the third group consists of nuclides that are continuously being made in another fashion that is not simple spontaneous [[radioactive decay]] (i.e., only one atom involved with no incoming particle) but instead involves a natural [[nuclear reaction]]. these occur when atoms react with natural neutrons (from cosmic rays, [[spontaneous fission]], or other sources), or are bombarded directly with [[cosmic ray]]s. the latter, if non-primordial, are called [[cosmogenic nuclide]]s. other types of natural nuclear reactions produce nuclides that are said to be [[nucleogenic]] nuclides.