A pair-conversion instrument detects high-energy gamma rays by providing an environment—generally a thin foil of dense metal, commonly tungsten—in which they tend to generate
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
-
positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
pairs, and then using standard particle-physics techniques such as a
microstrip detector A microstrip detector is a particle detector that consists of a large number of identical semiconductor strips laid out along one axis of a two-dimensional structure, generally by lithography. The geometrical layout of the components allows to accu ...
to detect these particles.
Usually there are several stacks of conversion foils and particle detectors placed on top of one another, so that the electron-positron pairs can be tracked as they pass through consecutive detectors; and at the bottom of the stack one commonly finds a
calorimeter capable of measuring the energy of the electron and the positron and inferring the energy of the incoming gamma ray.
An
anticoincidence shield of some kind is needed so that charged particles entering the device from outside are not confused with electron-positron pairs generated in the conversion foils.
One of the larger operational pair-conversion telescopes is the Large Area Telescope on
GLAST. Space-based pair-conversion detectors tend to make for rather expensive missions, since they unavoidably contain several hundred kilograms of lead or tungsten.
has a slide illustrating the design of LAT.
Photonics