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Converting From CPM to mR/hrMany radiation survey meters are calibrated to Cs137 or Co60. For Co60, it turns out that the conversion factor for the GM-10 to convert CPM to uR/hr is very close to 1 (0.93, but 1 is close enough for most purposes). This means that 1080 CPM is about equal to 1 mR/hr for Co60. For Cs137, it is closer to 1200 CPM for 1 mR/hr. For those who prefer to use Sieverts, 108 CPM is about 1 uSv/hr for C060, and for Cs137 it is around 120 CPM per 1 uSv/hr.For the GM 45, the numbers are closer to 3600 CPM for Co60 and 3000 CPM for Cs137, or 360 CPM and 300 CPM per per 1 uSv/hr, respectively. Our Rad software lets you enter in a multiplier, which you can use to convert the readings into whatever units you prefer. You would want to enter in the reciprocal of the number of CPM per unit. See the table below:
Cs137 Co60
mR/hr uSv/hr mR/hr uSv/hr
GM-10 0.000833 0.00833 0.000926 0.00926
GM-45 0.000333 0.00333 0.000277 0.00277
There is a graph available showing the relative sensitivity for other gamma energies. For example, at 60 keV (Americium 241) it is 7 for radiation coming through the side. So you would get about 7000 CPM for 1 mR/hr. It is closer to 5 for radiation coming into the front of the tube, through the mica window. There is a similar graph for the GM-45 also. You may also wish to take a look at our page that briefly describes the various units of radiation If you're interested in measuring your local radiation levels, our line of radiation detectors may be of interest to you.
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Thinking of buying a surplus CDV-700 or 715 detector? Be sure to read our report first.
