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- Geiger Counters
A typical Geiger counter consists of a Geiger-Mueller tube, a visual readout, and an audio readout. The Geiger-Mueller tube or detector is the heart of a Geiger counter. It is a type of ionization chamber that counts particles of radiation. That count is read by the user through a visual readout in the form of a traditional analog meter, or an electronic LCD (liquid crystal display) readout. These meters are available in different units, including mR/hr, or milli-Roentgens per hour (popular in the US and Israel), and µSv/hr, or micro-Sieverts per hour (popular in Canada and overseas ).
Most Geiger counters also have an audio readout that sounds one "click" for each particle count. These particles are emitted at random intervals, and a large number of particles produced in a short span of time sound almost like static from a radio. Play this sound file as an example. Geiger counters that have meter readouts in CPM, that is, Counts or Clicks per Minute, mimic the audible clicks in visual form. CPM is the unit normally used to measure Alpha and Beta radiation.
Digital Geiger counters not only offer visual digital readouts on an LCD display, but they typically also have audio ports for external speakers, as will as data ports for readout on computers and data loggers.
In contrast to vintage 1950's civil defense models, modern Geiger counters are built around transistorized, solid state electronics, and are powered by easily replaceable batteries.
Another feature found on some Geiger counters is an open window over the Geiger-Mueller tube. This allows for energy discrimination, that is, the determination of what type of radioactivity the unit is measuring, between Gamma and X-rays that are strong enough to pass right through the housing of the Geiger counter, versus Beta and Alpha radiation, which are too weak to pass through the housing, but can be read through the open window of the detector. Geiger counters don't incorporate any type of switch to discriminate between Gamma, X, Beta, or Alpha radiation, but instead make that distinction by the process described below:
Position the Geiger counter near the suspected source of radioactivity, without aiming the open window at the source. If you are getting a reading, it is most likely Gamma and/or X-radiation or high energy Beta, and can be read in terms of mR/hr or µSv/hr.
Next, place an 1/8" thick piece of aluminum between the instrument and the source. If the radioactive indication stops or decreases, it is most likely Beta radiation, and can be read in terms of CPM. Most common isotopes emit both Gamma and Beta radiation.
Now aim the open window of the Geiger counter at and immediately next to the suspected source of radioactivity. If this gives you a reading versus no detection through the housing itself, then the radioactivity is from Alpha, Beta, or low energy Gamma. Next, place a piece of aluminum foil between the open window and the radioactive source. If the radioactive indication stops, it is likely Beta radiation, and should be read in terms of CPM.
Now do the same test with a sheet of paper between the open window and the radioactive source. If the radioactive indication stops, it is most likely Alpha, and should be read in terms of CPM.
In the course of your readings, be careful not to contaminate the detector with radioactivity by physically touching the radioactive source or by holding the source above the open window of the Geiger counter.
Unless otherwise mentioned, all of our Geiger counters come pre-calibrated from the factory. You should re-calibrate your Geiger counter as often as your regulations require, or in any case, at least once a year. Follow the manufacturer's instructions in the operating manual that comes with your unit for guidance on re-calibration.
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