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activation analysis[‚ak·tə′vā·shən ə′nal·ə·səs]
a method of determining the qualitative and quantitative composition of matter based on the activation of atomic nuclei and on measurements of their radioactive radiation. This method was first used by the Hungarian chemists G. Hevesy and H. Levi in 1936. In activation analysis, the test material is irradiated (activated) by nuclear particles (neutrons, photons, alpha particles, and others) or by hard gamma rays for a certain length of time; then special equipment is used to determine the form and activity of each of the radioactive isotopes formed.
Each radioactive isotope exhibits its own peculiar characteristics—the half-life T½ and the energy of emission Eem—which never coincide with the counterpart characteristics of other isotopes; these characteristics are tabulated. Consequently, the tabular data can be used in identification when the problem is to determine the form of radiation and to measure Eem and/or T½ of the isotopes present in the activated specimen (that is, to find the atomic number and the mass number). Nuclear reactions that result in the formation of particular radioactive isotopes in a selected method of activation are usually well known, and by using them it is easy to determine the parent isotopes from which the radioactive isotopes detected in the activated specimen were formed—that is, to determine the original composition of the test material.
Quantitative activation analysis makes use of the fact that the activity of the radioactive isotopes after radiation of the sample is proportional to the number of nuclei of the original isotope participating in the nuclear reaction. Quantitative activation analysis can be carried out by either the absolute or the relative method. In the first case, the absolute activity of the isotope is measured, and, once the factors upon which its value depends are known—the exposure time, the number of activating particles passing through the specimen per unit of time, the effective cross section of the nuclear reaction (this characterizes the probability that nuclear reaction will occur), the isotope composition of the chemical element, the T½ of the radioactive element forming, and the time elapsed between the termination of irradiation and the moment activity measurements are begun—the original content of the element being analyzed is computed. The accuracy of the absolute method is not great (20–50 percent), and the method entails certain difficulties, so that it has not gained widespread acceptance. In the second case, a specially prepared standard, or series of standards, in which the content of the element to be determined is accurately known, is irradiated along with the test specimen under strictly identical conditions. Then the activity of the specimen is compared to the activities of the standards, and the required value is found by taking into account the fact that the number of radioactive atoms formed during irradiation is proportional to the content of the test element (when a series of standards is used, the determination is usually carried out against a calibration curve of the dependence of the activity upon the content of the element being analyzed). If the problem calls for determining the contents of several elements in a specimen, the activity of each of the isotopes activated in the specimen is compared to the activities of the corresponding standards.
An instrumental or radiochemical method can be used in activation analysis to determine qualitative and quantitative composition. Instrumental activation analysis involves investigating the radiation emitted by the radioactive isotopes formed, with the aid of radiotechnical equipment (usually including scintillation sensors). This method is a nondestructive, rapid method of analysis involving little labor and low cost, but its sensitivity is often inferior to that of the radiochemical method. Radiochemical activation analysis consists of chemical separation of the activated elements and determination of the activity of each of them. This method is appropriate for the simultaneous determination of a large number of different elements, but the completion of the chemical operations is very time-consuming.
Since the nuclei of many isotopes are mostly readily activated by neutrons, the sources of which are sufficiently varied and easily available, and since neutron activation analysis offers high sensitivity, neutron activation analysis has gained much greater popularity than has activation analysis using other nuclear particles or gamma rays. The differences in the effective cross section of individual isotopes in a nuclear reaction involving neutrons may extend to a factor of hundreds of thousands; because of this, neutron activation analysis has high specificity. Neutron activation analysis is used to determine trace quantities of impurities in materials used in the fabrication of nuclear reactors and rockets (for example, 10–4 percent hafnium in zirconium), in semiconductor technology (the sensitivity of neutron activation analysis to arsenic, the presence of which in germanium transistors must be strictly controlled, is as great as 10–10-10–11 g), and so on. Neutron activation analysis is well suited to the determination of such rare elements as gold—at contents as low as 10–9-10–10 percent—and platinum —as low as 10–5-10–6 percent.
An example of this is the determination, with the aid of neutron activation analysis, of the percentage of manganese in an aluminum alloy. Naturally occurring manganese consists of only one isotope, 56Mn, while naturally occurring aluminum consists of only one isotope, 27A1. When these isotopes are irradiated by neutrons, they yield the beta-active 57Mn, with half-life T½ = 2.58 hours and 28A1 with T½ = 2.3 min. Because of the low T½ of 28A1, the isotope decays almost completely within 15–20 min after irradiation is terminated, and the activity of the alloy will be determined by the presence of 57Mn in it. If a series of standards in which the percentage content of manganese is known is irradiated simultaneously with the test specimen and under strictly identical conditions, and the activity of the specimens and of the test alloy are then measured within a certain elapsed time after irradiation, a curve of the dependence of the activity upon the percentage content of manganese in the alloys can be plotted, and the required value can be easily found from the activity of the alloy being analyzed. The sensitivity of the determination will be higher in proportion to the level of neutron flux used and the effectiveness of activity measurements in the equipment.
Activation analysis based on nuclear reactions occurring in response to gamma radiation has received widespread use. For example, by measuring the neutron flux emitted by the test specimen after it has been irradiated with gamma rays, it is possible to detect the presence of 10–4 percent beryllium in a sample of 100 g mass. Determination of light elements whose isotopes respond poorly to activation by neutrons (carbon, nitrogen, and oxygen) can be performed by measuring the radiation from isotopes formed as a result of irradiation of the nuclei 12C, 14N, and 16O respectively by hard gamma rays. Activation analysis using charged nuclear particles (protons, deuterons, alpha particles, and others) also yields satisfactory results in some cases. For example, the determination of 10–7 percent boron in silicon, 10–5 percent niobium in tantalum, and so on have been performed successfully with the aid of accelerated protons. However, because of the lack of convenient radiation sources and several other factors, this activation analysis method has not yet become as popular as neutron activation analysis.
The great advantage of any form of activation analysis is the freedom from danger of contamination of the test specimen by impurities in the chemical reagents. The possibility of analyzing specimens without destroying them also renders activation analysis useful in monitoring the purity of finished products, in forensic science, in archaeology, and in other applications. The disadvantages of activation analysis include the need for expensive equipment, the fact that not all elements activate readily, and the need to take special precautionary measures.
REFERENCESTaylor, D. Neitronnoe izluchenie i aktivatsionnyi analiz. Moscow, 1965. (Translated trom English.)
Plaksin, I. N., and L. P. Starchik. Iaderno-fizicheskie metody kontrolia veshchestvennogo sostava: ladernye reaktsii i aktivatsionnyi analiz. Moscow, 1966.
Kuznetsov, R. A. Aktivatsionnyi analiz. Moscow, 1967.
S. S. BERDONOSOV