space charge limited current theory is a suitable model to explain condition in these polymeric materials. The ON-state was ohmic while the OFF-state obeyed Child-Langmuir law.Item Influence of dielectric capping layers on the crystallization kinetics of Ag5In6Sb59Te30 films(American Institute of Physics, 2004) Njoroge, W. K.; Dieker, H; Wuttig, M.AgInSbTe alloys are attractive storage materials for phase change recording utilizing both optical and electronic contrast. The demand to decrease the thickness of such storage layers increases the significance of the surrounding dielectric layers, which can have a profound impact on the crystallization process. Hence we have investigated the influence of different capping layers on the crystallization kinetics of Ag5In6Sb59Te30films by measuring the electrical resistivity of the films as a function of temperature employing the van der Pauw method. While the phase transition temperature was found to only increase slightly for capped Ag5In6Sb59Te30films, a profound impact on the activation barrier for crystallization was observed. Activation energies of 2.39±0.10eV and 3.24±0.12eV were determined for films capped with 5nm of ZnS-SiO2 and Si3N4, respective1y, in comparison with a value of 3.03±0.17eV for an uncapped film. Additional stress measurements reveal that this change in activation barrier is not caused by stresses induced by the dielectric films but must rather be due to the different activation barriers for heterogeneous nucleation at the different interfaces. In the amorphous state the temperature dependence of the sheet resistance follows an activation-type relation, where the activation energy is not affected by the capping layer.Item Study of angular correlation parameters using a distorted wave model(Kenyatta University, 2004) Singh, C. S.We have used a distorted wave model suggested by Singh in calculating the angular correlation parameters f... and R for electron-hydrogen collision. In this model the initial state distorted wave is generated by the static potential in the initial state and the final state distorted wave is generated by a potential taken as the sum ofone.:}}aif of the initial state static potential plus onehalf of the final state static potential. The results have been compared with other theoretical and experimental results.Item An investigation of the repeatability of calibration factors in gamma-ray spectrometry of geological materials(Elsevier, 2004-01) Mustapha, A.O.; Patel, J. P.; Rathore, I. V. S.; Hashim, N. O.; Otwoma, D.A NaI(Tl)-based gamma spectrometer for the analysis of geological materials was calibrated using the IAEA reference materials RGU-1, RGTH-1 and RGK-1. To simulate typical geological samples, two additional standards were prepared from aliquots of the three reference materials. The reproducibility of the instrument calibration factors (CFs) was tested by repeated measurements of the pure IAEA reference materials and the mixed samples in a reproducible counting geometry. The results were analysed using a two-way classification analysis of variance; it was found that the variance in the CFs is significantly higher between standards than it is between measurements. Allowance should be made for this when estimating uncertainties in measurements with the NaI(Tl) spectrometers.