Neutron activation analysis characteristics

The characteristics of radiochemical methods are well known [435]. An overview of the determination of elements by nuclear analytical methods has appeared [436]. Some selected reviews of nuclear methods of analysis are available charged particle activation analysis [437,438], instrumental neutron activation analysis [439-441] and ion-beam analysis [442]. 

Principles and Characteristics In neutron activation analysis (NAA) the sample is irradiated by neutrons. The principal reaction in NAA is ... 

Indium produces characteristic lines in the indigo-hlue region and may be detected by spectroscopic analysis. iVt trace concentrations In may be determined by lame-AA, fumace-AA, ICP-AES, x-ray fluorescence, or neutron activation analysis. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

Moreover the energies of these ( -partides (electrons) are known to be 1.39 MeV and that of the gamma-rays 1.38 MeV so that the measnned values of these magnitudes are characteristic of substances containing sodium. (Measurement of the y-radiadon is the usual procedure.) At least 70 of the elements can be activated in this way, by the capture of thermal neutrons, i.e.. by neutron activation analysis. An activation analysis follows a procedure similar to that shown ill Fig. 2. In almost all analyses, the sample materials are not treated before the bombardment, but are placed directly into the bombardment capsule or container. The length of the bombardment interval is usually determined by the half-life of the radionuclide used for the element of interest and the flux of nuclear particles. 

Neutron Activation Spectrometry. Another instrumental technique which has applicability to a wide range of elements is neutron activation analysis. In this method the sample (which could be orange juice without any prior sample treatment) is irradiated with a strong neutron flux. The elements of analytical interest are thus converted to unstable isotopes which decay with characteristic energies and thus measurement of the intensities results in analytical values for the elements of interest. There are some serious drawbacks to this method, however. The matrix can cause severe background effects especially when the sample contains large amounts of an element, like potassium, which is the situation with orange juice. In this event tedious chemical separations must be carried out to achieve adequate selectivity, accuracy... 

Each spectroscopic method has a characteristic application. For example, flame photometry is still applicable to the direct determination of Ca and Sr, and to the determination of Li, Rb, Cs and Ba after preconcentration with ion-exchange resin. Fluorimetry provides better sensitivities for Al, Be, Ga and U, although it suffers from severe interference effects. Emission spectrometry, X-ray fluorescence spectrometry and neutron activation analysis allow multielement analysis of solid samples with pretty good sensitivity and precision, and have commonly been applied to the analysis of marine organisms and sediments. Recently, inductively-coupled plasma (ICP)... 

For many years there was no sufficiently specific method for the identification of characteristic GSRs. One could not see metallic particles because of their small size (5-50 pm) and their presence was ascertained indirectly by means of colouring chemical reactions or such instrumental methods as atomic absorption spectroscopy (AAS), neutron activation analysis (NAA) or XRF. These methods, however, are... 

Neutron Activation Analysis. Magnesium-26 has a small cross section of 0.03 b. The product of irradiation with thermal neutrons is Mg (1 9.5m). As shown in Table 1, several elements commonly present in biological materials give rise to radioactive nuclides with radiations at energy levels close to those characteristic of Mg. Neutron activation was used in the first trials of Mg as an in vivo tracer when measurements were made with a well-type Nal-Tl crystal detector (14,21). Under these conditions the presence of sodium, altuninum and manganese in the samples interfered in the accurate detection of Mg, but could be reduced or eliminated by sample purification. 

Matters to be considered in the selection of a method of chemical analysis include the suite of elements for which the method is useful, and the lower limit of concentration at which each element can be detected and measured, as well as the accuracy of the analysis and the cost in money and time. Few analytical methods are applicable at the concentration levels required, and none is optimum by all these criteria. This paper describes one method in common use, neutron activation analysis (NAA). The essential characteristics of NAA will... 

Modern bulk analysis methods make possible non-destructive chemical identification, which means that the sample remains intact after the analysis. Such a procedure is provided by electron microprobe or X-ray fluorescence analyses, in which the sample is irradiated by electron beams or X-rays and the elemental composition is determined on the basis of induced characteristic X-ray emissions. These methods have been successfully employed to study both stratospheric (Junge, 1963) and tropospheric (Gillette and Blifford, 1971) aerosol particles. Neutron activation analysis is also widely used to identify the chemical composition of atmospheric particulate matter (e.g. Duceef ai, 1966 Rahn etal., 1971) this is also a non-destructive procedure. 

The use of activation methods for the analysis of biological material has been reviewed by Bowen ( 7). In Instrumental Neutron Activation Analysis, the dried sample is placed in the core of a nuclear reactor where it is bombarded with neutrons. Many of the elements present in the sample undergo nuclear reactions of which the most common are the (n, y) type. The products of these reactions are radioactive and decay with the emission of gamma photons of characteristic energy. 

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