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Answer to Question #1208 Submitted to "Ask the Experts"Category: Instrumentation and Measurements — Surveys and Measurements (SM) The following question was answered by an expert in the appropriate field: Q
I was wondering if you could tell me how a Geiger counter works. I am doing research on radon. In one of my sources it said that radon was used in the Geiger counter to detect other radiation.
A
The book Radiation Detection and Measurement by Dr. Glenn F. Knoll (from John Wiley & Sons, New York) is an excellent text on the subject of Geiger counters and other detectors. Geiger counter tubes can be designed and built with a 2 mg cm-2 mica window and will detect alpha particles (for example, from radon decay products) with a fair efficiency (~ 15 percent for a 4 MeV alpha). The "pancake" type tube is often used today with a 15 cm2 window area. These Geiger counter tubes basically count particles entering the sensitive gas volume by going into a complete gas discharge, which in turn generates about a 50 millivolt size electrical pulse. This makes the counting electronics simple, but there is no discrimination of particle type or energy of the alpha, beta, gamma, or x rays. The counting gas for modern Geiger counter tubes is normally neon, with a trace halogen (for example, Cl and/or Br) to help quench free electrons emitted from the cathode from causing double pulsing or continuous discharge. Radon is not used as the counting gas, nor am I familiar with any techniques where air samples are mixed in a flow counter arrangement. That's not to say it might not work, if calibrations were appropriate. There is an old radon measurement where one would take an air filter sample and then place the filter in intimate contact with window and make at least two gross counts over a known time interval (that is, with the radon decay products on the filter decaying in count rate). However, this is not a very accurate approach because of decay product equilibrium. Go to the Department of Homeland Security's Environmental Measurements Laboratory (EML) Web site for HASL-300, the online procedures manualfor various radionuclides, including radon. EML staff has done a lot of work in this area. Radon measurements can be made via Electret Ion Chamber, Alpha Track Detector, Charcoal Canister, Continuous Working Level Monitor, Continuous Radon Monitor, and Liquid Scintillation. There are many manufacturers of such equipment and instruments. You may want check with your state radiation control program, for example; the Pennsylvania Department of Environmental Protection Web site will link you to a list of certified testing labs. David J. Allard, CHP
Answer posted on 25 September 2001. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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