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Alpha particles, beta particles, gamma rays, and X-rays are measured using the par- ticle’s energy to produce an amplified pulse of electric current in a detector. These pulses are counted to give the rate of disintegration. Three types of detectors com- monly are encountered: gas-filled detectors, scintillation counters, and semiconduc- tor detectors. The gas-filled detector consists of a tube filled with an inert gas, such as Ar. When radioactive particles enter the tube, they ionize the inert gas, producing a large number of Ar+/e– ion pairs. Movement of the electrons toward an anode and the Ar+ toward a cathode generates a measurable electric current. A Geiger counter is one example of a gas-filled detector. A scintillation counter uses a fluorescent material to convert radioactive particles into easily measurable photons. For exam- ple, one solid-state scintillation counter, consisting of a crystal of NaI with 0.2% TlI, produces several thousand photons for each radioactive particle. Finally, in a semi- conductor detector the adsorption of a radioactive particle promotes thousands of electrons to the semiconductor’s conduction band, leading to an increase in its con- ductivity. Further details about radiation detectors, and the signal processors used to count particles.
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