Department of Physics
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Item Monitoring and High-Resolution Studies of Some Hydroxyl and Methanol Masers in the High Mass Star Forming Region G9.62 + 0.20 E(University of Ghana, 2020-07) Woode, B.This thesis presents the observational data reduction and results of an investigation into the 1:6 GHz OH and 6:7 GHz methanol masers in the G9:62+0:20E massive star forming region. Monitoring observations from the Hartebeesthoek Radio Astronomy Observatory (HartRAO) radio telescope were employed to establish correlated variabilities between the 6:7 GHz methanol masers and the left and right circular polarizations (LCP and RCP) of the mainline 1:6 GHz OH masers. This is the first reported incidence of periodic variations of the LCP and RCP components of the 1:6 GHz OH mainline masers associated with the 6:7 GHz methanol masers in this high mass star forming region. Correlated variability between the varying features in the LCP components of both hydroxyl masers and methanol masers was observed but the variability profiles were different for both species. Time lags were recorded between the methanol and the OH maser variabilties. Within the OH masers the variability profiles were preceded by a dip in flux density. The first high resolution observation of the L-band and Cband continuum of G9:62+0:20E were conducted with the e-MERLIN interferometric array. A flux density of 2:25 0:20 mJy and an angular size of 130 mas were measured for the C-band continuum. At L-band, an upper limit of 162 mJy on the integrated flux density was calculated at the position of the continuum. These results unlike previous observations reveal that the spectrum turns over steeply at low frequencies which could be explained with a truncated inverse square law density distribution model. The peak brightness temperature indicates that the continuum is optically thin at C-band and optically thick at L-band. In the context of the colliding wind binary model for periodic maser sources it is the variation of the seed photons from the background HII region that explains the variability. The continuum results in this thesis place firm constraints on models of the HII region that can be used to test the predictions of the colliding wind binary model in terms of the variability profiles of the OH and methanol masers.Item The Effects of Harmattan Dust and Rainfall on Microwave Signals in Parts of Ghana(University of Ghana, 2019-07) Acheampong, C.A.Microwave signal attenuation due to harmattan and rainfall are two of the major constraints in selecting microwave bands in Ghana. With customer complaints of signal loss during these seasons, telecommunication companies compensate for sudden losses in signal strength by amplifying the signal wave. This study investigates the effect of harmattan and rainfall on communication link for frequencies ranging from 10 to 100 GHz for two locations in Ghana, namely Axim and Tamale. Visibility and rain rate measurements were made over a two-year period in these locations (Axim and Tamale). The signal attenuation as a result of harmattan dust was calculated using the Zain Elabdin model for visibilities from measured data. The microwave attenuation was obtained for frequencies ranging from 10 GHz to 100 GHz. The rain attenuation was determined using the ITU-R Specific Attenuation Model for Rain (ITU-R P.838-3) for the design of microwave communication systems. The results showed that both harmattan dust and rainfall have a significant impact on signal attenuation in parts of Ghana. The effect of attenuation due to harmattan dust was much more evident in Tamale whiles rain attenuation was more evident in Axim. It was observed that the signal attenuation increased as the radius size increased for frequencies from 10 – 100 GHz. The signal attenuation due to harmattan dust was calculated for frequencies ranging from 10 – 100 GHz for both Tamale and Axim. The signal attenuation due to harmattan dust increased with increasing frequencies for both locations. Upon comparison, it was observed that Tamale had the higher attenuation values. Axim showed relatively low attenuation values from frequencies of 10 GHz to 100 GHz and ranged from 0.00009 dB/km to 0.03841 dB/km whiles signal attenuation values for Tamale increased from 0.00146 dB/km to a maximum value of 0.6272 dB/km. This graph shows that attenuation due to dust was more prevalent in the Tamale than in Axim. Furthermore, attenuation values calculated using measured rain rate were highest for high rainfall rates while the lowest attenuation values corresponded to the lowest rainfall rates, as shown in the ITU-R rain attenuation model.Item A Prototype of an Adaptive, Multi-Feedstock, Anaerobic Biomass Digester for Biogas Production(University Of Ghana, 2018-03) Ampomah-Benefo, K.In this work, the production of biogas from local biomass has been studied through the design, construction and operation of a single-stage, multi-feedstock anaerobic digester, with the aim of providing the scientific