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Annihilation radiation is a term used in Gamma spectroscopy for the gamma radiation produced when a particle and its antiparticle collide and annihilate. Most commonly, this refers to 511-keV gamma rays produced by a normal (negative) electron colliding with a positron.[1]
A Germanium detector spectrum showing the annihilation radiation peak (under the arrow). Note the width of the peak compared to the other gamma rays visible in the spectrum.

Annihilation radiation is not monoenergetic, unlike gamma rays produced by radioactive decay. The production mechanism of annihilation radiation introduces Doppler broadening.[2] The annihilation peak produced in a gamma spectrum by annihilation radiation therefore has a higher full width at half maximum (FWHM) than other gamma rays in spectrum. The difference is more apparent with high resolution detectors, such as Germanium detectors, than with low resolution detectors such as Sodium iodide detectors.

Because of their well-defined energy (511 keV) and characteristic, Doppler-broadened shape, annihilation radiation can often be useful in defining the energy calibration of a gamma ray spectrum.
References

Charlton M and Humberston JW. Positron Physics. Cambridge University Press, 2001, p. 6.
Gilmore, G., and Hemmingway, J.: "Practical Gamma Ray Spectrometry", page 13. John Wiley & Sons Ltd., 1995

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