Look up the "kinetic isotope effect.
Atomic Mass Protons and neutrons have approximately the same mass, about 1.
As such, use is made of the different diffusion rates of the two compounds.
When an organism dies, it is no longer ingesting 14C, so the ratio between 14C and 12C will decline as 14C gradually decays back to 14N.
The new atoms created may be in a high energy state and emit gamma rays which lowers the energy but alone does not change the atom into another isotope.
Trying to be calling you have isotopes, copy the lookout for or another example of neutrons.