Principles of Neutron Activation Analysis

Neutron activation analysis (NAA) came into being in 1936 when George von Hevesy and Hilde Levi published a new principle [3] for making analysis A sample was exposed to a stream of neutrons from a radium-beryllium source, and some of the atoms in the sample captured a neutron in their nucleus and became radioactive the composition of the sample could now be inferred from the measurement of the amounts and properties of such radioactive indicators. 

Of all analytical methods NAA is unique in that it is based on the excitation of the atomic nucleus rather than the surrounding electrons. This means that an element—or in fact a particular isotope of an element—is determined independently of its physical or chemical state. Each element is characterized by its isotopic composition and its corresponding ability to form neutron-induced radioactive indicators with individual half-lives and decay characteristics. 

Various sources of neutrons have been used for the activation of biological samples for analysis. 

Isotopic sources rely on the decay of one or more radionuclides generating neutrons directly or indirectly. Each type of isotopic source has its characteristic neutron energy, its output is determined by its activity, and such sources are in continuous operation. 

Examples of such sources used for analytical purposes are shown in Table 1. 

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Introduction to nuclear structure and the principles of neutron activation analysis... 

This method involves bombarding the sample with neutrons and measuring the radioactivity induced in the sample (commonly using gamma-ray spectrometry). In order to understand the principles of neutron activation analysis, some pertinent properties of neutrons and their interactions with matter will first be discussed. 

FIGURE 1.20 (a) A schematic presentation of the principle of neutron activation analysis (NAA). (Adapted from http //archaeometryjnissouri.edu/images/naa over figl.gif, accessed July 26, 2014.) (b) Schematic of a generic gamma spectrometer. 

Neutron activation analysis offers a highly sensitive technique for the quantitative determination of the rare earths. In some applications, it may also be one of the quickest and most convenient techniques. De Soete et al. (1972) have provided a detailed discussion of all aspects of neutron activation. A brief discussion will be provided here to enable the reader to assess the strengths and weaknesses of neutron activation analysis of rare earths. The principles of neutron activation analysis are quite simple. A sample is irradiated in a flux of neutrons, generally in a nuclear reactor, in which isotopes of the various elements absorb neutrons. Many of these reactions form radioactive products, the activity of which is measured and related to the amount of element present. 

The history of neutron activation analysis goes back to the middle of the 1930s when it was first described by G. Hevesy and H. Levi at the Niels Bohr Institute in Copenhagen. The principle of the technique is that elements can be made radioactive by exposure to neutron irradiation. Two types of physiological processes are associated with this activation one prompt and one delayed. Classically, neutron activation analysis is based on the detection of the delayed event, viz. the characteristic radiation emitted during the decay -with a specific half-life (ti/a) - of the unstable nuclei formed by (n,y) reaction. 

Kruger, P. Principles of Activation Analysis. New York Wiley-Interscience, 1971. A senior-level textbook covering various aspects of neutron activation analysis in fair depth. 

While studying the action of slow neutrons on rare earth elements, Hevesy and Levi (1936) found for the first time that the rare earth element Dy became highly radioactive after exposure to a source of neutrons - the principle on which the method of Neutron Activation Analysis is based. 

The uranium content in soil can be determined directly by some analytical methods that are mainly based on nuclear techniques (variations of neutron activation analysis, gamma spectrometry, x-ray fluorescence, or laser-ablation ICPMS), but the common, popular, and more accurate methods require digestion and dissolution of the entire soil sample or at least rely on leaching the uranium out of the sample matrix. In principle, the methods used for assaying uranium in minerals (see Chapter 2) are also suitable for soil characterization, but uranium is usually present in the latter only as a low-level impurity, usually below 100 pg U g. We shall first overview the procedures deployed for the treatment of soil samples prior to analysis and refer to the analytical devices used for the measuranent of the uranium content and isotopic composition in these studies. 

Uranium in nature may be measured either radiometrically or chemically because the main isotope - 238U - has a very long half life (i.e., relatively few of its radioactive atoms decay in a year). Its isotopes in water and urine samples usually are at low concentrations, for which popular analytical methods are (1) radiochemical purification plus alpha-particle spectral analysis, (2) neutron activation analysis, (3) fluorimetry, and (4) mass spectrometry. The radiochemical analysis method is similar in principle to that of the measurement of plutonium isotopes in water samples (Experiments 15 and 16). Mass spectrometric measurement involves ionization of the individual atoms of the uranium analyte, separation of the ions by isotopic mass, and measurement of the number of separated isotopic ions (see Chapter 17 of Radioanalytical Chemistry text). 

In many laboratories that have access to a nuclear reactor, neutron activation is used for the chemical analysis of rocks, minerals, petroleum, biological tissues, alloys, etc., and the technique is well suited for the determination of the concentrations of trace elements in polymers. Neutron activation analysis was used by Given et al. (1) in their studies of water tree growth in polymeric insulation and by Wu and Chen (2) in their studies of dopant-polymer interactions in MoCl5-dcped polyacetylene films. In this work the principles of the method are described and the possibilities are illustrated by means of measurements carried out on polyethylene. 

Antimony can be readily determined in petroleum at a concentration of less than 10 ng/g by instrumental neutron activation analysis with a 1-hr irradiation at a neutron flux of 1012 n cm"2 sec"1. The principle radiation arising from the 121Sb (n, y) 122Sb activation and subsequent decay of 122Sb occurs at 564 KeV. With a high resolution Ge(Li) detector antimony may be measured in the presence of 82Br (550 KeV) and 76As (559 KeV), which may be found in some petroleum matrices. 

