# Neutron Decay and the Weak Force

HeavyWater
I believe the neutron decays with a half life around 12 minutes via the weak mechanism. I'm wondering what causes a particular neutron to decay. At the moment the neutron decays, has it encountered (run into) a stray weak field? Or ... does the neutron decay just because of internal forces? If we could build something like a Faraday cage that would keep out the weak force, then would a neutron inside that Faraday cage decay?
Thanks,

Staff Emeritus
Gold Member
A neutron totally isolated in empty space, with no externally imposed fields, will still decay. The decay is not due to a stray field from outside. Yes, it decays because of internal forces.

HeavyWater
Thank you for your quick response bcrowell. I've done some more homework and am thinking about your response. I'm going to enter a follow up question--but I need to think about it some more.
Thanks

Staff Emeritus
A short answer is that a neutron decays because by decaying into a proton, it enters the lowest energy state possible for it, which is a proton. Neutrons in a nucleus of an atom don't decay (Except in unstable elements) because changing into a proton would cause the nucleus to enter a higher energy state due to the repulsiveness of protons to each other.

HeavyWater
Thank you Drakkith. You answered it so clearly, it sounds obvious now that you said it. I do have a little follow up to the question. I am aware that when the neutron decays in free space, it is via the Weak interaction. Does the neutron have to enter an external weak field for it to decay? That is, is an external weak field supplying the weak mechanism?

I saw a statement on one of these forums that any decay will occur if it is allowed by the laws of physics (by which I think they meant by the laws of the Standard model). If this statement is true (and I'm sure it is), then the neutron can decay in the absense of an external weak field.

Thanks to all of you for your patience,

Staff Emeritus
I don't know what an external weak field is, but I believe it will happen regardless.

HeavyWater
Thank you again Drakkith,
I was thinking of an "external weak field" as an analogy to an external electromagnetic field. The only other example I can think of is for the neutral pion. It decays by the electromagnetic mechanism. So, does the neutral pion have to reside in an external electromagnetic field to decay?
Thanks to you an everyone else for your patience

Staff Emeritus