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10.2: Radiation Detectors

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  • Radiation detectors convert energy from radioactive sources to electricity. Excessive radiation can be harmful to people, and humans cannot sense radioactivity. We can only measure it indirectly. For these reasons, radiation detectors are used as safety devices. Radiation can be classified as alpha particles, beta particles, gamma rays, or neutrons [37, p. 404]. Alpha particles are positively charged radiation composed of ionized nuclei of helium. Beta particles are high energy electrons. Gamma rays are high energy, short wavelength electromagnetic radiation. When these three types of radiation interact with air or another gas, they can excite or ionize the atoms of the gas. Flowing ions are a current, so this process converts the radiation to electricity. Types of radiation detectors include ionization chambers, Geiger counters, scintillation counters, and photographic film based detectors [37].

    Ionization chambers and Geiger counters work on the same principle. In both cases, a gas is enclosed in a chamber or tube, and a voltage is applied across the gas [37]. Incoming alpha particles, beta particles, or gamma rays, ionize the gas. Due to the applied voltage, positive ions flow to one of the electrodes, and negative ions flow to the other electrode thereby forming a current. Geiger counters operate at higher voltages than ionization chambers. The voltage between the electrodes in an ionization chamber may be from a few volts to hundreds of volts while the voltage is a Geiger counter is typically from 500 V to 2000 V [37]. Many smoke detectors are ionization chambers [152]. When no smoke is present, radiation from a weak radiation source ionizes air between the electrodes, and a current is detected on the electrodes. When smoke is present, it scatters the radiation, so no current is detected [152]. In an ionization chamber, each incoming radioactive particle causes a single atom to ionize. In a Geiger counter, an incoming radioactive particle causes an atom to ionize. Then, the ions formed ionize additional atoms of the gas, and these ions ionize additional atoms forming a cascading reaction powered and maintained by the voltage gradient which accelerates and separates the ion pairs. Geiger counters are often more sensitive due to this amplification of the current produced.

    Scintillation counters and photographic film based detectors involve an additional step in converting radiation to electricity. A scintillation counter is often made from a crystalline material such as sodium iodide [37]. Sometimes a phosphor is also used [5, p. 166]. Incoming radioactive particles excite, but do not ionize, the atoms of the material. These atoms then decay and emit a photon. Semiconductor or other types of photodetectors convert the photons to electricity [37]. In photographic film based detectors, incoming radioactive particles expose the film thereby changing its color [37]. Materials used in the film include Al\(_2\)O\(_3\) and lithium fluoride [153]. Again, photodetectors are used to convert the information recorded on the film to a measurable signal. Scintillation counters can be higher sensitivity than other types of radiation detectors, and they can be used to determine the energy of incoming radiation by spectroscopy [154]. The film based detectors can be worn as a ring or badge. These type of detectors are used, for example, by radiology technicians and by nuclear power plant employees. These detectors must be sent in to a lab to be analyzed, and both the amount and the type of radiation can be determined [153].