Nuclear detection devices
Nuclear detection devices are specialized instruments designed to identify and measure nuclear radiation and its properties. These tools play a critical role in forensic science and law enforcement by detecting radioactive materials and assessing the associated risks, especially in the context of ongoing threats from terrorism. Nuclear radiation consists of ionizing radiation, such as alpha particles, beta particles, and gamma rays, which can displace electrons from atoms, thereby creating ions. Common detection methods include Geiger counters and scintillation detectors, which utilize materials like sodium iodide or germanium to identify and amplify signals from radioactive emissions.
Customs agents employ detection portals equipped with neutron and gamma detectors to monitor incoming vehicles and cargo for illicit radioactive materials, including highly enriched uranium (HEU), which poses a significant security concern. Advanced detectors also allow for energy measurement of gamma rays, enabling scientists to differentiate between various isotopes based on their unique energy signatures. Recent developments in detector technology, such as cadmium zinc tellurium (CdZnTe), promise enhanced capabilities, including room-temperature operation. Overall, these devices are essential for safeguarding against the misuse of nuclear materials and ensuring public safety.
On this Page
Subject Terms
Nuclear detection devices
DEFINITION: Instruments used to detect nuclear radiation and to measure its properties.
SIGNIFICANCE: Forensic science can aid law-enforcement authorities in the detection of radioactive materials and in establishing the levels of danger such materials present so that appropriate warnings can be disseminated. Given ongoing threats of international terrorism, nuclear detection devices are important tools for locating lost or stolen radioactive materials that might otherwise be used as weapons in some form.
Atomic nuclei emit two types of radiation: ionizing and nonionizing. Alpha particles (two protons and two neutrons bound together, making a helium-4 nucleus), beta particles (high-energy electrons or positrons), and gamma rays (streams of high-energy photons) are ionizing radiation. Each of these particles can knock electrons away from their parent atoms, leaving them ionized. Geiger counters detect such freed electrons, as do detectors made from the semiconductorsgermanium and silicon.
When electrons recombine with the positive ions, they emit flashes of light (scintillations). A scintillation detector might consist of a large, transparent crystal of sodium iodide that is shielded from outside light and optically coupled to a phototube. The phototube detects and amplifies the scintillation, and then it converts the light flash into an electrical signal. The larger the crystal, the weaker the source that can be detected. Cylindrical crystals 7.6 centimeters (3 inches) in diameter, with associated electronics, can be mounted on a helicopter that then flies low to detect radioactive sources on the ground. Larger detectors can be made from scintillator plastics such as polyvinyl toluene (PVT).
Neutrons are nonionizing radiation, so they must be made to interact with something like helium 3 in such a way that ionizing radiation will be produced. Neutrons are emitted by heavy elements that spontaneously fission, such as plutonium and uranium. Customs agents at points of entry into the United States use portals—two columns, 2 to 4 meters (about 6 to 13 feet) high—containing helium-3 neutron detectors and PVT gamma detectors. People, cars, trucks with shipping containers, and trains pass through such portals, which can detect radioactive isotopes and plutonium but not highly enriched uranium (HEU). A small number of detector portals are available that can examine targets with X-rays or neutron beams; these can detect HEU. As it is generally thought that terrorists would find it easier to build bombs from HEU than from plutonium, it is particularly important that law-enforcement agencies be able to detect smuggled HEU.
Some detectors, including sodium iodide, germanium, and silicon, can measure the energy of gamma rays. The energy patterns are different for different atoms, so scientists can use the patterns as fingerprints to determine which isotopes are present. Germanium detectors have much finer energy resolution than do sodium iodides, but they must be held at very low temperatures (77 Kelvins, or -321 degrees Fahrenheit). A promising new detector material introduced during the early 1990s, cadmium zinc tellurium (CdZnTe), is a room-temperature superconductor. Improved detector portals will have energy-determining capability and will be able to determine which isotopes are present. Modern imaging gamma-ray cameras can spot a weak radioactive source up to 100 meters (328 feet) away.
Bibliography
Ahmed, Syed Naeem. Physics and Engineering of Radiation Detection. San Diego, Calif.: Academic Press, 2007.
Finck, Robert, et al. "Maximum Detection Distances for Gamma Emitting Point Sources in Mobile Gamma Spectrometry." Applied Radiation and Isotopes, vol. 184, June 2022, doi.org/10.1016/j.apradiso.2022.110195. Accessed 16 Aug. 2024.
Kleinknecht, Konrad. Detectors for Particle Radiation. 2d ed. New York: Cambridge University Press, 2001.
Saha, Gopal B. “Instruments for Radiation Detection and Measurement.” In Fundamentals of Nuclear Pharmacy. 5th ed. New York: Springer, 2004.
Tomono, Dai, et al. "First On-Site True Gamma-Ray Imaging-Spectroscopy of Contamination near Fukushima Plant." Scientific Reports, vol. 7, 3 Feb. 2017, doi.org/10.1038/srep41972. Accessed 16 Aug. 2024.