Nuclear spectroscopy
Nuclear spectroscopy is a highly sensitive analytical technique used to identify and measure tiny quantities of elements by detecting their gamma-ray emissions. This method relies on the analysis of radiation emitted by unstable atomic nuclei during gamma decay, a process where excess energy is released from the nucleus. Unlike X-rays, which originate from electron interactions, gamma rays have higher energy and are emitted directly from atomic nuclei.
The technique is particularly valuable in forensic science, as it can detect isotopes even in extremely minute quantities, making it more sensitive than most other detection methods. The equipment used in nuclear spectroscopy records gamma rays and displays their energies graphically, creating a spectrum that resembles a mountain range, with peaks indicating the intensity and energy of emissions. Each isotope produces a unique spectrum, allowing for precise identification.
Nuclear spectroscopy has been instrumental in notable criminal investigations, such as the Beltway Sniper case and the poisoning of Alexander Litvinenko, providing crucial evidence through the detection of radioactive materials. The ability to make non-radioactive samples radioactive via neutron bombardment further enhances its applicability in various contexts. Overall, nuclear spectroscopy serves as a powerful tool in both research and law enforcement, facilitating the identification of substances that are often undetectable by other means.
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Nuclear spectroscopy
DEFINITION: Technique for identifying tiny quantities of elements by detecting, recording, and graphically displaying the intensity and energy of their decaying gamma-ray emissions.
SIGNIFICANCE: The most sensitive type of spectroscopy, nuclear spectroscopy, is a valuable and frequently employed tool in forensic science because it can be used to identify and measure minute amounts of substances that may be undetectable through other techniques.
Nuclear spectroscopy is based on the analysis of radiation emitted by unstable atomic nuclei in elements. Gamma decay occurs when unstable atomic nuclei give off excess energy through a spontaneous electromagnetic process. Gamma rays resemble X-rays except they come from the nuclei of elements instead of electrons, and they generally have higher energy than X-rays.
Nuclear spectroscopy equipment displays a gamma ray’s detection and energy by placing a dot on a monitor screen. When the equipment detects another gamma ray of the same energy, it places a second dot above the first. Eventually, the monitor displays a graphical image resembling a mountain range, on which the vertical height of a peak represents the intensity of gamma rays from that energy, and the horizontal position represents the energies of the gamma rays. This graphical mountain range is the gamma-ray spectrum. The positions and relative peak heights differ among different isotopes, and the plotted spectra can be used to identify the isotopes emitting the radiation. Each element has a fixed number of protons in its nucleus; isotopes of the same element all have identical numbers of protons but different numbers of neutrons. The magic of nuclear spectroscopy is that it is more sensitive than almost all other methods of detection. It can detect the presence of an isotope, even if it is only a few parts per billion.
If an analyzed sample is not already radioactive, it may be made so through placement in a nuclear reactor, where neutron bombardment can make isotopes in the sample material radioactive. This technique was used in the so-called Beltway Sniper case in late 2002, when ten people were shot to death in the Washington, DC, area. After several bullet fragments found by investigators underwent neutron activation, nuclear spectroscopy showed that bullets used at different sniping sites probably came from the same source. Eventually, John Allen Muhammad and Lee Boyd Malvo were convicted of the murders.
Another notable example of the application of nuclear spectroscopy to a case occurred in Russia. On November 1, 2006, Alexander Litvinenko, a former Russian spy who had been critical of Russian president Vladimir Putin, was poisoned with the radioactive metal polonium 210 in London, England. He died three weeks later from the damage done to his internal organs by alpha radiation. A simple Geiger-counter examination would show that he had ingested some kind of radioactive material and find traces of it in places where he had been. However, the amount necessary to kill Litvinenko—a tiny fraction of a milligram— was too small to be identified by any method other than nuclear spectroscopy. Nuclear spectroscopy was used not only to identify the polonium 210 but also to determine the impurities in the sample taken from Litvinenko’s body. The sample was found to be unique to a particular nuclear reactor in Russia. However, Russia denied Britain’s request to extradite Andrei Lugovoi, the chief in Litvinenko’s murder.
Bibliography
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Ferguson, Charles D., and William C. Potter. The Four Faces of Nuclear Terrorism. New York: Routledge, 2005.
O’Hara, Charles E., and Gregory L. O’Hara. Fundamentals of Criminal Investigation. 7th ed. Springfield, Ill.: Charles C Thomas, 2003.
Varga, Z., et al. "Trends and Perspectives in Nuclear Forensic Science." TrAC Trends in Analytical Chemistry, vol. 146, Jan. 2022, doi.org/10.1016/j.trac.2021.116503. Accessed 16 Aug. 2024.