Neutron activation analysis (NAA): A sensitive analytical
technique useful for performing both
qualitative and
quantitative multi-
element analysis of major, minor, and
trace elements in samples from almost every conceivable field of scientific or technical interest
1. It is
performed by recording and studying the
gamma rays emmitted from a test sample which has been
bombarded with
neutrons.
For many elements and applications, NAA offers sensitivities that are superior to those attainable by other methods, on the order of parts per billion or better. In addition, because of its accuracy and reliability, NAA is generally recognized as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. Worldwide application of NAA is so widespread it is estimated that approximately 100,000 samples undergo analysis each year.
Neutron activation analysis was discovered in 1936 when Hevesy and Levi found that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons. From this observation, they quickly recognized the potential of employing nuclear reactions on samples followed by measurement of the induced radioactivity to facilitate both qualitative and quantitative identification of the elements present in the samples.
The sequence of events occurring during the most common type of nuclear reaction used for NAA, namely the neutron capture or (n,gamma) reaction, is as follows:
When a neutron interacts with a target nucleus via a non-elastic collision, a compound nucleus forms in an excited state. The excitation energy of the compound nucleus is due to the binding energy of the neutron with the nucleus. The compound nucleus will almost instantaneously de-excite into a more stable configuration through emission of one or more characteristic prompt gamma rays. In many cases, this new configuration yields a radioactive nucleus which also de-excites (or decays) by emission of one or more characteristic delayed gamma rays, but at a much slower rate according to the unique half-life of the radioactive nucleus. Depending upon the particular radioactive species, half-lives can range from fractions of a second to several years.
In principle, therefore, with respect to the time of measurement, NAA falls into two categories: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common; thus, when one mentions NAA it is generally assumed that measurement of the delayed gamma rays is intended. About 70% of the elements have properties suitable for measurement by NAA.
Information taken and adapted from Prof. Michael D. Glascock's web page:
www.missouri.edu/~glascock/naa_over.htm
His work is known and
loved by
archaeology students world-wide.
1 Fields and samples include:
Archaeology: pottery,
flint, bone and especially
basalt.
Anti-Cancer: blood samples,
tissue samples (they study the fate of things like
selenium and
iodine administered in dietary supplements and
medicines).
Geology: all kinds of
rocks.
Gene Thereapy: mousebone and mousetooth samples (they study how the bodies of
CF mice and
normal mice process minerals differently.
Semiconductors:any material...(they look for trace elements to verify purity of semiconductor)
Study of spread of uranium,thorium and hazardous wastes: earth and
ore samples.