and technological bases for the mass assembly and deployment of modular and scalable bio-digester system, using components obtainable in Ghana. A horizontal cylindrical plastic tank (1 m3) with torispherical ends has been developed into an air-tight (sealed) anaerobic digester. The factors considered in the design and operation include the type of organic material as feedstock and digester portability. The assembled digester was operated as a bench system for a cycle of 24 days under mesophilic temperature conditions. The maximum and minimum temperatures recorded during the operation of the digester were 38.5 oC and 32.4 oC respectively. The average operating temperature was 36.1 ± 1.5 oC. A slurry of fresh cow dung with calorific value of 26.45 MJ kg-1 was used as substrate for the study. The final composition of the biogas produced was 60.2 % CH4 and 39.7 % CO2, with the highest daily gas production of 0.474 m3, which was measured on the 18th day. A motorized stirrer and a thermostatically operated heat exchanger system were used to ensure there were no temperature and concentration gradients during the operation of the digester. The performance of the digester is deemed satisfactory and confirms that this pilot-scale prototype is a valid proof of concept and can therefore be used as the basis for the design and assembly of production-scale digester systems.Item Mode-Locked Erbium Doped Fiber Laser, Supercontinuum Generation in Solid Normal Dispersion Photonic Crystal Fibers and Supercontinuum Amplification(University Of Ghana, 2017-12) Hodasi, J.A.M.A mode-locked erbium doped fiber laser has been constructed and coherent octave spanning supercontinua have been generated and amplified. Mode-locking in the erbium-doped fiber laser, which produced parabolic pulses, was achieved by nonlinear polarization evolution. A 355 cm all-fiber ring cavity produced 73 fs pulses at a wavelength of 1572 nm with an average output power of 24 mW and a pulse energy of 0.46 nJ. A second all-fiber ring cavity of length 510 cm produced 63 fs pulses at a wavelength of 1604 nm and an average output power of 3.3 mW. The supercontinua were generated in all-solid, all-normal dispersion photonic crystal fibers (PCFs) specially designed to maintain normal dispersion over a wide bandwidth up to 3000 nm. The spectral broadening that leads to the supercontinuum arises from self-phase modulation and optical wave-breaking because of the normal dispersion character of the solid core photonic crystal fibers, giving a highly coherent supercontinuum pulse with a spectrum that spanned an octave, from 1100 nm to an upper wavelength limit of 2200 nm. The spectral envelope was generally smooth with a flat-top profile except for the central portion which contained cladding modes. The supercontinuum was used to seed an erbium-doped fiber amplifier (EDFA), a thulium-doped fiber amplifier (TDFA), and a thulium-holmium co-doped fiber amplifier (THDFA). The EDFA was pumped at 974 nm at a power of 650 mW and gave a net gain of 5.6 dB at a slope efficiency of 11.3%. The TDFA, which was pumped at 793 nm, amplified the supercontinuum over a bandwidth of 54 nm in forward pumping configuration whereas backward pumping gave a bandwidth of 40 nm. The amplified supercontinuum was centred at 1991 nm in forward pumping and at 1979 nm in backward pumping configuration. The widest bandwidth of amplified supercontinuum was obtained with the THDFA, which produced a 120 nm bandwidth pulse centred at 1.94 μm and which is compressible to a pulse width below 40 fs in an all-fiber system with 1 W maximum output power.Item Groundwater Velocity Field: Stochastic Analysis Of Major Groundwater Flowpaths And Implications For Contaminant Transport Within The Ga East And Adentan Districts, Ghana(University of Ghana, 2016-07) Hagan, G.BThe hydrogeological system of the Ga East and Adentan Districts of the Greater Accra Region of Ghana was studied using numerical modelling on the Groundwater Modelling System platform (version 10.0). Historical hydrogeological data and groundwater monitoring data on twenty wells drilled in 2012 were relied on in conceptualising the hydrogeological system of the study area. A single aquifer system made up of quartzite-schist formation was identified. The borehole logs reveal four lithologies, laterite, schist, quartzite, and clay. Subsequently, a calibrated steady-state groundwater flow model was developed for the terrain. The aquifer hydraulic conductivity estimates for about 90% of the terrain are lower than 15.0 m day−1. The observed outliers were attributable to the fractured and jointed quartzite within the weathered zone, which enhanced the conduits within the materials for rapid flow of groundwater. The estimated velocity field is in the range of 0.002 m day−1 to 11.2 m day−1, with about 90% of the area having velocities below 0.85 m day−1. The spatial distribution of the velocity field tied in well with the hydraulic conductivity field observed for the terrain. The estimated velocity field makes contaminant