Neutron Sources
Neutrons do not occur naturally as free particles unless they are produced continuously because
a free neutron is unstable to radioactive decay with a
mean life of about 15 minutes.
Thus,
for neutron capture reactions to play a role in stars there must be a source that makes neutrons.
For the slow capture process there are several nuclear reactions that can
occur in red giant stars that produce neutrons.
For the rapid capture process discussed next, the neutron
density that is required is far too high to be produced in nuclear reactions in normal stars. The only
possible sources are catastrophic, like supernova explosions.