particle physics - Is there a term for electron capture outside the nucleus?

My textbook says that electron capture is when an electron is 'captured' by a proton in the nucleus which causes them to turn into a neutron and an electron neutrino. The name kind of suggests it only works in the nucleus though.

Is there a term for electron capture happening between just a lone proton and electron?

Answer

The energetic requirements of $$e^- + p \longrightarrow n + \nu_e$$ are such that the electron needs around $1.4\,\mathrm{MeV}$ kinetic energy relative the proton for the reaction to proceed in free space (because of the difference in mass between the proton and the neutron). And even then this is a weak-mediated interaction, and so have a very small cross-section.

There are relatively few places where these conditions obtain (none that are friendly to unprotected humans). Where they do (mostly in the heart of stars) the reaction will be in equilibrium with the reverse reaction (beta decay of the neutron) with proton-neutron fusion removing nucleons from the equilibrium, and because of the small cross-section and the fleeting nature of moments when the preconditions are met the actual rate is trivial even compared to the very slow process of proton-proton fusion (if this event were common in the sun we'd see it in solar neutrino experiments, but we don't).