Fast, automated segmentation of the thalamic nuclei in the brain has long been desired as it provides for direct visualization of the target for certain procedures like Deep Brain Stimulation (DBS) that target a specific nucleus. It is also beneficial in the study of other pathologies that pertain to different nuclei. In this thesis, a novel approach to fast automated segmentation of thalamic nuclei called Shortened Template and THalamus for Optimal Multi Atlas Segmentation (ST THOMAS) was developed using the multi-atlas segmentation approach. It was designed with a focus on robustness and speed by making use of an averaged template for registration and cropping the inputs and the template. The performance of ST THOMAS was first evaluated on 7T MRI data by comparing with manual delineation (ground truth) by an expert neuroradiologist. Dice coefficients and Volumetric Similarity Indices were used as metrics. To extend the applicability of this method, 3T MRI data were also evaluated. Finally, applications to real time ventralintermideiate (VIM) nucleus targeting for DBS and study of the effects of alcoholism are demonstrated.

The rate of tidal disruption events (TDEs), R-TDE, is predicted to depend on stellar conditions near the super-massive black hole (SMBH), which are on difficult-to-measure sub-parsec scales. We test whether R-TDE depends on kpc-scale global galaxy properties, which are observable. We concentrate on stellar surface mass density, Sigma M-*, and velocity dispersion, sigma(nu), which correlate with the stellar density and velocity dispersion of the stars around the SMBH. We consider 35 TDE candidates, with and without known X-ray emission. The hosts range from star-forming to quiescent to quiescent with strong Balmer absorption lines. The last (often with post-starburst spectra) are overrepresented in our sample by a factor of 35(-17)(+21) or 18(-7)(+8), depending on the strength of the H delta absorption line. For a subsample of hosts with homogeneous measurements, Sigma M-* = 10(9)-10(10) M-circle dot/kpc(2), higher on average than for a volume-weighted control sample of Sloan Digital Sky Survey galaxies with similar redshifts and stellar masses. This is because (1) most of the TDE hosts here are quiescent galaxies, which tend to have higher Sigma M-* than the star-forming galaxies that dominate the control, and (2) the star-forming hosts have higher average Sigma M-* than the star-forming control. There is also a weak suggestion that TDE hosts have lower sigma(nu) than for the quiescent control. Assuming that R-TDE infinity Sigma M-*(alpha) x sigma(beta)(nu), and applying a statistical model to the TDE hosts and control sample, we estimate (alpha) over cap = 0.9 +/- 0.2 and (beta) over cap = -1.0 +/- 0.6. This is broadly consistent with RTDE being tied to the dynamical relaxation of stars surrounding the SMBH.

We present and analyse quasi-simultaneous multi-epoch spectral energy distributions (SEDs) of seven reverberation-mapped active galactic nuclei (AGNs) for which accurate black hole mass measurements and suitable archival data are available from the 'AGNWatch' monitoring programs. We explore the potential of optical-UV and X-ray data, obtained within 2 d, to provide more accurate SED-based measurements of individual AGN and quantify the impact of source variability on key measurements typically used to characterize the black hole accretion process plus on bolometric correction factors at 5100 angstrom, 1350 angstrom and for the 2-10 keV X-ray band, respectively. The largest SED changes occur on long time-scales (greater than or similar to 1 year). For our small sample, the 1 mu m to 10 keV integrated accretion luminosity typically changes by 10 per cent on short time-scales (over 20 d), by similar to 30 per cent over a year, but can change by 100 per cent or more for individual AGN. The extreme ultraviolet (EUV) gap is the most uncertain part of the intrinsic SED, introducing a similar to 25 per cent uncertainty in the accretion-induced luminosity, relative to the model independent interpolation method that we adopt. That aside, our analysis shows that the uncertainty in the accretion-induced luminosity, the Eddington luminosity ratio and the bolometric correction factors can be reduced (by a factor of two or more) by use of the SEDs built from data obtained within 20 d. However, (M) over dot and eta are mostly limited by the unknown EUV emission and the unknown details of the central engine and our aspect angle.

