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The Imaging of Extra-Galactic Low-Energy Gamma-Ray Sources: Prospects, Techniques, and Instrumentation


Finger, Mark Harold (1988) The Imaging of Extra-Galactic Low-Energy Gamma-Ray Sources: Prospects, Techniques, and Instrumentation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yagn-1j29.


The handful of extra-galactic low-energy gamma-ray sources so far observed are all active galaxies, which are expected to dominate future detections. The nature of these compact, highly luminous sources is at present not clear; however, they may be powered by massive black holes. Many of these sources may produce their peak luminosity in the 0.5 to 5.0 MeV energy band, and observation in this energy range will be important in revealing the nature of their central power-house.

Improved understanding of the nature of active galaxies will require detailed observations of 10-20 sources, while understanding of their gamma-ray luminosity function and its evolution will require the detection of ~100 sources. From x-ray number counts and the presently available information about active galaxy spectra, we estimate the hard x-ray and low-energy gamma-ray number source-flux relation N(>S) for active galaxies. Instruments capable of detecting ~100 active galaxies at low-energy gamma-ray energies are achievable. These instruments will, however, be observing sources with fluxes some 10⁻³ - 10⁻⁴ times lower than their instrumental background level, and will require careful control of systematic errors.

The angular resolution of an instrument, as well as its sensitivity, can limit the number of sources it can observe. We present an investigation of the angular resolution requirements for future low-energy gamma-ray instruments. We find that the strictest requirements arise not from the need to resolve detectable sources, but from the need to control the level of direction-to-direction fluctuations in the diffuse background level. We conclude that gamma-ray instruments capable of detecting 100 active galaxies must have sub-degree angular resolution.

We propose use of the coded aperture imaging technique as a method of achieving accurate control of systematic errors and fine angular resolution without unduly increasing the time needed to conduct full-sky surveys. This is a technique that employs a partially opaque mask to spatially modulate the source flux incident upon a position-sensitive photon detector. We present an analysis of coded aperture imaging for instruments that employ masks based on hexagonal uniformly redundant arrays. Rotation of such a mask allows complete, position-by-position background subtraction on short time-scales, and removes the periodic ambiguity inherent in uniformly redundant arrays.

An instrument, the Gamma-ray Imaging Payload, has been built that employs these imaging techniques. The primary detector of the instrument is a 41 cm diameter by 5 cm thick NaI(Tl) Anger camera. We describe the design and testing of the instrument in detail. Preliminary results from a balloon flight of the instrument are shown, demonstrating its imaging performance.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Physics
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Prince, Thomas A.
Group:Space Radiation Laboratory, Astronomy Department
Thesis Committee:
  • Prince, Thomas A. (chair)
  • Peck, Charles W.
  • Boehm, Felix H.
  • Phinney, E. Sterl
Defense Date:18 August 1987
Funding AgencyGrant Number
NASANGR 05-002-160
Record Number:CaltechTHESIS:09272011-105853397
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6690
Deposited On:28 Sep 2011 16:21
Last Modified:16 Apr 2021 23:24

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