OR You are a curator working with the ancient coin collection in a large museum. A donor has just given the museum a group of 50 gold coins presumably about 1500 years old. Are they genuine? OR You are a scientist working in the criminology laboratory of a large metropolitan city. A detective brings you a minute sample of paint taken from the clothing of a hit-and-run victim. He has a suspect whose automobile paint seems to match that sample. Can you determine his guilt or innocence? Neutron activation analysis can be used to solve each of these problems and many more. The solutions to these particular problems are explained on pages 19-46. The Atomic Fingerprint: Neutron Activation Analysis by Bernard Keisch WHAT IS NEUTRON ACTIVATION ANALYSIS? To understand neutron activation analysis, you should be acquainted with a few basic concepts. The nuclei of atoms are stable only when they contain certain numbers of neutrons and protons. The number of protons in an atom’s nucleus determines an element’s identity; the number of neutrons usually determines whether or not that atom is radioactive or nonradioactive (stable).[1] Thus, while all sodium atoms contain 11 protons, only those sodium atoms that contain 12 neutrons are stable. A radioactive sodium atom contains a different number of neutrons. For other elements, there may be more than one number of neutrons that results in stability; for instance, there are 10 stable atoms (isotopes) of tin, each containing a different number of neutrons in their nuclei. The fact that nuclei can absorb additional neutrons, which, in many cases, results in the conversion of a stable nucleus to a radioactive one, makes neutron activation analysis possible. Because radioactive nuclei decay in unique ways and yield radiations that are often distinct and can be measured even in very small amounts, measurements of these radiations can determine the kind and the number of radioactive atoms that are present. In the most common type of activation analysis, the neutron bombardment of a sample is performed in a nuclear reactor where the neutrons that strike the target atoms have been slowed down so that they have very little energy of motion. In this case, the usual reaction between the target atoms and a neutron results in the capture of the neutron and this creates a nucleus with an atomic weight of one more unit than it started with. Thus for sodium as found in nature (symbol ²³Na) sodium-23 + a neutron → radioactive sodium-24 + gamma rays[2] The numbers denote the atomic weight of the atom, which is the total number of protons and neutrons in its nucleus.