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→The general principle
*A: Yes, a neutron, standing alone, is unstable. It has a [[half-life]] of about 15 minutes. To see why this doesn't happen for neutrons inside a nucleus, keep reading.
*A: Yes, a neutron, standing alone, is unstable. It has a [[half-life]] of about 15 minutes. To see why this doesn't happen for neutrons inside a nucleus, keep reading.
The reverse transformation, turning a proton into a neutron, positron, and neutrino, has a negative energy budget, so it does not occur.
The reverse transformation, turning a proton into a neutron, positron, and neutrino, has a negative energy budget, so it does not occur. That is, it does not occur in isolation. But when the proton is in a nucleus, the energy situation of the entire nucleus must be considered, and this transformation can occur for certain isotopes. It is "beta plus" decay. It is a very rare type of decay of natural Potassium-40, and can occur in some radioactive isotopes. The most notable of these is artificially-prepared Oxygen-15, used in "PET (positron emission tomography) scans".
==Given that something can happen, how often does it happen?==
The probability, per unit time, of a beta transformation happening, varies enormously, and determines the substance's half-life. This probability is determined by how "close" the particles get to each other. They must be extremely close—the weak force is weak, and only acts over a distance of about the size of a nucleon, or 10<sup>-15</sup> meter. This distance is measured in terms of the size of the [[quantum mechanics|quantum mechanical wave function]]. It can actually be quite large (compared with the size of a nucleon) in electron capture, because electrons typically have a rather dispersed wave function.
*Q: How about neutron decay? How can the closeness of approach be theorized for a single particle?
*A: In such cases it is necessary to consider the more esoteric theory involving quarks, that this article is ignoring for the most part. In the diagram above, two quarks collided within the neutron, initiating the decay. One can think of larger particles like neutron as having quarks rattling around inside, ready to collide.
==References==
==References==