Editing Neutron activation analysis (section)

==Overview==
Neutron activation analysis is a sensitive multi-[[Chemical element|element]] analytical technique used for both [[Qualitative data|qualitative]] and [[Numerical data|quantitative]] analysis of major, minor, trace and rare elements. NAA was discovered in 1936 by [[George de Hevesy|Hevesy]] and Levi, who found that samples containing certain [[rare-earth elements]] became highly [[radioactive]] after exposure to a source of neutrons.<ref name="missouri1">[https://archaeometry.missouri.edu/naa_technical.html Overview of NAA<!-- Bot generated title -->]</ref> 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 [[Radionuclide|radioisotopes]] decay with emission of particles or, more importantly [[gamma ray]]s, 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&nbsp;mg (one-twentieth of a [[gram]]) is a sufficient sample, so damage to the object is minimised.<ref>[http://www.thebritishmuseum.ac.uk/science/text/techniques/sr-tech-naa-t.html] {{webarchive |url=https://web.archive.org/web/20050406230741/http://www.thebritishmuseum.ac.uk/science/text/techniques/sr-tech-naa-t.html |date=April 6, 2005 }}</ref> 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]].<ref>{{Cite web |url=http://www.ne.ncsu.edu/NRP/naa.html |title=Neutron Activation Analysis, Nuclear Services, NRP<!-- Bot generated title --> |access-date=2006-04-13 |archive-url=https://web.archive.org/web/20130411024207/http://www.ne.ncsu.edu/nrp/naa.html |archive-date=2013-04-11 |url-status=dead }}</ref> 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]] [[Nuclear fission|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 10<sup>12</sup> neutrons cm<sup>−2</sup> s<sup>−1</sup>.<ref name="pollard">Pollard, A. M., Heron, C., 1996, ''Archaeological Chemistry''. Cambridge, Royal Society of Chemistry.</ref> The type of neutrons generated are of relatively low [[kinetic energy]] (KE), typically less than 0.5 [[Electronvolt|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.<ref name="pollard" />