Gamma-ray spectroscopy: Galactic center observations and germanium detector development

We present observations d the Galactic Center region by the HIgh REsolution Gamma-ray and hard x-ray Spectrometer (HIREGS) during its third long duration ballon flight from Antarctica in January 1995. HIREGS is a high-resolution spectrometer (2.0 keV FWHM at 592 keV) covering three orders of magnitude in energy, from 20 keV to 17 MeV. The instrument is an array of 12 large germanium detectors, enclosed on the bottom and sides by a thick anticoincidence shield, and actively collimated to a large 24{dollar}spcirc{dollar} field-of-view. Narrow passive hard x-ray (3.7{dollar}spcirc{dollar} x 24{dollar}spcirc, {lcub}<{rcub}200{dollar} keV) collimators were added over half of the detectors for this flight, designed to allow HIREGS to observe with wide and narrow fields-of-view simultaneously. These combined observations allow us to directly measure and subtract the compact source spectra from the Galactic diffuse measurements. The resulting Galactic diffuse continuum is found to agree well with theoretical predictions, but is harder in the hard x-ray range than previous observations. The spatial distribution is found to be nearly flat, with upper limits set on the longitudinal gradient, and with no evidence of an edge in the observed region.; This flight also produced a serendipitous observation of a terrestrial {dollar}gamma{dollar}-ray burst, with emission extending up to the unprecedented energy of 10 MeV. This emission is most likely due to the precipitation of high energy electrons ({dollar}sim{dollar}10 MeV) produced through an impulsive mechanism in the Earth's magnetic tail. Precipitating electrons, however, generally originate in the trapped radiation belts, and at significantly lower energies, below {dollar}sim{dollar}1 MeV. Such a high energy source originating in the tail is difficult to comprehend in terms of our understanding of modem magnetospheric physics.; By analysis and modeling of current pulses produced in germanium detectors by the interaction of incident {dollar}gamma{dollar}-rays, we have developed methods of Pulse Shape Discrimination (PSD) to improve the (overall performance of germanium detectors, including improvements in sensitivity, spectral resolution, and spatial (imaging) resolution. PSD techniques we are developing for INTEGRAL, a large germanium spectrometer under construction by the European Space Agency for launch in 2001, are expected to significantly reduce the background, improving the sensitivity of the instrument. These techniques can also be used in INTEGRAL to correct for the effects of radiation damage in the germanium detectors due to extended exposure to cosmic rays and their secondaries, maintaining the high spectral resolution of the instrument even after it experiences severe radiation damage. Initial simulations using PSD techniques to improve spatial resolution, hence imaging capabilities, in germanium detectors are also presented, indicating a promising direction to follow for future {dollar}gamma{dollar}-ray imaging spectrometers.