Nuclear Spectroscopy
Nuclear spectroscopy is a powerful tool in the arsenal of scientists studying the structure of matter based upon the reactions that take place in excited atomic nuclei. Nuclear spectroscopy 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 that may only be present on the order of parts per billion in an unknown substance.
Nuclear spectroscopic technology allowing the identification of trace elements in soil and water samples has increasing use in ecological, agricultural and geological research. Applications of nuclear spectroscopic principles are important to the development of non-invasive diagnostic tools used by physicians.
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 emissions 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 measures the amounts or proportions of the components in a reaction or substance.
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 emissions 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 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.
One of most widely used methods of nuclear spectroscopy used to determine the elemental composition of substances is Nuclear Activation Analysis (NAA).
In neutron activation 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 fluorescence, 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 of the length 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 amounts 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 (NMR) is another form of nuclear spectroscopy that is widely used in medicine.
NMR is based on the fact that a proton in a magnetic field had two quantized spin states. The actual magnetic field experienced by most protons is, however, slightly different from the external applied field because neighboring atoms alter the field. As a result, however, a picture of complex structures of molecules and compounds can be obtained by measuring differences between the expected and measured photons absorbed. NMR spectroscopy as 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).
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