Hi JohnFolton,
A photon is a fundamental particle, and photons do not decay. They can be absorbed and re-emitted, but they do not decay.
The decay referred to in the science of radiometric dating is the decay of atoms. An atom has a nucleus of protons and neutrons and is surrounded by a cloud of electrons. An atom is said to decay when the nucleus of an atom
*spontaneously* emits some of its particles, and the result is a lighter atom.
Decay is spontaneous. Decay happens without any discernable cause. Decay is not the result of collision of sub-atomic particles with an atom's nucleus.
The proper words for collision events are fission and fusion, depending upon whether the end result is a heavier (fusion) or lighter (fission) atom.
--Percy