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Geiger-Müller tubes on Wikipedia

The original design of the g-m tube by Hans Geiger and E.W. Mueller in 1928 hasn't changed much and the tube's sensor function remains the same. Radiation, as it passes through the GM tube, ionizes the gas within the tube. The ionization initiates a momentary avalanche of electrons accelerate by the high voltage potential used to power the GM tube. This avalance, create a momentary conductive path between the wire at the center of the tube (Anode) and the wall of the tube (Cathode), resulting in a 'click' sound. By measuring the number of the clicks, the instrument indicates the radiation levels.The GM tube sensor is the heart of the Geiger counter and to be a Geiger counter, the device has to contain a Geiger-Muller (GM) tube.

Inexpensive GM tubes like the SBM-20 Russian Geiger counter tube shown to the right, only detect beta, x-ray and gamma radiation. So if your Geiger counter uses one of these styles GM tubes it is blind to all alpha particle radiation.

More expensive GM tubes, like the LND-712 Geiger Counter tube have a thin mica window that allows alpha particle radiation to penetrate into the interior of the GM tube and ionize the gas for detection. It is generally more fragile than the beta-gamma GM tubes because of the thin mica window. The mica window allows one to detect the alpha radiation from radium and polonium. This type of Geiger counter may also be used for prospecting, experiments, and general field work. In addition, it can measure total radiation from materials including alpha radiation. Most laboratory grade Geiger counters use this style tube.

Selection of different Geiger-Müller tubes on sale

Alpha Particle Radiation were found to be the nuclei of helium atoms, two protons and two neutrons bound together. Alpha rays have a net positive charge. Alpha particles have the weak penetrating ability; a couple of inches of air or a few sheets of paper can effectively block them.

While alpha particles are stopped by a piece of paper, the situation changes once alpha radiation is inside the body, then the effects are more harmful than beta or gamma radiation. (See “Q” factor in testing food for radioactivity.) So materials that radiate Alpha particles are harmful if ingested or inhaled. But are not dangerous when external to the human body.

Beta Radiation were found to be electrons, identical to the electrons found in atoms. Beta rays have a net negative charge. Beta rays have a greater penetrating power than Alpha rays and can penetrate 3mm of aluminum.

Gamma Radiation and X-rays are high-energy electromagnetic radiation (high energy photons). This classification of radiation has the greatest penetrating power. High energy gamma rays are able to pass through several centimeters of lead and still be detected on the other side. Gamma rays are produced naturally from the decay of some radioactive materials. X-rays on the other hand, is man-made radiation used in medicine and dentistry. While x-rays are man made electromagnetic radiation their frequency is so high that the radiation is also ionizing.