Answer to Question #3280 Submitted to "Ask the Experts"

First, there really isn't an easy direct comparison with a Geiger counter reading to radon concentration in air. 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 radiation detectors. Some Geiger counter tubes can be designed and built with a 2 mg/cm² mica window that will detect alpha particles (for example, from radon decay products) with a fair efficiency (~15 percent for a 4 MeV alpha). The "pancake" type G-M tube has a 15 cm² window area. These counter tubes basically count particles entering the sensitive gas volume by causing 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.

There is an old radon measurement approach (that is, the circa 1956 Kusnetz method) where one would take an air filter sample and then place the filter in intimate contact with a ZnS scintillator 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, one is only measuring alpha particles with a ZnS detector, unlike the Geiger counter, and this is not a very accurate approach because of the unknown degree of radon decay product equilibrium.

You can go to the U.S. Department of Energy's Environmental Measurements Laboratory (EML) Web site for the HASL-300 manual, which has good descriptions of various health physics survey techniques and radiochemical methods. This link to the EML online HASL-300 procedures manual includes radon. EML staff have done a lot of work in this area. Radon measurements can be made via Electret Ion Chamber, Pulse 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.

Many radon-measurement service providers can supply an inexpensive charcoal canister for a short-term measurement or an alpha track etch detector to make a long-term radon measurement. Given the relatively low 6.2 pCi/L measurement before the renovations, you might want to do a year average with an alpha track etch detector. That should cost about $25.

You may want check with your state radiation control program; it may perform the measurement or provide a list of radon testers. For example, see the Pennsylvania Bureau of Radiation Protection - Radon Web site. Here you will find links to a list of certified testing labs and other helpful information.

David J. Allard, CHP