The principle of activation analysis is that elements can be made radioactive by exposure to radiations such as neutrons, protons, or high-energy photons. Two physical processes, prompt and delayed, are associated with activation. For example, in the reaction between thermal neutrons and sodium the gamma rays are emitted within 10−12sec of the absorption of the neutron. Several analytical techniques, such as X-ray fluorescence spectrometry, utilize prompt radiation, but these will not be discussed here. Activation analysis proper is concerned with the delayed event resulting from the decay of the activated nuclide. Thus in the reaction above, sodium-24 is radioactive and decays with a half-life of 15 hr, giving off characteristic beta and gamma radiation and forming stable24Mg.