Nuclear Detection Devices
█ LARRY GILMAN
Nuclear detection devices, also termed radiation detectors, are systems designed to detect the presence of radioactive materials. These materials may take the form of gases, particles suspended in air, or solid metals (often alloys of uranium or plutonium). Detection of nuclear materials is needed for safety monitoring of all facilities handling nuclear materials, for the interdiction of nuclear smuggling, and for arms-control monitoring of peaceful nuclear programs to detect any diversions of fissile material to bomb-building programs.
Although radioactive materials can be (and, in the laboratory, often are) detected by direct chemical assay, it is far easier in practice to detect them at second hand by measuring the radiation they emit. Nuclear materials emit two kinds of radiation as the nuclei of their atoms spontaneously break apart: fast particles (i.e., neutrons, electrons, and ions) and electromagnetic radiation (i.e., X rays and gamma rays). Different nuclear materials emit different blends of these radiation types. This radiation, unless blocked by layers of matter (shielding), reveals the presence of the nuclear material. The use of nuclear detection devices or radiation detectors is thus, key to monitoring for the presence of radioactive substances. The arms-control monitoring programs of the International Atomic Energy Agency, for example, depend heavily on both automated and hand-carried detection devices that seek to measure the telltale radiations emitted by nuclear materials.
Furthermore, radiation can cause illness, injury, or death. A single fast particle, X ray, or gamma ray can damage a DNA molecule so that a healthy cell is converted to a cancer cell, and sufficiently large numbers of particles or rays can disturb enough of a cells' molecules to kill it. Therefore, nuclear detection devices are also used to alert to releases of radioactive material, whether deliberate (e.g., caused by a "dirty bomb") or accidental (e.g., material escaping from a nuclear power plant, waste-storage facility, or fuel-reprocessing plant).
To be detectable, radiation must be partly or wholly absorbed by ordinary matter. Radiation is said to have been absorbed by a mass of material when it has given up most or all of its energy to that material; radiation that is difficult to absorb (e.g., neutrino flow) is correspondingly difficult to detect. There are several different radiation-absorption phenomena, each of which is exploited in the design of a different class of detection devices. The most important form of absorption is ionization, that is, the separation of neutral atoms in the absorbing medium into free electrons (negatively charged) and free ions (positively charged atoms lacking one or more electrons). All forms of radiation mentioned above can cause ionization. Ionization, in turn, can be detected in numerous ways. One way is chemical, as ions, because they lack electrons, readily combine with other atoms to form new molecules. In a photographic film, this recombination appears as the chemical change known as exposure. Film-badge dosimeters measure radiation by accumulating chemical changes in response to ionizing radiation.
A more precise and continuous measure of ionizing radiation is obtained by electronic amplification of individual ionization events. The best known of the tools that measures radiation in this way is the Geiger counter. In a Geiger counter, a voltage is placed across a chamber filled with gas (usually argon or xenon); this causes an electric field to exist between one end of the chamber and the other. When a fast particle or high-energy ray passes through the chamber, it ionizes neutral atoms, that is, splits them up into free electrons and positively-charged ions. Under the influence of the electric field, the electrons accelerate toward one end of the chamber and the ions toward the other. If the electrical field is strong enough, it accelerates them enough so that when they strike other atoms in the gas they ionize them as well. The electrons and ions thus produced may also be accelerated enough to cause ionization, and so on. The resulting brief avalanche of charged particles constitutes a pulse of electrical current that can be detected, amplified, and counted by appropriate circuitry. In the audio output circuit of a Geiger counter, a single ionization event is amplified to produce the device's trademark "click." Although the arrival
of any one ray or particle is a randomly timed event, the average rate of such arrivals, smoothed over time, gives an accurate idea of how much radiation is present.
Another type of radiation-detection device is the scintillation detector. Certain crystals, when struck by a single high-energy photon or particle, produce a scintillation, that is, a flash of light consisting of thousands or tens of thousands of visible photons. In the early twentieth century, one method of measuring radiation was to count scintillation rates under a microscope; modern detectors use electronic circuits for the same purpose.
The interactions of radiation with semiconducting crystals such as silicon can also be measured. Semiconducting radiation detectors have the advantages of small size, high sensitivity, and high accuracy.
█ FURTHER READING:
Delaney, C. F. G., and E. C. Finch. Radiation Detectors. New York: Oxford University Press, 1992.