Natural radioactivity is part of the environment and people receive between 100 and 500 millirems each year from natural sources. The actual amount received varies with certain activities and situations that increase the yearly exposure to radiation. In general, the exposure for the average person is about 130 millirems per year. When radiation passes through the probe of a Geiger counter it ionizes some of the gas atoms in the tube, releasing free electrons. The electrons are accelerated by an electric field in the tube and an avalanche of free electrons creates a pulse of current that is amplified and measured.
More radiation means more avalanches, so the pulses are an indirect means of measuring radiation.
Each avalanche makes a pop or click when the circuit includes a speaker or earphone. The activity of a radioactive source is a measure of the number of nuclear disintegrations per unit of time. See Table 15.4. All nuclei with an atomic number greater than 83 are outside the upper right limit of the band of stability. Emission of an alpha particle reduces the number of protons by two and the number of neutrons by two, moving thee nucleus more toward the band of stability.