Neutron Activation Analysis - Overview

Overview

Neutron activation analysis is a sensitive multi-element analytical technique used for both qualitative and quantitative analysis of major, minor, trace and rare elements. NAA was discovered in 1936 by Hevesy and Levi, who found that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons. This observation led to the use of induced radioactivity for the identification of elements. NAA is significantly different from other spectroscopic analytical techniques in that it is based not on electronic transitions but on nuclear transitions. To carry out an NAA analysis the specimen is placed into a suitable irradiation facility and bombarded with neutrons. This creates artificial radioisotopes of the elements present. Following irradiation the artificial radioisotopes decay via the emission of particles or more importantly gamma rays, which are characteristic of the element from which they were emitted.

For the NAA procedure to be successful the specimen or sample must be selected carefully. In many cases small objects can be irradiated and analysed intact without the need of sampling. But more commonly a small sample is taken, usually by drilling in an inconspicuous place. About 50 mg (one-twentieth of a gram) is a sufficient sample, so damage to the object is minimised. It is often good practice to remove two samples using two different drill bits made of different materials. This will reveal any contamination of the sample from the drill bit material itself. The sample is then encapsulated in a vial made of either high purity linear polyethylene or quartz. These sample vials come in many shapes and sizes to accommodate many specimen types. The sample and a standard are then packaged and irradiated in a suitable reactor at a constant, known neutron flux. A typical reactor used for activation uses uranium fission, providing a high neutron flux and the highest available sensitivities for most elements. The neutron flux from such a reactor is in the order of 1012 neutrons cm−2 s−1. The type of neutrons generated are of relatively low kinetic energy (KE), typically less than 0.5 eV. These neutrons are termed thermal neutrons. Upon irradiation a thermal neutron interacts with the target nucleus via a non-elastic collision, causing neutron capture. This collision forms a compound nucleus which is in an excited state. The excitation energy within the compound nucleus is formed from the binding energy of the thermal neutron with the target nucleus. This excited state is unfavourable and the compound nucleus will almost instantaneously de-excite (transmutate) into a more stable configuration through the emission of a prompt particle and one or more characteristic prompt gamma photons. In most cases this more stable configuration yields a radioactive nucleus. The newly formed radioactive nucleus now decays by the emission of both particles and one or more characteristic delayed gamma photons. This decay process is at a much slower rate than the initial de-excitation and is dependent on unique half-life of the radioactive nucleus. These unique half-lives are dependent upon the particular radioactive species and can range from fractions of a second to several years. Once irradiated the sample is left for a specific decay period then placed into a detector, which will measure the nuclear decay according to either the emitted particles, or more commonly the emitted gamma rays.