Changes

*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.  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".

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?==

==Given that something can happen, how often does it happen?==