A. This conversion is necessary, as we are required by our NRC license to document wipe test results in dpm. Many counters will do the conversion for you. If yours does not, you'll need to do the conversion manually before recording results. Following is the formula to use to convert cpm to dpm: dpm = cpm - BKG efficiency As you can see, you will need to count a background vial (be sure it is in the first position IF your LSC has first position background subtraction!) and know the efficiency of your liquid scintillation counter (LSC) for the isotopes you're counting. Many LSC operating manuals will list this information. If yours does not, you can use the following common efficiencies: P-32 (95%) S-35 (80%) H-3 (50%) C-14 (60%) For more assistance in determining efficiencies, please call Radiation Safety.