Re: 511 kev line - why not 1022 kev?
From: Chris Dams (chrisd_at_gamow.sci.kun.nl)
Date: 08/27/04
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Date: Fri, 27 Aug 2004 17:05:05 +0000 (UTC)
Dear Christian,
"Christian" <putin_news@yahoo.de> writes:
>When an electron and a positron annihilate, one speaks of the 511 kev peak.
>But a mass equal to 1022 kev gets destroyed, so i am wondering why we see a
>peak at 511 kev?
This is because electron-positron annihilation should yield at least
two photons. (As a side-note: more than two photons should be possible
but should only happen in a tiny minority of annihilation events.) So,
each photon gets half the energy. That it is impossible to produce just
one photon is because one can always get to the rest frame of the
electron-positron system. In that frame the center of mass is at rest.
The resulting single photon, should therefore also be at rest. However,
photons move with the speed of light. Hence, it is not possible to
produce a single photon.
Best,
Chris
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