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24 May 2012

Answer to Question #3497 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

For some time, I've wanted to obtain a Geiger-Müller (GM) counter. The less-expensive models do not seem to be particularly rugged or accurate, while the better ones are (in my opinion) overpriced. Thus, I have decided to construct my own instrument. I'm an electrical engineer, so this is not an unreasonable solution. I plan to use an enclosure made of ABS (acrylonitrite butadiene styrene) plastic, and the end-window GM tube will be mounted within this case. I will drill a hole in the end of the case to allow alpha particles to reach the tube's window unimpeded. My question is this: should I also cut a hole in the bottom of the case to allow beta particles access to the side wall of the tube—or will those pass through the ABS plastic?

If you're going to build your own Geiger-Müller counter, you may want to purchase or borrow a copy of the textbook, by Glenn Knoll, Radiation Detection and Measurement. You may want to also review some of the other Q&A on G-M tubes—for example, Q761, Q1590, and Q2255. To answer your question, you need that hole exposing the 2 mg/cm² mica window on the GM tube to detect alpha particles. Beta particles will also pass thru this window, as will more-penetrating gamma rays. A GM counter will not discriminate between alpha, beta, or gamma rays. If you place a sheet or two of paper over a source emitting primarily alphas, you'll see the count rate response go to "background" levels. Beta emitters have a wide range of energies, for example, from 18 keV (max) from tritium (³H) to 2.28 MeV for ⁹⁰Y. The tritium beta you will not detect, even with an end window GM tube, but a "thin (side) walled" GM will detect many mid- to high-energy betas. However, I'm not aware of anyone who manufactures a mica end window GM tube that also has a standard 30 to 60 mg/cm² thin side wall. Bottom line, don't worry about opening up the side wall with the end window GM tube.

David J. Allard, CHP

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