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Basics - Poster 3
Another radioactive emission : Positron Annihilation Radiation and Pair Production
Positron decay
The positron ( or antielectron ) is the antiparticle ( or the
A Z
A
X Y +
0e
Z-1
+ 1 antimatter counterpart ) of the electron , having a positive charge . In this process , a proton becomes a neutron and a positron is emitted . However , as will be seen below , positrons can arise from another process .
0 + 1
e
The theoretical prediction of the existence of positrons started
with Dirac in 1928 . Dmitri Skobeltsyn first observed the positron in 1929 , while using a Wilson cloud chamber , but it took another couple of years before its significance was fully understood .
Carl D . Anderson discovered the positron in 1932 , for which he won the Nobel Prize for Physics in 1936 . Anderson also coined the term positron . The positron was the first evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate . A magnet surrounded this apparatus , causing particles to bend in different directions based on their electric charge . The ion trail left by each positron appeared on the photographic plate ( right ) with a curvature matching the mass-to-charge ratio of an electron , but in a direction that showed its charge was positive .
A positron travels only a short distance ( typically less than 1 mm in tissue ) during which it loses kinetic energy , until it decelerates to a point where it can interact with an electron . The encounter
Electron-Positron Annihilation annihilates both electron and positron , producing a pair of annihilation ( gamma ) photons , each with 511 keV of energy
511keV photon moving in opposite directions .
0 -1 e
This phenomenon is the basis for some of the modern Nuclear Medicine techniques . Positron emitters used in nuclear medicine are produced in a cyclotron and have a short half-life , such as
0 + 1 e carbon-11 (~ 20 min ), nitrogen-13 (~ 10 min ), oxygen-15 (~ 2 min ), and fluorine-18 (~ 110 min ).
511keV photon
Pair Production
0 -1 e
Pair production is the opposite of electron-positron annihilation . This requires a photon with energy in excess of 1.022 MeV .
This process is what Anderson was observing in his cloud chamber when he discovered the positron .
At the moment pair production is really only used in research as an indicator of other events . photon E > 1.022MeV
0 + 1 e
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