In chemistry, neutron activation analysis is a technique used to very accurately determine the concentrations of elements in a sample. The particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artefacts.
The sample is introduced into the intense radiation field of a nuclear reactor. The sample is thus bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it.
Some elements form isotopes that have very short half-lives, and so cannot be measured using this method. Others have exceptionally long half-lives, and so can be measured, but have to be measured over long periods.