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[Truncated abstract] Infrared radiation detectors find application in a wide range of military and civilian applications: for example, target identification, astronomy, atmospheric sensing and medical imaging. The greatest sensitivity, response speed, and wavelength range is offered by infrared detectors based on HgCdTe semiconductor material, the growth and characterisation of which is the subject of this thesis. Molecular Beam Epitaxy (MBE) is a versatile method of depositing layers of semiconductor material on a suitable crystalline substrate. In particular, MBE facilitates the growth of multilayer structures, thus allowing bandgap engineered devices to be realised. By modulating the bandgap within the device structure it is possible to improve the sensitivity or increase the operating temperature of photodetectors when compared to devices fabricated on single layer material. Furthermore, dual-band detectors may be fabricated using multi-layered HgCdTe material. The bulk of this thesis is concerned with the development of the MBE process for multilayer growth, from modelling of the growth process to characterisation of the material produced, and measurement of photoconductive devices fabricated on these wafers. In this thesis a previously published model of HgCdTe growth by MBE is reviewed in detail, and is applied to the growth of double layer heterostructures in order to determine the optimum method of changing the mole fraction between layers. The model has been used to predict the change in the temperature of the phase limit when the mole fraction and growth rate change suddenly as is the case during growth of an abrupt heterostructure. Two options for growth of an abrupt heterostructure were examined (a) modulating the CdTe flux and (b) modulating the Te flux. The change in the phase limit temperature between the layers was calculated as being 4:1±C for option (a) and 5:2±C for option (b) when growing a Hg(0:7)Cd(0:3)Te/Hg(0:56)Cd(0:44)Te heterostructure

The transmission of neon line radiation through the positive column of a neon dc glow discharge has been measured. Six lengths of the column were used and a graphical comparison of the theoretical and experimental transmissions were made. This permitted a determination of the absorption coefficient of the gas, for the case of Doppler-broadened spectral lines. The relative transition probabilities for transitions with the same lower level were obtained from the values of the absorption coefficients. Radial variation of the density of absorbing atoms and the presence of isotopes in the column were taken into account. The relative intensities of several pairs of spectral lines emitted by neon gas excited by a pulsed electron beam have been measured. The neon was at a low pressure (.1 mm Hg) and excited for a short time (200 nsec) to suppress self-absorption of the emitted radiation. The relative transition probabilities for lines with the same upper level were determined from the intensity measurements. A weighted averaging technique was used to connect the relative transition probabilities of the absorption and emission measurements and a complete set of relative transition probabilities was obtained. The probabilities were placed on an absolute scale using the results of a lifetime measurement recently made by van Andel /17/. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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A method for determining percent cloud cover by taking two consecutive images using different color contrast filters, specifically a red filter and a blue filter, is described. This method (RF/BF estimate) involves dividing the "red" image by the "blue" image, pixel by pixel, and determining the percentage of pixels in the resultant image with intensity values of one. This is the percent cloud cover. The RF/BF estimate is based on the different effects of molecule and particle scattering of electromagnetic waves in the visible portion of the spectrum. These two types of scattering, Rayleigh and Mie scattering respectively, are described as they relate to this methodology. The RF/BF estimates are compared to visual estimates of percent cloud cover using the northern portion of 100 Central Florida skies collected during the period from January 28, 1988 to February 25, 1988. A strong correlation r=0.94 was found between the RF/BF and visual estimates. Advantages of the method are discussed along with implications and suggestions for follow-up studies.

The Earth Radiation Budget Experiment (ERBE) and the Clouds and Earth's Radiant Energy System (CERES) are systems that measure the earth's radiation budget, or the difference between earth-emitted radiation and radiation absorbed from the sun. Both these systems use three scanning radiometers based on a Cassegrain telescope design. Each of the three radiometers is sensitive in one of three wavelength intervals: shortwave, longwave, and total. A numerical model has been developed to investigate the radiative and optical characteristics of these instruments. The Monte-Carlo ray-trace method has been applied to the ERBE and CERES scanning radiometer instruments to calculate radiation exchange. In a radiative analysis, the surfaces of both instruments are found to emit a significant amount of radiation to the detector. In an optics analysis, both the ERBE and CERES instrument are found to have excellent directional selectivity. The field-of-view limits in the design of both instruments are verified using results from the optical analysis. / Master of Science

In 1984, the Earth Radiation Budget Experiment (ERBE) was started by the National Aeronautics and Space Administration (NASA) to provide data to the meteorological community to predict long-term weather and climate trends. Three satellites employing nonscanning active cavity and scanning thermistor bolometer radiometers are orbiting the Earth to monitor its radiative emission. A numerical model has been formulated to better understand the performance of the ERBE scanning radiometer and to aid future radiometric design and calibration procedures. The Monte Carlo method is applied to the ERBE scanning radiometer to spectrally characterize its optical and radiative performance. The optical analysis reveals that the ERBE scanning radiometer design successfully limits the amount of energy that reaches the active sensor to the designated instrument field of view. Distribution factors between the diffuse-specular surfaces of the scanning radiometer are calculated using the Monte Carlo method and are then used to perform the radiative analysis. This analysis shows that less than three percent of the radiation emitted from the passive surfaces of the radiometer reaches the active sensor, an acceptable level for radiometric instrumentation used in space. / Master of Science

In this study, two methods of analysing activity concentrations of natural radionuclides (U, Th and K) in soil are critically compared. These are the Window Analysis (WA) and Full Spectrum Analysis (FSA). In the usual WA method, the activity concentrations are determined from the net counts of the windows set around individual &gamma / -ray peaks associated with the decay of U, Th and K. In the FSA method, the full energy spectrum is considered and the measured spectrum is described as the sum of the three standard spectra (associated with U, Th and K, respectively), each multiplied by an unknown concentration. The concentrations are determined from the FSA and correspond to the activity concentrations of U, Th and K in the soil. The standard spectra derived from separate calibration measurements using the HPGe detector, represents the response of the HPGe to a Marinelli sample beaker containing an activity concentration of 1 Bq/kg.