Answer to Question #3610 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

I perform surveys and the specifications say that areas must be less than 200 dpm. All my readings are in cpm. The efficiency of the GM detector I use is stated as +/-10%. I am reading the background at about 20 cpm. Having the formula dpm = (cpm indicated - cpm background)/detector efficiency, I am getting a result that 200 dpm is the same as 200 cpm, using 0.9 for detector efficiency (for 90%). I have a feeling that 200 dpm should not convert perfectly over to cpm like this. Am I making an error somewhere?

Your calculation is correct, and it just works out that given this background count rate and detector efficiency, 200 cpm is exactly equivalent to 200 dpm. If you change the gross count rate (that is, "indicated") above or below 200 cpm, you'll note the result will begin to deviate from the original number to a greater degree. I'd also note that with contamination surveys, you want to compare the result to some surface contamination limit, for example, 1,000 dpm per 100 cm square, which is a common regulatory criteria for most beta/gamma emitters (from Nuclear Regulatory Commission Regulatory Guide 1.86). Thus, one should know the detection area of the GM probe you're using to further adjust observed readings. I suspect someone has done this calculation by setting 200 dpm as survey limit.

David J. Allard, CHP

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