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11 October 2008

Answer to Question #761 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:

Even with a thin absorber in a GM one can detect radioactivity (alpha-beta-gamma) from the ground and water or there is a specific instrument to do that?

Geiger-Mueller (GM) tubes are interfaced with many different health physics instruments and are produced by numerous manufacturers. (Links to these manufacturers can be found on the HPS Web site, as many are HPS Affiliate Members.) The GM tubes are normally placed in the instrument or probe and come in many different overall sizes. Several basic designs of thick-wall gamma, thin-wall (~ 15 mg/cm sq.) beta/gamma, and mica window (~ 2 mg/cm sq.) alpha/beta/gamma detecting GM tubes are available. The "pancake" style mica window GM tube or probe is perhaps the most versatile and useful for health physics applications. The technical parameters for the "pancake" GM tube are as follows:

a radiation causes a complete gas discharge and large electronic output pulse; thus
it will not discriminate alpha/beta/gamma radiation;
a thin covering of aluminum foil over the window would attenuate the alpha radiation;
the window is ~ 15 cm square in area, and multiplying by factor of 6.7 will yield results per 100 cm sq;
it has a Navy 8767 MIL Spec ⁹⁹Tc beta efficiency of 0.10 count per disintegration (c/d);
it will typically detect betas of ¹⁴C at 0.03 c/d and ⁹⁰Sr/⁹⁰Y at 0.30 c/d;
it will detect a 4.6 MeV ²³⁰Th alpha radiation if in near contact with the source (eff. ~ 0.10 c/d);
it has a poor gamma detection efficiency, but will detect 10 keV and above photons;
and the ¹³⁷Cs gamma response is about 3,500 counts/minute per milliroentgen/hour.

Lastly, it is important to note that any instrument will need to be calibrated to National Institute of Standards and Technology traceable standards prior to use, and the above data are only approximate. These standard sources are most often plated, or have minimal self absorption due to aluminized mylar coverings. If the radioactive material is distributed in a soil or water matrix, the appropriate reduction in detection efficiency will need to be determined. David J. Allard, CHP

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