So like it wouldn't be a decay if it went neutron → neutron, if that makes sense. There is one “main” diagram which goes neutron → neutron and it looks like a s

So like it wouldn't be a decay if it went neutron → neutron, if that makes sense. There is one “main” diagram which goes neutron → neutron and it looks like a straight line with no vertices and it is by far the most probable thing, most neutrons just stay neutrons. So there are two reasons that a free neutron outside of a nucleus takes so long to become a proton, and you can kind of visualize it like pulling a molecule of air through an air filter or so, the first reason that this particular setup takes so long is that this particular air filter is really thick, and the second reason is that the fan you're using is not very strong. The “wall being thick” has to do with this intermediate particle, and that’s what I was alluding to above. The wall is thick because you need to create this W- boson. The problem is that this boson has about twice the mass of the neutron itself, call it Bohb because it’s a Big Ol’ Honking Boson. There's just nowhere near the energy in the system to create this thing directly. And in quantum mechanics that is okay because quantum systems can “tunnel” through states that they cannot directly actually occupy: but it generally takes longer and longer the more and more energy you need to borrow, and this is a lot of energy to borrow. The other thing is the weak blower, and that has to do with what “pressure” or “energy difference” drives the decay. In this case the driver is the mass difference: down-quarks are just intrinsically about 2 MeV heavier than up-quarks and that is enough to cover the 0.5 MeV of an electron and a neutrino., so you have something like 1.5 MeV left over to spread across the universe. By itself that number doesn't mean anything, though—what means something is the ratio of the initial to the final masses, which is something like 939.57 MeV : 938.78 MeV, so the final mass is only 0.08% lighter than the initial mass. The reaction rate goes like some high power—a fifth or sixth power—of this ratio, so when one side has like half the mass of the other side then the reaction happens very very fast because there is so much pressure driving it. But in this case the masses are so close to equal that the reaction takes something like hundreds of times longer than you might otherwise expect from just the thickness of the barrier alone.