Neutron-activation analysis

Neutron activation analysis (NAA) is a qualitative and quantitative analytical technique for the determination of trace elements in a variety of complex sample matrices. It can be performed in a variety of ways depending on the element and its levels to be measured, as well as on the nature and the extent of interference from other elements present in the sample. 

Next to education and training, NAA is the most simple and widely used application of research reactors. Almost any reactor of a few tens of kilowatts and above is capable of irradiating samples for some sort of NAA. In addition, many of the uses of trace element identification can be directly linked to potential economic benefits. Therefore, NAA should be looked at as a key component of most research reactor strategic plans. 

The NAA technique can be broadly classified into two categories based on whether chemical separations are employed in the analytical procedure. Under favourable conditions, the experimental parameters such as irradiation, decay and counting times can be optimized so that the elements of interest can be determined without the need of physical destruction of the sample by chemical treatments. This process is called non-destructive NAA. However, it is more commonly referred to as instrumental NAA (INAA) and is the most widely used form of NAA. 

The INAA technique generally involves a neutron irradiation followed by a decay period of several minutes to many days. In some cases, cyclic INAA (CINAA) is employed whereby a sample is irradiated for a short time, quickly transferred to a detector and counted for a short time. This process of irradiation-transfer-counting is repeated for an optimum number of cycles. The INAA technique uses reactor neutrons that are a combination of thermal and epithermal neutrons. When mostly epithermal neutrons are used for irradiations, the technique is called epithermal INAA (EINAA). 

for Microelements in tissue of animals, plants and fishery products. 

2 Neutron activation analysis fM.W. Rophael M.A. Malati, Chem.Labor Betrieb. 34(1983)51) 

An intimate mixture of an a emitter and Be will produce neutrons with a range of energies. When these are moderated i.e. slowed to thermal energy, they can induce an (n,y) reaction in some nuclei. This can be represented by  

The isotopic product, being richer in neutrons, is likely to be a p emitter and its activity can be measured without prior separation from the target. High energy neutrons induce other nuclear processes. Although the activity of X builds up as neutron irradiation continues, it will simultaneously decay usually by the process  

1 Analytical Instrumentation Handbook, 2 ed., Ed.G.W.Ewing, Marcel Dekker New York, 1997. 

Skogg J.J.Leary, Principles of Instrumental Analysis, 4 ed. Saunders College Publishing, Fort Worth, 1992. 

Although many other types of nuclear reaction are possible as a result of high neutron fluxes, these two are the ones of prime importance in radioanalytical chemistry. The two principal requirements for a reaction to be useful analytically are that the element of interest must be capable of undergoing a nuclear reaction of some sort, and the product of that reaction (the daughter) must itself be radioactively unstable. Ideally, the daughter nucleus should have a half life which is in the range of a few days to a few months, and should emit a particle which has a characteristic energy, and is free from interference from other particles which may be produced by other elements within the sample. 

Detection levels for NAA can be as low as 1.5 x 10 5ppb for very sensitive elements in a suitable matrix (Glascock, 1994). More routinely, one would expect to see figures of the order of lOppb or better, up to perhaps 10 ppm, for trace elements in geological or biological material. It is important to realize that not all elements can be analysed by normal NAA. As with all analytical 

For a number of reasons, neutrons have found the widest applications as bombarding particles in activation analysis. First, the cross sections for reactions of nuclides with neutrons, yielding primarily (n, 7) reactions, but also (n,a) (n,p), (n, 2 n), and (n. n ) reactions, are often large. The type of reaction depends on the energy of the neutron and is chosen individually for each case, on the basis of the following considerations  

A suitable reaction product with not too long a half-life (preferably below one year) and with decay parameters compatible with the available counting equipment A sufficiently high cross section The absence of important interfering reactions yielding the same radionuclide Easy treatment of the sample after irradiation (limited matrix activity) 

Second, intense neutron sources (nuclear reactors, neutron generators, isotopic sources) were generally available. Third, the high penetration depth of neutrons allows a nearly homogeneous flux density of the entire sample to be analyzed. 

If a target with nuclides of A per cubic centimeter is placed in a neutron beam q (number 

The cross section is expressed as an area in square centimeters or, more practically, in bam (1 b=10 - cm-). 

There are several configurations of this technique that include instrumental neutron activation analysis (INAA) where the sample is measured without any chemical treatment and radiochemical NAA (RNAA) where post-irradiation separation is done. 

FIGURE 1.19 (a) A schematic presentation of the principle of operation of x-ray fluorescence (XRF). (From http //museumbulletin.files.wordpress.eom/2012/08/xrf lgl.jpg, accessed July 26, 2014.) (b) XRF spectra of two hair samples from a person exposed to elevated uranium levels in drinking water in Finland (solid line) and from a control in Israel (dashed line). (Finnish sample provided courtesy of Laina Salonen from STUK.) 

