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 semiconductors germanium and silicon. Other gas-filled detectors, such as ionization chambers and proportional counters, measure radiation by collecting the electrical charge produced by ionization within a controlled gas volume.

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 polyvinyltoluene (PVT).

Neutrons do not ionize atoms directly, so they must be made to interact with materials like helium-3 in such a way that detectable ionizing radiation will be produced. Neutrons are emitted by heavy elements that spontaneously fission, such as plutonium and uranium. Neutron detection systems may also use materials containing boron-10 or lithium-6, which capture neutrons and emit charged particles that can be measured. 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. Some portal systems incorporate alternative neutron-sensitive materials in order to maintain detection capability without reliance on helium-3. 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. Portal monitoring systems may incorporate spectroscopic analysis to distinguish naturally occurring radioactive materials from special nuclear materials.  Germanium detectors have much finer energy resolution than do sodium iodides, but they must be operated at cryogenic  temperatures. A promising new detector material introduced during the early 1990s, cadmium zinc tellurium (CdZnTe), is a room-temperature semiconductor. Advances in crystal growth and detector design have improved the energy resolution and stability of CdZnTe-based detectors.  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  weak radioactive sources at significant distances, depending on source strength, shielding, and environmental conditions.  Some gamma-ray imaging systems use Compton scattering techniques to determine the direction and approximate location of radioactive sources.

Bibliography

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