# Why $\beta ^+$ decay occurs inside the nucleus?

The positron decay is given by

$p \to n + e^+ + \nu$

'Since the mass of the proton is less than a neutron, the process can only take place inside a nucleus.'

Can someone explain why this is so? And why exactly is a neutrino released in this case? While an antinutrino is released in beta decay

$n \to p + e^- + \bar \nu$

Note :if possible please explain the first part qualitatively(for my understanding).

• The identification of the normal versus anti-neutrino is addressed in this old answer of mine, and the rest is at least partially addressed in physics.stackexchange.com/q/60357/520 and links therein. – dmckee --- ex-moderator kitten Mar 13 '17 at 5:07
• is it correct to say that $\beta^+$ decay occurs inside the nucleus, because it can undergo 'quantum tunneling' there, by absorbing the ambient energy to cross its potential barrier? As is explained in the second answer in :physics.stackexchange.com/questions/60357/beta-decay – Lelouch Mar 13 '17 at 5:57
• thanks for the great simple answer to the second part as well. – Lelouch Mar 13 '17 at 5:59
• I suppose it is defensible at some level, but it is neither clear nor generalizable. – dmckee --- ex-moderator kitten Mar 13 '17 at 6:00
• @Schrodingers The Majorana nature of neutrino s is speculative at this point. The standard model calls for it, but the standard model also has them massless which is known to be untrue. And so far the neutrino-less double beta decay searches are null. – dmckee --- ex-moderator kitten Mar 13 '17 at 6:58