█ K. LEE LERNER
Nuclear spectroscopy is a powerful tool in the arsenal of scientists and forensic investigators because it allows detailed study of the structure of matter based upon the reactions that take place in excited atomic nuclei. It is a widely used technique to determine the composition of substances because it is more sensitive than other spectroscopic methods and can detect the trace presence of elements in an unknown substance that may only be present on the order of parts per billion. Nuclear spectroscopic analysis techniques provided forensic investigators with evidence that linked several of what were eventually to be known as the Washington area "sniper shootings" in late 2002.
Basic principles. A number of methods can be used to excite atomic nuclei and then measure their decaying gamma ray emissions as the atoms return to normal energy levels (i.e., their ground state). The emissions are then analyzed and separated into an emission spectrum that is characteristic for each element. Excitation can be accomplished by colliding nuclei, heavy ion beams, and a number of other methods, but the fundamental purpose remains to measure the spectral properties of a sample as a tool to learn something about the quantum structure of the atoms in the sample.
Like other forms of spectroscopy, the fundamental measurements of nuclear spectroscopy involve recording the emission or absorption of photons by atoms. The specific emissions or absorptions reflect the energy levels, spin states, parity, and other properties of an atom's structure (e.g., quantized energy levels). A qualitative analysis identifies the components of a substance or mixture. Quantitative analysis, on the other hand, measures the amounts or proportions of those components. Because each element—and each nuclide (i.e., an atomic nucleus with a unique combination of protons and neutrons)—emits or absorbs only specific frequencies and wavelengths of electromagnetic radiation, nuclear spectroscopy is a qualitative test (i.e., a test designed to identify the components of a substance or mixture) to determine the presence of an element or isotope in an unknown sample.
In addition, the strength of emission and absorption for each element and nuclide can allow for a quantitative measurement of the amount or proportion of the element in an unknown. To perform quantitative tests, that is, to measure amounts of an element present, the measured spectrum needs to be narrowed down to analysis of photons with specific energies (i.e., electromagnetic radiation of a specific wavelength or frequency). Quantitative computation using Beer's Law is then applied to the measured intensities of photon emission or absorption. Many other spectroscopic methods use this technique (e.g., atomic absorption spectroscopy and UV-visible light spectroscopy) to determine the amount of a element present.
Nuclear activation analysis. One of most widely used methods of nuclear spectroscopy used to determine the elemental composition of substances is Nuclear activation analysis (NAA). In this type of analysis the goal is to determine the composition of an unknown substance by measuring the energies and intensities of the gamma rays emitted after excitation and the subsequent matching of those measurements to the emissions of gamma rays from standardized (known) samples. In this regard, neutron activation analysis is similar to other spectroscopic measurements that utilize other portions of the electromagnetic spectrum. Infrared photons, x-ray florescence, and spectral analysis of visible light are all used to identify elements and compounds. In each of these spectroscopic methods, a measurement of electromagnetic radiation is compared with some known quantum characteristic of an atomic nucleus, atom, or molecule. With NAA, of course, high-energy gamma-ray photons are measured.
Neutron activation analysis involves a comparison of measurements from an unknown sample with values obtained from tests with known samples. Depending on which elements are being tested for, the samples are irradiated with energetic neutrons. The process of radioactivity results in the emission of products of nuclear reactions (in this case, gamma rays) that are measurable by instruments designed for that purpose. After a time (dependent on the duration of radiation) the gamma rays are counted by gamma ray sensitive spectrometers. Because the products of the nuclear reactions are characteristic of the elements present in the sample and a measure of the amounts present, neutron activation analysis is both a qualitative and quantitative tool. Although NAA usually involves the measurement of gamma rays emitted from the radioactive sample, more complex techniques also measure beta and positron emissions.
Nuclear magnetic resonance. Nuclear magnetic resonance (NMR) is another form of nuclear spectroscopy that is widely used in medicine and in forensic analysis. NMR is based on the fact that a proton in a magnetic field has two quantized spin states. The actual magnetic field experienced by most protons is, however, slightly different from the external applied field because neighboring atoms serve to alter it. As a result, a picture of complex structures of molecules and compounds can be obtained by measuring differences between the expected and measured photons absorbed. NMR spectroscopy is an important tool used to determine the structure of organic molecules.
When a group of nuclei are brought into resonance—that is, when they are absorbing and emitting photons of similar energy (electromagnetic radiation, e.g., radio waves, of similar wavelengths)—and then small changes are made in the photon energy, the resonance must change. How quickly and to what form the resonance changes allows for the non-destructive (because of the use of low-energy photons) determination of complex structures. This form of NMR is used by physicians as the physical and chemical basis of a powerful diagnostic technique termed Magnetic Resonance Imaging (MRI). MRI can also be used for noninvasive examinations for concealed substances or implanted objects.
█ FURTHER READING:
deGraaf, R. In Vivo NMR Spectroscopy: Principles and Techniques. New York: John Wiley & Sons, 1999.