The Geiger counters we feature detect some or all of the four major types of ionizing radiation, namely Alpha, Beta, Gamma, and X-rays. Both Gamma and X-rays are part of the electromagnetic spectrum, specifically at its high frequency, short wavelength end. That same spectrum also includes the more familiar ultraviolet light, visible light, infrared rays, microwaves, and radio waves, listed in order of decreasing frequency and increasing wavelength from Gamma and X-rays.
As the four major types of ionizing radiation go, Gamma and X-rays are very powerful and potentially very dangerous. They can pass through virtually anything, and are effectively shielded or absorbed only by materials of high atomic weight such as lead. Because of their penetrating ability, X-rays are used to see inside the human body, destroy cancer cells in radiation therapy, or analyze the internal structure of rocks and minerals, for instance.
Gamma rays are produced naturally by the sun and other bodies in outer space, their transmission to earth being known as "cosmic radiation". Certain minerals that make up part of the earth containing the radioactive elements Uranium and/or Thorium also emit Gamma rays. This cosmic radiation, along with these radioactive earth minerals, combine to produce the "background count" of a Geiger counter. That is, even when a Geiger counter is removed from a specific radioactive object, the meter will still register a background level or count of radioactivity. This might typically be in the range of 20 counts per minute, but will vary depending upon where it is on the earth. The background count should always be factored in or "subtracted" from the overall reading derived from a specific radioactive source. X-rays, being very similar to Gamma rays, are produced from man-made sources such as X-ray tubes, arcs, and lamps.
While not part of the electromagnetic spectrum, Alpha and Beta particles are the two other types of ionizing radiation detectable by Geiger counters. Alpha radiation consists of positively charged particles emitted from the nucleus of an atom in the process of decay. These particles are also very dense which, with their strong positive charge, precludes them from penetrating more than an inch of air or a sheet of paper. Because of this, Alpha particles are not a serious health hazard, except when they are emitted from within the body as a result of ingestion, for instance, when their high energy poses an extreme hazard to sensitive living tissue. Radioactivity of this nature is difficult to detect using a standard Geiger counter, but since most Alpha-emitting substances also produce Gamma rays, this can allow for detection in some cases.
Beta radiation consists of negatively charged particles emitted from an atom in the process of decay. These particles are relatively light and can penetrate somewhat better than an Alpha particle, though still only through a few millimeters of aluminum.
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