Neutron activation analysis (NAA) is a sensitive analytical technique useful for performing both qualitative and quantitative multi-element analysis of major, minor, and trace elements in samples from almost every conceivable field of scientific or technical interest. For many elements and applications, NAA offers sensitivities that are superior to those attainable by other methods, on the order of parts per billion or better. In addition, because of its accuracy and reliability, NAA is generally recognized as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. Worldwide application of NAA is so widespread it is estimated that approximately 100,000 samples undergo analysis each year.
The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons, instrumentation suitable for detecting gamma rays, and a detailed knowledge of the reactions that occur when neutrons interact with target nuclei. Brief descriptions of the NAA method, reactor neutron sources, and gamma-ray detection are given below.
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The most common type of nuclear reaction used for NAA is the neutron capture or (n,gamma) reaction. When a neutron interacts with the target nucleus via a non-elastic collision, a compound nucleus forms in an excited state. The excitation energy of the compound nucleus is due to the binding energy of the neutron with the nucleus. The compound nucleus will almost instantaneously de-excite into a more stable configuration through emission of one or more characteristic prompt gamma rays. In many cases, this new configuration yields a radioactive nucleus which also de-excites (or decays) by emission of one or more characteristic delayed gamma rays, but at a much slower rate according to the unique half-life of the radioactive nucleus. Depending upon the particular radioactive species, half-lives can range from fractions of a second to several years.
In principle, therefore, with respect to the time of measurement, NAA falls into two categories: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common; thus, when one mentions NAA it is generally assumed that measurement of the delayed gamma rays is intended. About 70% of the elements have properties suitable for measurement by NAA.
Although there are several types of neutron sources (reactors, accelerators, and radioisotopic neutron emitters) one can use for NAA, nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes due to the materials used to moderate (or reduce the energies of) the primary fission neutrons. However, most neutron energy distributions are quite broad and consist of three principal components (thermal, epithermal, and fast).
The thermal neutron component consists of low-energy neutrons (energies below 0.5 eV) in thermal equilibrium with atoms in the reactor's moderator. At room temperature, the energy spectrum of thermal neutrons is best described by a Maxwell-Boltzmann distribution with a mean energy of 0.025 eV and a most probable velocity of 2200 m/s. In most reactor irradiation positions, 90-95% of the neutrons that bombard a sample are thermal neutrons. In general, a one-megawatt reactor has a peak thermal neutron flux of approximately 1E13 neutrons per square centimeter per second.
The sensitivities for NAA are dependent upon the irradiation parameters (i.e., neutron flux, irradiation and decay times), measurement conditions (i.e., measurement time, detector efficiency), nuclear parameters of the elements being measured (i.e., isotope abundance, neutron cross-section, half-life, and gamma-ray abundance). The accuracy of an individual NAA determination usually ranges between 1 to 10 percent of the reported value. Table I lists the approximate sensitivities for determination of elements assuming interference free spectra.
Estimated detection limits for INAA using decay gamma rays. Assuming irradiation in a reactor neutron flux of 1E11 neutrons per square centimeter per second.
| Sensitvity (nanograms) | Elements |
|---|---|
| 0.1 | Dy, Eu |
| 0.1 - 1 | In, Lu, Mn |
| 1 - 10 | Au, Ho, Ir, Re, Sm, W |
| 10 - 100 | Ag, Ar, As, Br, Cl, Co, Cs, Cu, Er, Ga, Hf, I, La, Sb, Sc, Se, Ta, Tb, Th, Tm, U, V, Yb |
| 100 - 1E3 | Al, Ba, Cd, Ce, Cr, Hg, Kr, Gd, Ge, Mo, Na, Nd, Ni, Os, Pd, Rb, Rh, Ru, Sr, Te, Zn, Zr |
| 1E3 - 1E4 | Bi, Ca, K, Mg, P, Pt, Si, Sn, Ti, Tl, Xe, Y |
| 1E4 - 1E5 | F, Fe, Nb, Ne |
| 1E6 | Pb, S |
The use of neutron activation analysis to characterize archaeological specimens (e.g., pottery, obsidian, chert, and limestone) and to relate the artifacts to source materials through their chemical signatures is a well-established application. For example, the "fingerprinting" of obsidian artifacts by NAA is a nearly 100 percent successful method for determining prehistoric trade routes since sources of obsidian are easily differentiated from one another through their chemical compositions.
High-specific activity radiotracers, produced by neutron activation, have been used with great success to study biochemical processes in the small animal model. For example, Selenium-75, having a specific activity of 1000 Ci/g has been used to advance the discovery of dependent enzymes and other biologically important proteins. Trace-element and mineral nutrition are important aspects of human and animal health. NAA has been used to characterize a wide variety of samples for their elemental content. The basic nutritional requirement at the cellular level can be studied using NAA and radiotracer techniques. NAA is one of two methods that can be used to study nutritional bioavailability and absorption of essential trace elements in the human using enriched stable isotopes.
As the United States continues to confront its legacy of nuclear weapons waste, analytical techniques must be developed and employed to characterize a wide variety of sample matrices which may contain significant concentrations of actinide and rare earth elements. NAA and gamma-ray spectroscopy are important techniques in this effort. For example, epithermal NAA has been shown to be a powerful tool in the characterization of uranium over a wide range of concentrations (sub-ppm to several percent) in samples which may also have a rare earth content of 10 percent or greater.
Samples such as hair, nails, blood, urine, and various tissues are analyzed by NAA for both essential and toxic trace elements to determine their effect on disease outcomes. For example, it is has been demonstrated that the selenium concentration in human nails is an accurate monitor of the dietary intake of Se. As a consequence, the nail monitor has been extensively used to study the protective effect of dietary Selenium against cancer and heart disease in numerous prospective case-control studies nested in large well-characterized cohorts such as the Nurses' Health Study sponsored by the National Institutes of Health and Harvard University.
Forensic laboratories are often called upon to analyze evidence for the investigation and prosecution of criminal cases. The excellent sensitivity of detection available by neutron activation analysis facilitates analysis of the extremely small evidence samples (e.g., gunshot residues, bullet lead, glass, paint, hair, etc.) typically found at crime scenes. For example, NAA was used to analyze samples of bullet lead from the assassination of President John F. Kennedy.
Analysis of rock specimens by neutron activation analysis assists geochemists in research on the processes involved in the formation of different rocks through the analysis of the rare earth elements (REEs) and other trace elements. For example, the discovery of anomalously high iridium concentrations in 65-million-year old limestone deposits from Italy and Denmark could only have been accomplished by NAA. The NAA findings support the theory that extinction of the dinosaurs occurred soon after the impact of a large meteorite with the earth.
The behavior of semiconductor devices is strongly influenced by the presence of impurity elements either added intentionally (doping with B, P, As, etc.) or contaminants remaining due to incomplete purification of the semiconductor material during device manufacture. Small quantities of impurities present at concentrations below 1 ppb can have a significant effect on the quality of semiconductor devices. The impurity levels of interest are such that the NAA technique is often the only method with adequate sensitivity for measuring the impurity concentrations. For example, NAA has been used to identify and eliminate the sources of contamination in semiconductor devices produced by several different companies, thereby saving these companies millions of dollars.
Many agricultural processes and their consequences, such as fertilization and herbicidal and pesticidal control, are influenced by surface and sub-surface movement, percolation and infiltration of water. Stable activatable tracers, such as bromide, analyzed by NAA, have allowed the soil scientist to quantify the distribution of agricultural chemicals under a wide variety of environmental and land use influences.
