The main processes taking place have names like pair annihilation (particles and antiparticles destroying each other with the result being gamma-ray emission), Compton scattering (wherein a charged particle collides with a photon - changing the energy and direction of the photon), and synchrotron emission (the photons emitted when relativistic charged particles travel in a magnetic field). The means by which gamma-ray photons are created are also related to how these photons are detected. The nonthermal processes are the key to how instruments detect high-energy photons. Unlike optical and x-ray photons, gamma rays cannot be focussed by a mirror - they simply pass through thin materials. Paradoxically, one of the reasons gamma rays are valuable is that they are less affected by their environment than lower energy photons, thus providing a more pristine view of the source. Interactions within gamma-ray detectors such as pair creation, Compton scattering and nuclear excitation - those processes which give rise to the photons at the cosmic source - are used to measure the time of gamma-ray arrival and energy. Gamma-ray instrumentation relies on detection techniques developed in the world of high-energy physics. Phototubes . . . spark chambers . . . scintillators . . . collimators . . . these are some of the terms in the lexicon of gamma-ray instrument builders. Putting all these devices together into a coherent mission to investigate the unexplored gamma-ray universe was the challenge of the Compton Gamma Ray Observatory Project. 

RIGHT: Some of the fundamental physical interactions which result in gamma-ray line emission. Other processes can produce gamma rays over broad energy ranges

Some of the fundamental physical interactions which result
in gamma-ray line emission
back  home next