transport significant in the domain. For instance, contaminants/pollutants leached into the aquifer zone through recharge by rainfall from the landfill sited at Pantang will travel at approximately 0.85 m day−1 towards Oyarifa, Teiman and Ayimensa communities along an identified flowpath. Seven principal groundwater flowpaths were identified using the particle tracking technique. The travel times along the flowpaths from recharge to discharge locations ranged from 7 years to 833 years. The cross-section cutting technique revealed cases of local and intermediate flow systems, and identified potential recharge areas. The effective aquifer recharge through precipitation ranges from 2.70×10−5 m day−1 to 8.10×10−5 m day−1, representing 1.2% and 3.6% of the average annual precipitation in the area. The water budget shows that the flow system conserved mass, an indication that the flow model was indeed calibrated under steady-state conditions. The water budget also indicates that the current abstraction levels of groundwater can be sustained by recharge through rainfall to the aquifer system with minimal net drawdown in the hydraulic head. Stochastic simulations carried out on the calibrated model indicates that the model is unique for the aquifer recharge, hydraulic conductivity and hydraulic head. Three management scenarios which are centred on variations in groundwater recharge and abstraction were simulated to investigate the sustainability of the groundwater resource. The first scenario indicates that the existing recharge rates can sustain up to a three times increase in the current rates of abstraction of groundwater for both domestic and commercial purposes. The second scenario shows that, when the current rates of recharge decline by half or more, there would be considerable drawdown if the current abstraction rates were to be sustained solely by groundwater resource. The third scenario reveals that, with rainfall being the main source of recharge to the aquifer, a reduction in the current rainfall figures coupled with an increase in the current abstraction rates by three to five times would result in considerable drawdown.Item Synthesis and Characterization of Cupric Oxide Nanoparticles for Photovoltaic Applications(University of Ghana, 2015-03) Yankson, A.A; Kuditcher, A; Amuzu, J.K.A; Hagoss, G.G; University of Ghana, College of Basic and Applied Sciences, School of Physical and Mathematical Sciences, Department of PhysicsA simple inexpensive chemical route has been identified and used to synthesize cupric oxide nanoparticles suitable for photovoltaic applications. X-ray diffraction analysis showed the synthesized nanoparticles to be a pure cupric oxide phase. The particle size and particle size distribution of the cupric oxide nanoparticles were obtained by transmission electron microscopy (TEM) whereas the crystallite size and crystallite size distribution were obtained by X-ray diffraction peak broadening analysis. The particle size was found to be between 20 nm and 60 nm, an indication that cupric oxide nanoparticles are dominant in the sample produced and analysed. The particle size distribution obtained from cumulative percentage frequency plots features a log-normal function. Absorbance measurements and analysis show that the material has an absorbance peak around 314 nm and an average energy bandgap of 1.48 eV, making it a good candidate for photovoltaic applications. A thin film of average thickness of 1.47 microns made, was used to determine the conductivity of the material, which is 8.541×10−2 Ω−1cm−1. Incident radiation of various wavelengths from the highest (600 nm) to the lowest (300 nm), caused decrease in resistance of the thin film sample, an indication that the material responds favourably to visible radiation making it suitable for photovoltaic and photo electronic applications. We emphasize that a low cost approach has been used successfully to synthesize cupric oxide nanoparticles with the suitable optical and electrical properties required for application in the photovoltaic and photo electronic industries.Item Characterization and Sources of Air Particulate Matter at Kwabenya, Near Accra, Ghana(University of Ghana, 2000-05) Aboh, I.J.K.; Tetteh, G.K.; Osae, E.K.; Kakane, V.C.K.; University of Ghana, College of Humanities , School of Arts , Department of Philosophy and ClassicsGravimetric, reflectometric and elemental analyses have been carried out on airborne particulate matter sampled in a semi-rural area of Kwabenya, near Accra-Ghana. The PM 10 aerosols were sampled using a Gent sampler, size segregating the aerosol into coarse (PM10.2.5) and fine (PM2.5) fractions. The data and derived information were generated from 216 days of sampling spanning a period of about 14 months, 28th December 2005 to 12th February 2007. The particulate matter (PM) at Kwabenya was dominated by the coarse particulates and showed low levels during the Rainy season and high levels during the Harmattan period. The levels measured during the 2006/07 Harmattan