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The Question

(Submitted July 19, 1997)

I've been reading up on the recent positron findings at our galaxy center and wondered if we know that electron-positron annihilation always yields energy in the gamma-ray band?

My background in this area is limited to college physics, and book (net) research in plasma and astrophysics.

The Answer

Thank you for your question. The answer is yes, we know the physics of electron-positron interactions quite well, because it has been measured in particle physics labs. As it turns out the mass of an electron (9.1E-28 grams) times the speed of light squared (E = m c2) is 8.12E-07 ergs of energy. In more common units this is 511 keV (kilo electron volts).

When an electron and positron annihilate they produce 2 photons, each with 511 keV of energy (so no net energy is gained or lost). When we observe a spectral emission line at 511 keV, we can be pretty sure it is caused by this positron/electron interactions.

Jonathan Keohane
for Ask an Astrophysicist

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