CGRO Special Exhibit
The Compton Gamma Ray Observatory Mission
The Compton Gamma Ray Observatory was the second of NASA's Great Observatories following the Hubble Space Telescope. It was launched from the space shuttle Atlantis on April 5, 1991. Its large weight and low-earth orbit necessitated a number of orbit reboosts throughout its lifetime, which were all accomplished successfully. The observatory was named after Arthur Holly Compton, who won the Nobel prize in physics for work on scattering of high energy photons by electrons. Three of the four of the CGRO instruments use this principle to detect gamma-rays.
Diagram of the Compton Gamma Ray Observatory
Instrumentation
Compton had four instruments which covered the high energy electromagnetic spectrum from 30 keV to 30 GeV: BATSE, OSSE, COMPTEL, and EGRET.
The Burst And Transient Source Experiment (BATSE) detected and located transient sources such as gamma ray bursts and outbursts from other sources (including solar flares) all over the sky. Eight BATSE detectors, one facing outward from each corner of CGRO, together viewed the entire sky from 20 keV to 1000 keV. BATSE detected gamma rays using NaI crystals, which flash in visible light when struck by gamma rays.
The Oriented Scintillation Spectrometer Experiment (OSSE) measured the distribution of the energy emitted from a number of gamma-ray sources, and as such studied nuclear lines in solar flares, radioactive decay of nuclei in supernova remnants, and matter-antimatter annihilation taking place near the center of our galaxy. OSSE consisted of four NaI scintillation crystals, and was sensitive to gamma rays with energies ranging from 50 keV to 10 Mev. Each of the detectors could be pointed individually. For most instances, observations of a gamma ray source were alternated with observations of nearby blank sky so as to be able to determine the background gamma ray emission.
The Imaging Compton Telescope (COMPTEL) constructed images of gamma-ray sources, which was useful in the study of active galaxies, supernova remnants, and diffuse gamma ray emission from giant molecular clouds. COMPTEL used two layers of gamma ray detectors to study gamma rays ranging from 1 to 30 MeV. The top layer was filled with a liquid scintillator which scattered a gamma-ray photon and lowered its energy. The scattered photon was absorbed in the lower layer, and the direction and energy of the original gamma-ray photon was determined.
The Energetic Gamma Ray Experiment Telescope (EGRET) detected the highest energy gamma rays of all the instruments on CGRO – 20 MeV to 30 GeV. Its sensitivity allowed for studies of diffuse gamma ray emission, gamma ray bursts, cosmic rays, pulsars, and blazars. The EGRET instrument produced images using high voltage, gas-filled spark chambers. Gamma rays entering the chamber produced pairs of electrons and positrons. The path of the pairs and their energies were recorded, allowing scientists to determine the direction and energy of the original incoming gamma ray. From this, an image of the source could be constructed.
Published: May 2000
Text Reviewed: September 2018