were very high. The mass concentration for the measuring period were in the following ranges; coarse (PM10-2 .5 ) fraction (0.16 - 1794.01 ^ig/m3); PM2 .s(fme) fraction (0.50 - 430.23 ng/m3) and PM10 (0.87 ng/m3 to 2064.89 |ig/m3). Additional information about the ambient air was obtained through the subsequent determination of elemental concentration using energy dispersive x-ray fluorescence (EDXRF) analygia^and black carbon (BC) concentration through the “black smoke method”. The/^lpfnents iafe^ti|led and quantified with the Quantitative X-ray Analysis System (QXAS^ package software were: Al, Si, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr an^ i^ itL ^ i^ co a rse fraction. The following elements were identified and quantified in the fine fraction: Al, Si, S, K, Ca, Ti, Mn, Fe, Cu, Zn, Br, Rb, Sr and Pb. Validation of the quantitative methods with the standard reference filter SRM2783 gave very good agreement (within ±15%) for most elements analysed except for Ni (±43%)which was very close to the detection limit. The elemental concentrations in the two fractions vary from season to season. Using simple correlation analysis some elements correlate, the elemental correlations also vary from season to season, for example during the Harmattan S, Cl, V, Br and Sr correlated very well but during the Rainy season S did not correlate with V and Br. This could serve as possible source indicators. The BC concentration in the fine fraction (ranging from 0.01 to 5.97 figm"3) was generally higher than in the coarse fraction and comparable to data from some developed countries. A receptor model using principal component and regression analysis was used to identify sources contributing to the air particulate matter at Kwabenya. The species used in the model were mass, BC and elemental concentrations. The following major sources were identified in the coarse aerosol: Soil/Dust, Biomass/LDT and Sea aerosol. In the fine aerosol the following sources were identified: Soil/Dust, Biomass/LDT and some industrial sources. The contribution of the sources to the PM load varied from season to season, There was very good agreement between the experimental and model data (mass, BC and elemental concentrations). Comparing the data with WHO limit (50 ^grrf3 for 24-hour mean) and Ghana EPA guideline limit (70 Hgm' 3 for 24 hours) for PMio, a total of 185 and 130 days respectively out of 216 days had values above these limits. For PM2 5 a total of 60 days had values exceeding the WHO limit (25 (agm' 3 for 24-hour mean). The levels of S, Ni and Pb were also comparable to industralised countries. There is the need for some mitigation measures to curb the emission of these elements and fine BC.Item Characterization and Source Apportionment of Airborne Particulate Matter in Some Urban Neighbourhoods of Accra, Ghana(University of Ghana, 2014-07) Hughes, A.F.; Kakane, V.C.K.; Amuzu, J.K.A; Agyei, E.K.; University of Ghana, College of Humanities , School of Arts , Department of Philosophy and ClassicsA year-long campaign which accounted for seasonal differences have been conducted to examine the levels, chemical composition and sources of ambient particulate matter in multiple neighbourhoods of varying socio-economic status in Accra, Ghana. Between September 2007 and August 2008, simultaneous measurements of PM2.5 and PM10 aerosols at five monitoring sites in four neighbourhoods (Asylum Down, East Legon, Nima and James Town/Ussher Town) were done for one 48-hour period every six days. Harvard Impactor with polytetrafluorethylene (PTFE) Teflon filter of 37 mm supported by a Whatman drain disc were used to sample air particles in the Accra neighbourhoods. Gravimetric analysis and energy dispersive X-ray fluorescence (EDXRF) were used to determine the chemical composition and concentration of the aerosol particles. The mean mass concentration values for PM2.5 obtained at the five sites during the study period varied from 45.9 μg m-3 to 74.8 μg m-3. The mean mass concentration for PM10 also varied from 93.9 μg m-3 to 134.8 μg m-3. These levels were all substantially higher than the EPA (Ghana) guidelines values and other international air quality standards from WHO, USEPA and EU. Weak relationships were obtained between PM and weather parameters. Crustal elements were most abundant during the seasonal Harmattan period between late December and early February when Saharan dust is transported across West Africa. Enrichment factor analysis was used to provide an initial indication of the species of anthropogenic origin in the measured elemental composition. Source contributions were analysed using positive matrix factorization (PMF) model separately for Harmattan and non-Harmattan periods because large changes to source profiles is expected during the Harmattan period. Anthropogenic sources resolved by PMF model (biomass burning, solid waste burning, resuspended dust and traffic/industry emissions) during both Harmattan and non-Harmattan periods had significant influence on the four neighbourhoods in Accra.