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07 February 2012

Answer to Question #236 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Conversion of survey meter readings to dose

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

I am using ³²P mounted on glass microscope slides (irradiated apatite grains). I need to record dose in microSievert but my measurements are in cps. I have a Berthold LB 1210B survey geiger counter with a counting area of 100 cm2. For this instrument, the calibration factor for ³²P is .026 Bq /cm /s. The source is a sheet about 1.5 cm2 and let's say that I am measuring at 1 cm. Can you give me a simple conversion formula? Thanks!

It appears that the calibration factor given is incorrect. It is therefore assumed that the calibration factor for ³²P is closer to 0.5 c/s per Bq--which would be similar to other thin-window GM detectors. As such, please refer to my work site and "Ask the Experts" question #82 for brief discussions of some beta dose-rate conversion factors. The values discussed in the above links are for pancake detectors and assume uniform irradiation across a surface area equal to 15.5 cm². The data can easily be scaled for other thin-window detectors and exposure areas as well. For an exposed area equal to 1.5 cm², the number of counts observed per unit dose absorbed will be an order of magnitude less- or about 24,000 to 30,000 cpm per mGy/h (400 cps per mSv/h).

Robert L. Coleman Oak Ridge National Laboratory

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