Answer to Question #82 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 have a GM counter that reads in dps for beta and I need to record my results in mrem h^-1. What is the formula?

It depends somewhat on the type of GM detector being used and possible differences in calibration. For a side-window type detector, a dose-rate conversion factor of 2000 cpm/mrad/h (open-window) has been used by our group in the past for measurement of beta dose rates from ²³⁸U. I have personally never seen the data on which this conversion was based, however it is highly consistent with observed values for GM-pancake data that has been collected since. For a pancake-type GM detector, values have been measured (at ORNL) and have ranged from about 2300 up to 4000 cpm/mrad/h with an open-window probe.

The most probable response for a pancake and the selection that is most often used by our group is 2400 cpm/mrad/h and appears to be valid over a fairly broad beta energy range. Note that these conversion factors are for estimating dermal dose by beta irradiation which means that it is the dose rate at 70 um beneath the surface of the skin. Also note that the conversion assumes that count losses are negligible or have already been accounted for otherwise.

Robert Coleman
Oak Ridge National Laboratory

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