The term PCNAA is used when preconcentration precedes the neutron activation while if epithermal neutrons are used to excite the sample the acronym given is ENAA. The monitoring of the delayed neutrons emitted after excitation is termed DNAA. All these NAA procedures are nondestructive techniques used for characterizing solid (and in some cases also liquid) samples. However, a neutron source and a suitable detector are required and the sample can become quite radioactive after irradiation. The sensitivity of NAA techniques varies widely among different elements and sample preparation and post-irradiation methods employed. Several specific examples of NAA application for analysis of uranium in different matrices will be presented in the appropriate chapters. 

One of the more widely used techniques of the last 50 years has been NAA, sometimes defined as instrumental neutron activation analysis (INAA). NAA uses gamma-rays, the highest energy end of the spectrum, to measure a wide variety of elements to parts-per-million concentrations. At the same time, NAA requires a nuclear reactor - not readily available to many researchers. 

NAA is gamma ray spectroscopy that uses the slow thermal neutrons from a nuclear reactor to excite the nucleus of an atom. When an atom absorbs a thermal neutron, its atomic mass increases by one and the nucleus becomes unstable. One or more nuclear reactions then take place that release gamma-rays with energies characteristic of the particular nuclear decay reactions, along with other radiation (Fig. 4.14). While 

NAA is based on the production of radioisotopes of the element of interest (this is the activation step), which is done by neutron capture, hence the name neutron activation analysis. The nuclear reactions usually occur with energy release in the form of y-rays, which are analyzed to determine the elements present. Most of the neutrons in nuclear reactors are in thermal equilibrium with the surrounding atoms, and capture of these neutrons by common elements produces suitable radioisotopes. An example of a neutron capture reaction is as follows  

In Equation 7.18, is an electron and v a neutrino. The product isotope 20 Ca can be obtained in one of several energy levels, with 82% yield in the ground state and nearly 18% yield in an excited state 1.525 MeV above the ground one (Helmke 1996) nuclei in this state will eventually decay to the ground state with the emission of 1.525 MeV photons, which are detected and measured to quantify K in the sample. 

NAA is a powerful technique for elmental analysis in complex samples such as soils, despite the aforementioned limitations, and has been applied in a number of studies (Steinnes 2000 Grosheva, Zaichick, and Zaichick 2007 Srivastava et al. 2011). In the past years, NAA applications have diminished somewhat in favor of other techniques such as ICP-MS (Tsukada et al. 2005). 

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. 

In principle, therefore, NAA falls into two categories with respect to the time of measurement (1) PGNAA in which the measurements take place during irradiation and/or (2) DGNAA, w here the measurements follow 

The qualitative characteristics of NAA are the energies of the emitted y-rays E) and the half-life of the nuclide (T1/2) while the quantitative characteristic is the intensity (7), which is the number of y quanta of energy E measured per unit time. 

The evaluated database for prompt y-rays from radiative capture of thermal neutrons, by elements from Hydrogen to Zinc, has been compiled by IAEA (Reddy and Frankie 2003). This database includes the energies of 

The raw data consists of counts per 30 seconds from a digital counter, which can be converted to weight % of the element of interest. 

Principle method for determining total oxygen directly. 

Fast (about 10 minutes per analysis). Repeat analysis on weighed sample requires only 1 minute). 

CAS Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 

Neutron activation analysis (NAA) is a sensitive analytical technique useful for performing both qualitative and quantitative multielement analysis of major, minor, and trace elements in samples from almost every conceivable field of scientific or technical interest. NAA was discovered in 1936 by Hevesy and Levi. They found that Dy in samples became radioactive when bombarded by neutrons. From this observation, they quickly recognized the potential of employing nuclear reactions for the determination of elements present in different samples. Neutron activation analysis was soon established as a method of qualitative and quantitative element analysis. 

Edited by Chunying Chen, Zhifang Chai and Yuxi Gao 

Although there are several types of neutron sources (reactors, accelerators, and radioisotope neutron sources) that can be used for NAA, nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. 

Phosphorus can be determined through the radionuclide P formed by the reaction  

6 = natural isotopic abundance a = cross-section at the most probable 

Interference of sulphur and chlorine can be avoided by irradiating the samples in a well thermalized reactor flux i.e. where the fast neutron flux is very low. 

Simultaneous determination of phosphorus and sulphur by fast neutron activation 

Simultaneous determination of sulphur and phosphorus is feasible by a double irradiation technique. The theoretical background of this method was described by Op de Beeck and Hoste (13). 

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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. 

