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This thesis contains complementary aspects of the Seyfert phenomenon, each of which is analysed to bring a better understanding of present unification theories. Observations of the nuclear regions of various types of Seyfert galaxies were mostly made at infrared wavelengths which allow the study of dusty environments and provide new information on the physical conditions of these objects. For example, near infrared spectroscopy of Seyfert 2 galaxies revealed that there is a subclass of type 2 Seyferts with hot IR excess at ∼3μm with broad IR emission lines suggesting that some Seyfert 2s do in fact contain a hidden Seyfert 1 nucleus. Additional spectropolarimetry showed that the scattering screens, postulated in the standard model, are not always present in Seyfert 2s. At mid infrared wavelengths, it was found that the 10μm nuclear emission of Seyferts with broad emission lines is intrinsically brighter than that of Seyferts with no broad lines. The extended 10mum emission shows that Seyfert 2 galaxies present enhanced star-formation when compared to Seyfert 1s. Both results pose obstacles for present unification ideas and I discuss possible interpretations to these observations. Seyfert galaxies were also observed at radio wavelengths to study their large scale emission of 1-0 CO. Surprisingly, this emission usually related with star formation activity was found to be similar in both types of Seyfert galaxies and therefore does not explain why Seyfert 2 galaxies have enhanced star formation as concluded in the 10mum study. A study of galaxy morphology and companions in this set of Seyferts shows at a significant statistical level that Seyfert 2s present a higher incidence of asymmetric morphologies compared to Seyfert 1s and field galaxies, and therefore are undergoing gravitational perturbations which may induce star formation. Near infrared spectroscopy of a large sample of Seyfert galaxies is analysed to study the excitation mechanisms of [FeII] and H₂ lines in the NLR of Seyfert galaxies, a subject which is in great debate at present. Here I present some results indicating that shock excitation is likely, but not always, the dominant mechanism that excites these IR lines.

A new method of calculating accurate visible radiance values in the solar aureole region is derived in this study. The method interpolates between the radiance values output by a Gauss-Seidel flat atmosphere radiative transfer model using the radiative transfer equation itself. The method thereby produces accurate radiance values for arbitrary directions with only a slight increase in the required computational effort. The radiance and diffuse irradiance are then calculated for a variety of specified model atmospheres. The impact of polarization and the vertical stratification of aerosol particles is also investigated under various atmospheric conditions, and found to be relatively small under most realistic conditions. It is also found that some care must be taken when choosing the maximum aerosol radius at which to truncate the aerosol size distribution employed in the model atmosphere, to ensure that one does not exclude significant contributors to the scattered radiance field.

A study is performed to assess the utility of a computer numerically controlled (CNC) mill to produce missing tissue compensating filters and for the delivery of intensity-modulated beams for inverse treatment planning. A computer aided machining (CAM) software is used to assist in the design and construction of such filters. Geometric measurements of stepped and wedged surfaces are made to examine the accuracy of surface milling. Results show that the deviation of the filter surfaces from design does not exceed 1.5%. Effective attenuation coefficients are measured for CadFree and Cerrobend in a 6 MV photon beam. The ability of the CNC mill to accurately produce surfaces is further verified with dose profile measurements in a 6 MV photon beam. Dose profiles, measured beneath the test phantoms and beneath a flat phantom are compared to those produced by a commercial treatment planning system. Agreement between measured and predicted profiles is within 2%, indicating the viability of the system for filter production.

The dosimetric characteristics of a standard Varian 52-leaf multileaf collimator (MLC) and BrainLAB m3 micro-multileaf collimator (micro-MLC) have been investigated for square, rectangular, and irregular fields for 6 MV and 18 MV photon beams provided by a Varian Clinac 2300 C/D linear accelerator (linac). The percentage depth dose data and the conventional collimator factor are unaffected by the addition of MLC or micro-MLC shaped field unless, in the latter case, the tertiary field is much less than the jaw setting. However, relative dose factors for a given MLC or micro-MLC field size depend on the jaw setting. The penumbra is generally sharpest for fields defined by the micro-MLC and the least sharp for fields defined by the MLC. Average transmission values were found to be between 1.5% and 2.5%. Comparison and evaluation of two treatments, one delivered using the MLC and the other using the micro-MLC, for the same radiosurgical target volume are described.

