Annihilation radiation is a term used in
Gamma spectroscopy for the photon radiation produced when a
particle and its
antiparticle collide and
annihilate. Most commonly, this refers to 511-k
eV photons produced by an
electron interacting with a
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 ...
. These photons are frequently referred to as
gamma rays, despite having their origin outside the nucleus, due to
unclear distinctions between types of photon radiation. Positively charged electrons (Positrons) are emitted from the nucleus as it undergoes β+ decay. The positron travels a short distance (a few millimeters), depositing any excess energy before it combines with a free electron. The mass of the e- and e+ is completely converted into two photons with an energy of 511 keV each. These annihilation photons are emitted in opposite directions, 180˚ apart. This is the basis for
PET scanners in a process called coincidence counting.
Annihilation radiation is not monoenergetic, unlike gamma rays produced by
radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
. The production mechanism of annihilation radiation introduces
Doppler broadening.
[ Gilmore, G., and Hemmingway, J.: "Practical Gamma Ray Spectrometry", page 13. John Wiley & Sons Ltd., 1995] The annihilation peak produced in a photon spectrum by annihilation radiation therefore has a higher
full width at half maximum (FWHM) than decay-generated gamma rays in spectrum. The difference is more apparent with high resolution detectors, such as
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
detectors, than with low resolution detectors such as
Sodium iodide
Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
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
{{DEFAULTSORT:Annihilation Radiation
Antimatter
Gamma rays