Radiation Detectors

Radiation detectors are used for a variety of purposes including medical diagnoses, radioactive dating measurements, measurement of background radiation, activity level and radiation dose. Radiation passing through matter can cause severe damage. The degree and type of damage depends on several factors including the type and energy of the radiation and the properties of the medium. Radiation damage in biological organisms is primarily due to ionization effects in cells. The normal operation of the cell may be disrupted when highly reactive ions or radicals, thought to lead to cancers, are formed as the result of ionizing radiation. Radiation detectors can be used to monitor area or personal exposure.

Radiation detectors can use many different technologies, the most popular being ionization chambers, proportional counters, Geiger-Mueller counters, scintillation detectors, semiconductor diode detectors, and dosimeters. An ionization chamber detector applies an electric field across a volume of gas. Electrons become attached to neutral molecules to form negative ions. When an electric field is applied to the gas, the ions drift along the lines of force to produce an ionization current. Proportional counters work by increasing the electric field strength at the center electrode of a pulse ionization chamber above a certain level. The size of the output pulse from the chamber increases but is still proportional to the initial ionization. This method allows detection of a very low initial ionization, even down to a single ion pair. Geiger-Mueller counters use the ionization of a medium as the basic detection process. They consist of a cylindrical metal tube that is filled with a gas at low pressure and a long wire along the axis of the tube. When a high-energy particle or photon enters the tube through a thin window at one end, some of the atoms of the gas become ionized. An avalanche of electrons results in the region of intense electric field near the wire, producing a current pulse at the output of the tube. After the pulse is amplified it can be used to trigger an electronic counter or delivered to a loud speaker, which clicks every time a particle is detected.

Scintillation radiation detectors use a solid or liquid material whose atoms are easily excited by incoming radiation. These excited atoms emit visible light when they return to their ground state. Common materials used as scintillators are crystals of sodium iodide and certain plastics. If such a material is attached to one end of a photomultiplier tube the photons emitted by the scintillator can be converted to an electric signal. This pulse can be sent to an electronic counter. The scintillator device is much more sensitive than the Geiger counter, mainly because of the higher density of the detecting medium. A semiconductor diode detector is a reverse-bias P-N junction. As an energetic particle passes through the junction, electrons are excited into the conducting band and holes are formed in the valence band. The internal electric field sweeps the electrons toward the positive (n) side and the hole toward the negative (p) side. This creates a pulse of current that can be measured with an electronic counter. A dosimeter is a detector designed to measure radiation dose or exposure. Dosimeter types are pocket (ion) chambers, film badge, thermoluminescent device (TLD) and neutron moderation dosimeters.

Some important specifications for radiation detectors are the range they measure and what type of radiation they detect, whether alpha particles, beta particles, gamma rays or X-rays. Also important is the type of output. Are these detectors measuring dose, dose rate or count rate? Outputs can be analog or digital, including computer signals.

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