Neutron Activation Analysis

Neutron activation analysis is a technique for determining the identities and quantities of the elements present in a sample of material. The technique is based on a well-known principle of nuclear chemistry: when an element is bombarded with neutrons, some of the atoms may capture neutrons and incorporate them into their nuclei. Those atoms that do so have the same atomic number (that is, have the same number of protons in their nuclei and are therefore the same element) as before, but have an atomic mass one higher. Bombarding atoms of sodium-23 with neutrons, for example, converts them to atoms of sodium-24.

In most cases, the heavier atoms formed in this reaction are radioactive. These atoms usually decay with the emission of a beta particle and a gamma ray. The energy associated with the decay process is characteristic of the radioactive isotope in question. For example, a gamma ray released in the decay of sodium-24 has an energy of 2.75 or 1.37 MeV. By observing the products of radioactive breakdown in the sample, one can determine which radioactive isotopes were created by neutron bombardment and hence which elements were present before bombardment.

In practice, nuclear activation analysis is carried out by placing the sample to be examined in a nuclear reactor. The neutrons available in the reactor bring about the n/c (neutron/gamma ray) reactions described above. The radioactive sample is then removed from the reactor and examined with a gamma ray spectrometer. This device measures the type and intensity of radiation released by the sample. These data can then be compared to standard tables to determine which elements and the amounts of each are present in the sample.

Neutron activation analysis is valuable as a nondestructive form of analysis. It can be used without fear of damaging or destroying the material being tested. It is also a very precise form of analysis, permitting the detection of very small quantities of an element. One application that illustrates these strengths is in the analysis of archaeological materials that are too fragile or too valuable to expose to other analytical techniques.