Arsenic levels below 10 ng/g can be readily detected in petroleum by instrumental neutron activation analysis. The most convenient technique involves direct gamma counting based on the 75As (n, y) 76As reaction with a principle radiation of 559 keV. After a 1-hr irradiation at a neutron flux of 1012 n cm"2 sec-1, the arsenic may be counted in a relatively short time. The method requires a high resolution Ge(Li) detector to avoid interference from bromine (550 keV) or antimony (564 keV). 

Earlier methods used to determine mercury in biological tissue and fluids were mainly colorimetric, using dithizone as the com-plexing agent. However, during the past two to three decades, AAS methods - predominantly the cold vapor principle with atomic absorption or atomic fluorescence detection - have become widely used due to their simplicity, sensitivity, and relatively low price. Neutron activation analysis (NAA), either in the instrumental or radiochemical mode, is still frequently used where nuclear reactors are available. Inductively coupled plasma mass spectrometry (ICP-MS) has become a valuable tool in mercury speciation. Gas and liquid chromatography, coupled with various detectors have also gained much importance for separa-tion/detection of mercury compounds (Table 17.1). 

X-ray fluorescence spectrometry, gas chromatography and neutron activation analysis (NAA). An older book edited by Hofstader, Milner and Runnels on Analysis of Petroleum for Trace Metals (1976), includes one chapter each on principles of trace analysis and techniques of trace analysis and others devoted to specific elements in petroleum products. Markert (1996) presents a fresh approach to sampling, sample preparation, instrumental analysis, data handling and interpretation. The Handbook on Metals in Clinical and Analytical Chemistry, edited by Seiler,... 

The fundamental principle behind analysis by activation analysis is activation or excitation of an atomic nucleus by exposure to radiation such as neutrons, protons or high-energy photons with subsequent measurement of emitted sub-atomic particles or radiation. The most common aspect of the technique involves activation with neutrons in a nuclear reactor and measurement of delayed emitted gamma rays, denoted neutron activation analysis, either instrumental neutron activation analysis (INAA) or neutron activation followed by radiochemical separation (RNAA) in which the element of interest is chemically separated from the matrix after irradiation to provide for better, unimpeded counting. 

Neutron activation analysis is an invaluable technique for trace element determinations in biological matrices. Probabiy its most important advantage is its relative freedom from errors due to extraneous additions of exogenous materiai from reagents, equipment, or laboratory environment. Characteristics which contribute further to the popularity of the technique are its outstanding sensitivity, excellent specificity, and multielement capability. In principle, the technique is able to produce relatively unbiased and precise measurements — at least in competent hands. That it is, however, necessary to warn against uncritical expectations is illustrated by the grossly inconsistent results obtained in several laboratories. 

Since neutron activation analysis is the most widely used of the activation techniques, the technical portion of this discussion will be concerned with its principles and technology. 

Abstract This chapter presents the basic principles of activation analysis and details its different types. Emphasis is given to instrumental neutron activation analysis and radiochemical separations for the determination of trace and ultra-trace elements. Location sensitive analysis is also included. 

The principle of reactor operation and the different reactor types will be discussed in the following. Neutrons are the most important agent in driving a reactor. Also neutrons for the scientist are the most important product of a research reactor, because they can be used for various analytical purposes (neutron activation analysis, structural analysis through neutron diffraction, etc.). Therefore, at first, some neutron physics will be presented as of relevance for the existing two main types of reactors ("thermal and fast ). 

A variety of techniques based on different physical principles have been used for trace element measurements. The most commonly used include neutron activation analysis (NAA) [1], atomic absorption spectrometry [2,3], and mass spectrometry [4-7]. The two distinct advantages primarily responsible for the selection of NAA in earlier studies appeared to be the option to determine several trace elements simultaneously and the elimination of complex chemical separation steps. The poor precision values obtained by NAA have recently necessitated prechemical separation, which introduces problems of analyte loss, contamination, and blank correction. However, the major drawbacks are the requirement of a nuclear reactor facility, the slow turnaround of the samples, and the relatively high cost of analysis. Nonetheless, NAA is a well-established, multielemental, nondestructive technique with detection limits for most elements in the 1-50 p.g/liter range. This topic is covered by Heydom in Chap. 13 of this book. 

Basically, the I content in the purified iodine fraction can be measured by different techniques. Due to the low specific activity of this long-lived radionuclide, direct 3 , y and X-ray measurement techniques show only a moderate detection capability better detection limits can be obtained by determination of the I mass present in the sample. Here, laser-induced fluorescence spectrometry offers in principle favorable results however, when this technique is applied, the difficulties associated with the preparation of the h chemical species at very low iodine concentrations have to be taken into consideration. The most sensitive I determination technique is neutron activation analysis, which leads to the formation of the... 

In particular, the neutron activation analyst, to whom efficiency curves may be an irrelevance, is not weU served by most spectrum analysis packages. As far as activation analysis is concerned, there is much evidence to show that absolute analysis, calculating concentrations from first principles, is much less accurate than comparative analysis. Apart from aU of the problems which derive from having to use efficiency calibration curves, there are specific problems associated with defining and measuring neutron fluxes and cross-sections which make absolute analysis not worthwhile, in my opinion (although there are those who have devoted a considerable amount of effort into developing absolute neutron activation analysis procedures who would dispute that). For that reason, almost every activation analysis involves irradiation of samples and standards. A direct comparison between them is the simplest solution. 

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