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I am confused by a simple fact about the $\beta^{-}$ decay of ${}^{60}{\rm Co}$ nucleus. According to Wikipedia, the most likely decay branch is to an excited state of ${}^{60}{\rm Ni}$, see the diagram:

Decay scheme of Co-60

But the energy indicated on the diagram (0.31 MeV) is less than the electron rest mass (0.511 MeV)! How is that possible?

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up vote 4 down vote accepted

When they show a decay with a certian amount of energy, this energy is net of the masses of the particles. So you get Co = $\beta$ + Ni + 0.31 MeV, the energy is attached to the ejected beta particle.

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Fair enough, thanks. So 0.31 MeV is just the kinectic energy of the electron and the anti-neutrino. –  Slaviks Mar 28 at 11:45

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