The concentration of Mn in steel can be determined by a neutron activation analysis using the method of external standards. A 1.000-g sample of an unknown steel sample and a 0.950-g sample of a standard steel known to contain 0.463% w/w Mn, are irradiated with neutrons in a nuclear reactor for 10 h. After a 40-min cooling period, the activities for gamma-ray emission were found to be 2542 cpm (counts per minute) for the unknown and 1984 cpm for the standard. What is the %w/w Mn in the unknown steel sample ... 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

Numerous methods have been pubUshed for the determination of trace amounts of tellurium (33—42). Instmmental analytical methods (qv) used to determine trace amounts of tellurium include atomic absorption spectrometry, flame, graphite furnace, and hydride generation inductively coupled argon plasma optical emission spectrometry inductively coupled plasma mass spectrometry neutron activation analysis and spectrophotometry (see Mass spectrometry Spectroscopy, optical). Other instmmental methods include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

Thermal neutron activation analysis has been used for archeological samples, such as amber, coins, ceramics, and glass biological samples and forensic samples (see Forensic chemistry) as weU as human tissues, including bile, blood, bone, teeth, and urine laboratory animals geological samples, such as meteorites and ores and a variety of industrial products (166). 

MetaUic impurities in beryUium metal were formerly determined by d-c arc emission spectrography, foUowing dissolution of the sample in sulfuric acid and calcination to the oxide (16) and this technique is stUl used to determine less common trace elements in nuclear-grade beryUium. However, the common metallic impurities are more conveniently and accurately determined by d-c plasma emission spectrometry, foUowing dissolution of the sample in a hydrochloric—nitric—hydrofluoric acid mixture. Thermal neutron activation analysis has been used to complement d-c plasma and d-c arc emission spectrometry in the analysis of nuclear-grade beryUium. 

Instiximental neutron activation analysis (INAA) is considered the most informative and highly sensitive. Being applied, it allows detecting and determination of 30-40 elements with the sensitivity of 10 -10 g/g in one sample. The evident advantage of INAA is the ability to analyze samples of different nature (filters, soils, plants, biological tests, etc.) without any complex schemes of preliminai y prepai ation. 

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

Thorinm-232 is the only non-radiogenic thorium isotope of the U/Th decay series. Thorinm-232 enters the ocean by continental weathering and is mostly in the particulate form. Early measurements of Th were by alpha-spectrometry and required large volume samples ca. 1000 T). Not only did this make sample collection difficult, but the signal-to-noise ratio was often low and uncertain. With the development of a neutron activation analysis " and amass spectrometry method " the quality of the data greatly improved, and the required volume for mass spectrometry was reduced to less than a liter. Surface ocean waters typically have elevated concentrations of dissolved and particulate 17,3 7,62... 

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). 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

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 Instrumental Neutron Activation Analysis... 

The HFBR at Brookhaven National Laboratory is a heavy water moderated and cooled reactor designed to provide an intense beam of neutrons to the experimental area. In addition using thimbles i oiitaincd within the vessel, it provides isotopic production, neutron activation analysis, ami muiemi irradiations. It began operation in 1965 at a power of 40 MW to be upgraded to 60 MW m 19S2. 

The neutron activation analysis of the polymer reveals that initiation is effected predominantly by chlorine atoms. No retardation or inhibition were detected,... 

Neutron activation analysis of a polymer suggests that when Py is used as the electron doner (D), the initiation proceeds through the Cl atom, but when D = DMSO, both Cr and DMSO residues are the primary radicals produced from the photoexcited ion-pair complex. The following reaction scheme is proposed ... 

Nitronium tetrafluoroborate is very hygroscopic. It is stable as long as it is anhydrous, but it is decomposed by moisture, and all transfers should be in a dry box. Its purity can be checked by conventional elemental analysis. However, because of the hygroscopic nature of the salt, the submitters have found it convenient to use neutron activation analysis (B, F, N, O) of samples... 

Radioactive nuclei are used extensively in chemical analysis. One technique of particular importance is neutron activation analysis. This procedure depends on the phenomenon of induced radioactivity. A sample is bombarded by neutrons, bringing about such reactions as... 

In the modern forensic chemistry laboratory (Figure B) arsenic is detected by analysis of hair samples, where the element tends to concentrate in chronic arsenic poisoning. A single strand of hair is sufficient to establish the presence or absence of the element. The technique most commonly used is neutron activation analysis, described in Chapter 19. If the concentration found is greater than about 0.0003%, poisoning is indicated normal arsenic levels are much lower than this. 

Harbottle, G. Neutron Activation Analysis in Archaeological Chemistry. 157,57-92 (1990). 

Comparison of Various FNAA Techniques for Assay of Synthetic Octol Samples Precision of Single-Axis Rotation FNAA for Assay of Octol Plant Samples Fast Neutron Activation Analysis for Nitrogen in Explosives by... 

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