An excerpt from the Chart of the Nuclides below shows stable, non radioactive isotopes of a few elements shaded with gray, and the remaining isotopes (radioactive) unshaded. Radioactive isotopes (radionuclides) can be generated by introducing a neutron (such as produced in operating a nuclear reactor) into the nucleus of the atom. Detection of this radiation allows identification, quantification, and/or distribution of elements in a sample.
Neutrons do not react strongly with most materials; but there are notable exceptions where the likelihood of interaction is very strong. One consequence of this difference is illustrated below as a thickness of material that will stop 1/2 of incident neutrons.