Neutron Activation Analysis NAA

Activation analysis is based on the production of radionuclides by nuclear reactions. The specific activity is given by the equation of activation, 

From eqs. (15.2) and (15.3) it is evident that the limits of quantitative determination depend on the cross section a of the nuclear reaction, the flux density 0 of the projectiles and the ratio f/fi/2 (time of irradiation to half-life). 

The following possible modes of neutron activation can be distinguished  

Reactor neutrons are most frequently used for activation analysis, because they are available in high flux densities. Moreover, for most elements the cross section of (n,y) reactions is relatively high. On the assumption that an activity of lOBq allows quantitative determination, the lower limits of determination by (n,y) reactions at a thermal neutron flux density of lO cm s are listed in Table 17.2 for a large number of elements and two irradiation times (1 h and 1 week). Detection limits of the order of 10 to g/g are, in general, not available by other analytical methods. 

The spectrum of neutron energies in nuclear reactors is, in general, relatively broad. Furthermore, it varies with the type of reactor, and in the same reactor with 

Activation analysis is a process useful for performing both qualitative and quantitative multi-element analysis from almost every field of scientific or technical interest. The elemental concentrations in the sample are calculated following an irradiation step of the unknown sample and a comparator standard containing 

The total activity generated in the sample will diminish with time following a decreasing curve corresponding to an envelope of the superimposed activities of the individual radionuclei present. The (5 spectrum being continuous, the composition of the elements cannot be deduced by a simple analysis of the radiation. 

The key for identification comes from the y emission which accompanies the emission and is characteristic to each radionuclide. This emission spectrum occurs in the same spectral range as X-ray fluorescence. 

There are several types of neutron sources one can use for NAA, but megawatt nuclear reactors with their intense flux of 10 to 10 neutrons m s from uranium fission, offer the highest available sensitivities for most elements. Neutron energy distribution is quite broad, but for conventional NAA, low-energy neutrons (energies below 0.5 eV) are chosen that represent 90—95 per cent of the neutron flux. Sufficient levels of activation are reached in a few minutes, even if the isotope formed has a long half-life. The procedure imposes that the sample to be treated must be thermally stable. It is enclosed in a tube with a comparator standard of known concentration, before being introduced in a reactor beam port. 

Small sources of neutrons — containing an a-emitter (several p.g Am or Sb) encapsulated in a beryllium envelope — have been developed to avoid these restrictions. The nuclear reaction generating the neutron is the following  

Most of the transition elements that are of primary interest in the semiconductor industry such as Fe, Cr, Mn, Co, and Ni, can be analyzed with very low detection limits. Second to its sensitivity, the most important advantage of NAA is the minimal sample preparation that is required, eliminating the likelihood of contamination due to handling. Quantitative values can be obtained and a precision of 1-5% relative is regularly achieved. Since the technique measures many elements simultaneously, NAA is used to scan for impurities conveniently. 

It is applicable to plastic packaging materials, where purities with respect to mobile ions, such as Cl and Na, can be checked. In addition, a-particle precursors, such as U and Th, can be determined in solid plastics with sub-ppb detection limits. 

It is important to note that the neutron capture probability, called the cross section a, is vasdy different for various elements. Excellent sensitivity for Au is due largely to its high cross section (a = 100 barns 1 barn = 1 x 10 cm ). Other elements, such as Pb, have low cross sections and much poorer detection limits. 

Elements with multiple stable isotopes may produce several radioisotopes that can be measured to assure the accuracy of the analysis. For example, Zn has five stable isotopes. The isotope Zn will produce the radioisotope Zn, and Zn will produce the radioisotope Zn. Both of these radioisotopes can provide an independent measurement of the Zn concentration and therefore can be used to check the consistency and quality of the analysis. On the other hand, Zn will produce Zn, which is nonradioactive and therefore cannot be used in NAA. 

At the end of the irradiation, the samples are withdrawn from the reactor and y-ray spectroscopy is carried out. Most often the laboratory performing the y-ray spectroscopy is located in a different city, in which case the samples are shipped and the reactor serves as a neutron source only. Many reactors also have y-ray spectroscopy capability so that measurements can be made at the reactor site as well. 

