sentences:  The source of the neutrons in these reactions is often secondary neutrons produced by alpha radiation from natural uranium and thorium in rock.Because nucleogenic isotopes have been produced later than the birth of the solar system (and the nucleosynthetic events that preceded it), nucleogenic isotopes, by definition, are not primordial nuclides.However, nucleogenic isotopes should not be confused with much more common radiogenic nuclides that are also younger than primordial nuclides, but which arise as simple daughter isotopes from radioactive decay.