THE DEVELOPMENT OF THE GODDARD COMPTON GAMMA-RAY TELESCOPE AND NEUTRAL HYDROGEN OBSERVATIONS TOWARDS THE PUPPIS WINDOW OF THE GALAXY. (VOLUMES I AND II)

The dissertation is composed of two parts, each treating a separate topic of current interest in astronomy. In Part One, the development of a new instrument designed to conduct balloon-borne observations in the medium energy gamma-ray ((gamma)-ray) region is described. Based on the Compton coincidence principle, the NASA/Goddard Advanced Compton Telescope (ACT), is intended for the study of point sources of cosmic (gamma)-rays in the energy range from approximately 0.5 to 30 MeV. The telescope design incorporates several features aimed at improving instrument performance. Chief among these is the application of the Anger camera technique to determine (gamma)-ray interaction positions within the scintillation detectors. The results of a Monte Carlo computer simulation of instrument performance are summarized. The experimental objectives and primary observational targets (Seyfert I galaxies) of the first balloon flight of the Goddard ACT are discussed. The value of the results anticipated from the experiment are summarized in terms of constraints which may be imposed on currently viable models of (gamma)-ray emission from Seyfert galaxies.; In Part Two, the results of a 21 cm neutral hydrogen survey, carried out in a region of the galactic disk known as the "Puppis Window," are reported. Observations were conducted on the NRAO 43 m radio telescope on an area of the Milky Way which contains much less absorbing interstellar dust and associated gas than is found in more typical regions of the Galaxy. The presence of numerous, sometimes expanding, HI shells and filaments, attests to the turbulent nature of the interstellar medium. Reports of optical spiral tracers at distances up to 8 kpc from the Sun make this area of the galactic disk ideal for the comparison of optical and radio data. Correlations of specific HI features with optical spiral tracers are discussed, along with the corresponding implications regarding the spiral structure in this quadrant of the Galaxy. Gas-to-dust ratios determined along lines-of-sight to five distant optical features agree with gas-to-dust ratios for globular clusters, indicating a uniform Galaxy-wide value for this quantity. Total HI integrated brightness levels are calculated and presented for the entire region surveyed. Finally, the association of the young pulsar PSRO740-28 with an HI shell in Puppis is established, and evidence pointing towards its possible origin in a "second-generation" supernova event is discussed. (Abstract shortened with permission of author.)...