# When does electron capture occur and when does positron emission occur?

I’ve been told that electron capture occurs when there isn’t enough energy to produce a positron by beta plus decay. Exactly why is this the case? Why does it take more energy for positron emission than electron capture?

• I've added links that (I think) explain more clearly what you're asking. Please roll back me edit if I have got this wrong. Jul 19, 2015 at 9:57

Electron capture: $p+e^-\rightarrow n+\nu_e$
Beta plus decay: $p\rightarrow n+e^++\bar{\nu_e}$

Let's check the masses of both sides of the processes:
Electron capture, initial state: $m_p+m_e=938.78 \frac{MeV}{c^2}$
Final state: $m_n=939.56 \frac{MeV}{c^2}$
The difference: (Final minus initial) $0.78 \frac{MeV}{c^2}$

Beta plus decay: (Positron emission)
Intial state: $m_p=938.27 \frac{MeV}{c^2}$
Final state: $m_n+m_e=940.07 \frac{MeV}{c^2}$
The difference: (Final minus initial) $1.8 \frac{MeV}{c^2}$

You see that you need to add more energy in order to do beta plus decay than to capture an electron.

Also note: A free proton can't emit a positron and turn into a neutron, but a free proton can capture an electron.

A free proton can't emit a positron and turn into a neutron, but a free proton can capture an electron.

this is not true, EC only happens in many p's and many n's atoms.

but free p can only have positron emmision for a weak interaction, or the sun will burn too fast if it is done by EC {electron capture}

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• What does a free proton mean? Feb 26, 2017 at 11:39