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Neutron activation analysis (NAA) is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on neutron activation and therefore requires a source of neutrons. The sample is 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. A 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 artifacts. NAA can also be used to determine the activity of a radioactive sample. If NAA is conducted directly on irradiated samples it is termed Instrumental Neutron Activation Analysis (INAA). In some cases irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest, this technique is known as Radiochemical Neutron Activation Analysis (RNAA). NAA can perform non-destructive analyses on solids, liquids, suspensions, slurries, and gases with no or minimal preparation. Due to the penetrating nature of incident neutrons and resultant gamma rays, the technique provides a true bulk analysis. As different radioisotopes have different half-lives, counting can be delayed to allow interfering species to decay eliminating interference. Until the introduction of ICP-AES and PIXE, NAA was the standard analytical method for performing multi-element analyses with minimum detection limits in the sub-ppm range.〔 Accuracy of NAA is in the region of 5%, and relative precision is often better than 0.1%.〔 There are two noteworthy drawbacks to the use of NAA; even though the technique is essentially non-destructive, the irradiated sample will remain radioactive for many years after the initial analysis, requiring handling and disposal protocols for low-level to medium-level radioactive material; also, the number of suitable activation nuclear reactors is declining; with a lack of irradiation facilities, the technique has declined in popularity and become more expensive. ==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.〔http://archaeometry.missouri.edu/naa_overview.html〕 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 with 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.〔http://www.thebritishmuseum.ac.uk/science/text/techniques/sr-tech-naa-t.html〕 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.〔http://www.ne.ncsu.edu/NRP/naa.html〕 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.〔Pollard, A. M., Heron, C., 1996, ''Archaeological Chemistry''. Cambridge, Royal Society of Chemistry.〕 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 the 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「neutron activation analysis」の詳細全文を読む スポンサード リンク
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