CELESTIAL GAMMA RAY BURSTS DETECTOR DEVELOPMENT AND MODEL SIMULATIONS

Celestial gamma-ray bursts are a poorly understood astrophysical phenomenon. These transient events were discovered over twenty years ago, yet their origin is still an unsolved mystery. At present no quiescent counterpart to a gamma ray burst source has been conclusively identified, partly because the poor angular resolution of gamma ray detectors and the short durations of the bursts make it difficult to determine precise source positions. (A few precise source positions have been determined by analysis of burst arrival times at widely separated detectors.) The High Energy Transient Experiment (HETE), described by Ricker, et al. (1992), is a new gamma ray astronomy satellite designed to overcome these difficulties. It can determine precise source positions by simultaneously observing a gamma ray burst with gamma ray x-ray, and ultraviolet (UV) instruments and utilizing the better angular resolutions available with the x-ray and UV instruments. In the first part of this dissertation I present experimental research which contributes to the development of a UV-sensitive solid-state imaging detector for the HETE satellite. The detector is a thinned, backside-illuminated charge-coupled device (CCD). The UV quantum efficiency (QE) is very sensitive to the results of the back-surface treatment, which stabilizes and protects that surface. As part of the detector development I designed and built an instrument to measure the quantum efficiency of a CCD over the wavelength range of 200--500~nm. With this instrument I measured the QE of seven prototype devices that were manufactured with three different back-surface technologies. I derived a statistical test to measure the mean number of electrons per photon, which increases from unity with increasing photon energy above a threshold of ~3.65~eV (340 nm). This effect is critically important when making photometric measurements at these wavelengths with solid state detectors. I also developed a simple physically-motivated model of the back surface, which provides adequate fits to the measured QE. I find that the best back-surface technology yields CCDs that have stable QEs of >40% in the HETE UV band of 220-310 nm. This is significantly better than the QE of 20% required by the HETE UV instrument (Ricker, et al. 1992). This encouraging result enhances the ability of the HETE UV instrument to detect a gamma-ray burst, which will ultimately lead to the discovery of the underlying physical sources. While the origin of gamma-ray bursts is unknown, the rapid variability and hard spectra indicate that the sources are compact objects. Many different models of gamma-ray bursts assume that the bursts originate from neutrons stars. Blaes, et al. (1990) put forth the idea that the natural evolution of a slowly-accreting, isolated neutron star leads to the formation of density inversions which might become unstable and thereby lead to a gamma-ray burst. However, the recent measurements of the gamma-ray burst distribution reported by Meegan, et al. (1992) rule out many galactic models. Recent theoretical work is split between galactic halo models and cosmological models, many of which still associate gamma-ray bursts with neutron stars. In any event, slowly-accreting neutron stars should exist in the galaxy. Their evolution is the focus of the second part of this dissertation. I present computational research on the evolution of this class of slowly accreting neutron stars. I describe an evolution code, which simulates the crust of a slowly accreting neutron star, and report on the evolution of the structure, composition, density inversions, and stored energy of fifteen different models. This evolution code is a version of ASTRA, an evolution code originally developed by Rakavy, et al. (1967). It is based on the version developed by Joss (1978) to simulate thermonuclear flashes in the crust of an accreting neutron star. The major changes are a new set of thermodynamic equations, a new nuclear reaction network, and a new thermal conductivity algorithm. The thermodynamic equations are based on a temperature-dependent equation of state for degenerate matter. The nuclear reaction network is based on the relevant pycnonuclear and electron-capture reactions, and it includes the first estimates for the reaction rates of some extremely neutron-rich nuclei. The simulated models can be divided into two classes. The first set of models assumes a constant accretion rate of 1010 for eight different neutron stars with masses ranging from 0.1 to 2.8 solar masses and radii ranging from 7 to 150 km. The second set of models assumes eight different constant accretion rates ranging from 1010 to 1016 for a neutron star with a mass of 1.4 and a radius of 10 km. I find that the crust of a slowly-accreting neutron star stores up to ~1045 ergs in the non-equilibrium nuclear composition. This is a sufficient energy source for gamma-ray bursts in the galactic halo if one percent of this energy released as gamma rays. One possible trigger mechanism for releasing this energy is the density inversions which occur naturally during the evolution of the crust. These density inversions store ~1041 ergs of gravitational energy. If one of them becomes unstable and overturns, the liberated energy could rapidly heat the crust and thereby release the stored nuclear energy. In my simulations the observed density inversions never exceed the threshold for instability described by Blaes, et al. (1992). However, the full range of density inversion has yet to be explored.