Analysis and results are presented from EXOSAT observations of Seyfert type active galactic nuclei (AGN). The sample chosen for study axe 48 hard X-ray selected Seyfert type AGN including all 29 of the emission line AGN in the Piccinotti sample making this the largest such survey to date. EXOSAT, its detectors and methods of data analysis axe detailed, with emphasis on background subtraction techniques. One of the most important new results from EXOSAT data is the discovery that rapid variability (factor 2 over a few thousand seconds) is common in AGN. No evidence is found in the sample sources for any characteristic timescales of variability. Combining EXOSAT LE and ME data allowed us to obtain broad band X-ray spectra over the range 0.1-10 keV. Spectra in the ∼ 2-10 keV range are found to be well described by a simple power-law. The distribution of spectral indices across the sample can be approximated by a Gaussian distribution of mean a = 0.70 and a = 0.17. The spectra are not all consistent with this mean at the 90% confidence level and there must some intrinsic spread in AGN slopes. This spread can be constrained to 0.14. EXOSAT has also revealed a significant number of sources with complex soft X-ray spectra. These include spectra with a second spectral component at soft X-ray energies and sources with ''leaky" absorbing columns. Evidence that soft excess components occur in at least 50% of Seyfert type AGN, together with detection of rapid variability in the soft component provides the strongest support to date for an accretion disc model for AGN.

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Ph.D. (Physics) / Please refer to full text to view abstract

Philosophiae Doctor - PhD / The faint radio population consisting of star forming galaxies (SFG) and radio-quiet active galactic nuclei (AGN) is important in the study of galaxy evolution. However, the bulk of the faint population is below the detection threshold of the current radio surveys. I study this population through a Bayesian-stacking technique that I have adapted to probe the radio luminosity function (RLF) below the typical 5σ detection threshold. The technique works by fitting RLF models to radio flux densities extracted at the position of galaxies selected from an auxiliary catalogue. I test the technique by adding Gaussian noise (σ) to simulated data and the RLF models are in agreement with the simulated data for up to three orders of magnitude (3 dex) below the detection threshold (5σ). The source of radio emission from radio quiet quasars (subset of AGN) is widely debated. I apply the technique to 1.4-GHz flux densities from the Faint Images of the Radio Sky at Twenty-cm survey (FIRST) at the positions of the optical quasars from the Sloan Digital Sky Survey (SDSS). The RLF models are constrained to 2 dex below the FIRST detection threshold. I found that the radio luminosity where radio-quiet quasars emerge coincides with the luminosity where SFGs are expected to start to dominate the RLF. This Implies that the radio emission of radio-quiet quasars and radio-quiet AGN, in general, could have a significant contribution from star formation in the host galaxies.

No description available.

Philosophiae Doctor - PhD / The progressive development of scintillator detectors has made it possible to perform direct electronic lifetime determination up to a few hundred nanoseconds. The 2" by 2" LaBr3:Ce detectors provide a combination of excellent time resolution and good energy resolution. Recently a fast timing array has been commissioned at iThemba LABS, Cape Town consisting of eight 2" by 2" LaBr3:Ce detectors. Test measurements using radioactive sources produced at the laboratory were conducted. Six 2" by 2" LaBr3:Ce detectors were coupled to the AFRODITE array as their rst in-beam experiment. AFRODITE consisted of eight HPGe clover detectors as well as two 3.5" x 8" LaBr3:Ce detectors. A particle telescope was used to select the desired reaction channels. The reactions of interest 45Sc(p,d)44Sc and 45Sc(p, )42Ca were carried out at a beam energy of 27 MeV. The current analysis also investigates the rare earth nucleus 150Gd which was populated through 150Sm( ,4n)150Gd at a beam energy of 48 MeV.

<p> A study of the inter-multiplet Ml transition probability for states populated by the (n,γ) reaction in a number of odd-odd nuclei has been performed. Using a high resolution delayed coincidence system and specially developed calibration and data analysis techniques, lifetime measurements have been made for low energy transitions in 28Al, 32P, 40K, 46Sc, 52v and 56Mn. A theoretical model which uses the static magnetic dipole moments of the corresponding odd-A nuclei as the base states has been investigated and was shown able to produce a very satisfactory agreement with the experimental inter-multiplet B(Ml) values.</p> / Thesis / Doctor of Philosophy (PhD)