The treatment head of the Clinac-18 medical linear accelerator was modelled using-the EGS4 Monte Carlo simulation package. Photon-energy spectra for fields ranging from 2 x 2 cm$ sp2$ to 20 x 20 cm$ sp2$ in size were generated and the primary and scatter spectra were analyzed separately. The generated x-ray spectra were used in the calculation of the percent depth dose (PDD) distributions for flattened and unflattened 10 MV x-ray beams in a water phantom at a source-surface distance of 100 cm for the various field sizes. The agreement between calculated and measured depth doses is excellent. / Measurements of the dose in the build-up region show that the depth of dose maximum (d$ sb{max}$) increases with increasing field size for fields up to 5 x 5 cm$ sp2$ for both the flattened and unflattened beams. As the field size is increased beyond 5 x 5 cm$ sp2,$ d$ sb{max}$ decreases with increasing field size for the flattened x-ray beam while remaining nearly constant for the unflattened beam. Additionally, the surface dose of the flattened beam is found to approach that of the unflattened beam for large field sizes. Calculations show that the decrease in d$ sb{max}$ as the field size is increased above 5 x 5 cm$ sp2,$ and the rapid increase in the surface dose for the flattened x-ray beam with increasing field size, are due to the degradation of the flattened-beam parameters caused by low-energy photons produced in the flattening filter.

In this thesis, the use of the Fricke-gelatin dosimeter in NMR-based radiation dosimetry is investigated. The relationship between the proton spin-lattice relaxation rate and the absorbed dose for the Fricke-gelatin dosimeter is determined using pulsed NMR at 25 MHz. This relationship is used to calculate the NMR dose sensitivity. Practical considerations of Fricke-gelatin dosimetry, such as the spontaneous conversion of ferrous to ferric ions and the effects of oxygen and sodium chloride on the dose response, are determined. Sensitization of the dosimeter by bubbling with a nitrous oxide/oxygen gas mixture is investigated. The system is modelled assuming a multi-site fast exchange between water bound to gelatin, water bound to ions, and water in the bulk. Preliminary aspects of the model are investigated and compared with experimentally determined data. Finally, the utility of the dosimeter is demonstrated by magnetic resonance imaging a Fricke-gel irradiated with an $ sp{192}$Ir radioactive seed.

Results of spectral analysis and time series analysis of the transient source x-ray nova Velorum 1993 (GRS 1009-45) and x-ray nova Ophicuhus 1993 (GRS 1716-249) are presented. These data were accumulated using the OSSE (Oriented Scintillation Spectrometer Experiment) low-energy gamma ray telescope on the Compton Gamma Ray Observatory. Spectral observations of the two x-ray novae showed the presence of gamma ray emission to greater than 100 keV, which is thought to be a signature of a binary system with an accreting black hole. Observations of GRS 1009-45 shows that it was in a gamma-ray soft state during the one-day OSSE observation, which occurred during the exponential decay phase of the outburst. The spectrum of GRS 2716-249 acquired during the plateau stage of outburst indicated a hard state, while spectra acquired during the sawtooth outbursts suggest that a hard to soft transition occurs as the decay progresses. The 40-200 keV pre-outburst luminosity of GRS 1716-249 is shown to be two orders of magnitude less than that estimated during the plateau-stage observation. The only significant power density spectrum was that acquired for GRS 1716-249 during the plateau stage of outburst. It is consistent with the sum of two QPO peaks at 0.128 and 0.32 Hz and a red noise ($\nu\sp{-1}$) component. This power spectrum is consistent with other black hole candidate power spectra for which the source is in the low state. Power spectra for GRS 1009-45 and GRS 1716-249 during the peak of a sawtooth outburst are also presented. They are consistent with band-limited white noise. Corresponding autocorrelations are shown for all observations.

Current brachytherapy dosimetry protocols assume that a physical source may be approximated by a point source. A new brachytherapy dosimetry protocol, recently proposed by the American Association of Physicists in Medicine Task Group 43, has the advantage of using functions derived solely from measurements performed in the medium and uses a more realistic source geometry than the point source approximation. The aim of this work is to obtain the dosimetric functions required by this new protocol for both a low and a high dose-rate Iridium-192 brachytherapy source through dose measurements in a water-equivalent phantom. / Dose measurements have been performed using lithium fluoride thermoluminescent detectors positioned in a polystyrene phantom at distances from the source that vary from 1 cm to 10 cm, with 1-cm intervals, and at angles that vary from 0$ sp circ$ to 170$ sp circ$ with 10$ sp circ$ intervals. / Our experimental results have clearly shown that the point-source approximation model can overestimate the dose to water, especially for the high dose-rate source, where we have found that differences between point-source estimates and exact measured values can differ by almost 30% for points along the longitudinal axis of the source.