Many elements become radioactive when exposed to neutron bombardment inside a nuclear reactor. This is the basis of an extremely sensitive analytical method neutron activation analysis (NAA). The induced radioactivity is examined by y-ray measurement With computerized data processing it is possible to measure more than thirty elements simultaneously without chemical processing. For many elements and applications, NAA offers sensitivities of the order of parts per biUion (pbb). 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. 

NAA has been used in many applications. One very special one is the determination of the iridium content in the earth s cmst and in a 1-cm thick clay layer round the earth. This day layer was formed 65 million years ago and defines the boundary between the Cretaceous and the Tertiary pericxls of geological history. Shells offoraminifera are found in all marine environments and in abundance in the limestone above and below the spedal day layer. But not in the layer itself. In addition it was shown by NAA that the iridium content of the limestone layers above and below is 0.3 ppb while the concentration in the layer is 10-40 pbb. This is the source of the theory of a giant asteroid collision with the earth at this time, a collision that brought the end of the dinosaur era. 

Burchard, The History and Apparatus of Blowpipe Analysis, The Mineralogical Record 1994, 25, pp. 251-277 Goran Jonsson and Elisabeth Nilsson, Vdgldra och optik [Wave science and optics], (in Swedish), 2002, Teach Support, 

Wollaston, Philosophical Transactions of the Royal Society (London), 

David Alter, On Certain Physical Properties of Light Produced by the Combustion of Different Metals in the Electric Spark, Refracted by a Prism, American Journal of Science, 1854,18, pp. 55-57. 

---

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

Neutron Activation Analysis (NAA) is one of the analytical methods recommended for low level Mo determination in biological materials. 

The chemical composition of particulate pollutants is determined in two forms specific elements, or specific compounds or ions. Knowledge of their chemical composition is useful in determining the sources of airborne particles and in understanding the fate of particles in the atmosphere. Elemental analysis yields results in terms of the individual elements present in a sample such as a given quantity of sulfur, S. From elemental analysis techniques we do not obtain direct information about the chemical form of S in a sample such as sulfate (SO/ ) or sulfide. Two nondestructive techniques used for direct elemental analysis of particulate samples are X-ray fluorescence spectroscopy (XRF) and neutron activation analysis (NAA). 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

Neutron activation analysis (NAA) is a supreme technique for elemental analysis (Section 8.6.1). Other nuclear analytical techniques, such as PIXE (Section 8.4.2) and RBS, also find application in investigations of diffusion processes [445]. 

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

Neutron activation analysis (NAA) is a technique for the qualitative and/or quantitative determination of atoms possessing certain types of nuclei. Bombarding a sample with neutrons transforms some stable isotopes into radioactive isotopes measuring the energy and/or intensity of the gamma rays emitted from the radioactive isotopes created as a result of the irradiation reveals information on the nature of the elements in the sample. NAA Is widely used to characterize such archaeological materials as pottery, obsidian, chert, basalt, and limestone (Keisch 2003). 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

It was not until the application of neutron activation analysis (NAA) that the problem of overlapping sources could be tackled. NAA is a highly sensitive and essentially non-destructive technique, although samples have to be taken which remain radioactive for some time after analysis. The use of NAA in characterizing obsidian was first demonstrated in the early 1970s (Aspinall... 

How do analyses obtained using one technique (in this case OES) relate to those collected using a different technique (principally neutron activation analysis, NAA), covering a different suite of elements ... 

Membrane morphology is studied with scanning electron microscopy (SEM) thereby providing an Insight into the relationship between asymmetric membrane preparation, structure, and performance (29,3A). The extent of ion exchange of the salt form of the SPSF membranes is studied with atomic absorption spectroscopy (AAS), neutron activation analysis (NAA), and ESCA. AAS is used for solution analysis, NAA for the bulk membrane analysis, and ESCA for the surface 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). 

Neutron activation analysis (NAA), based on the interaction of the object material with fast neutrons, has been used in the identification and determination of the content of elements present in pigments, coins and alloys, stone, glass, and pottery [26]. Multi-elemental analysis (about 20 elements) can be performed on small samples off less than 5 mg, with sensitivities in the ppm range. The requirement of a nuclear reactor, the handling of radioactive materials, and the time-consuming procedures required for preparing the samples are the main drawbacks of this technique. 

---