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B.    50 µR/hr is correct

C.    30 CPM is also correct


According to the SOG, decontamination would be achieved at 49 µR/hr or 29 CPM.


There have been some published recommendations for the use of 1000 CPM as a decontamination level. In order to use activity accurately, you must first know the isotope and then calibrate your instrument to that isotope. Otherwise, using a detector set up for activity is like using an LEL meter and not knowing the flammable material. Until you know the flammable material and use the provided conversion chart, the number means nothing as far as the actual percent of LEL. It's only providing a yes or no answer that a flammable is present. This is the same situation with radioactive isotopes. A number on a detector measuring activity simply gives you a yes or no answer and tells you nothing about the actual amount of gamma radiation present, unless it has been calibrated to that isotope. And there are no conversion charts for activity based on a particular isotope and there are no simple mathematical equations to do a conversion.

Here is an example:

1 curie of activity of americium-241 would result in a gamma exposure rate of 320 mR/hr at 3 feet from the source, while 1 curie of activity of cobalt-60 would result in a gamma exposure rate of 1400 mR/hr at 3 feet from the source! The actual exposure rate for Co-60 is nearly 5 times greater than Am-241 for the same amount of activity!


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