Answer to Question #1001 Submitted to "Ask the Experts"

One microcurie is equivalent to 2.22 × 10⁶ dpm or 2,220,000 dpm. To convert from dpm to microcuries, divide the number of dpm by the above factor; that is, if C is the number of dpm, activity in microcuries is: C / 2.22 × 10^6.

Example calculation: C = 1.776 × 10⁷ dpm Equivalent activity in microcuries: (1.776 × 10⁷ dpm) / (2.22 × 10⁶ dpm/microcurie) = 8 microcuries.  

If this question had dealt with the conversion of count rate to activity, we would first had to have made use of an appropriate counting efficiency to convert from a count rate to an activity. Note that it would be mistaken to use the observed count rate as the activity in a source, in dpm in this example, because radiation counters do not 'see' every disintegration due to such factors as the geometric relationship between the radiation counter and the source, the stopping power of the medium in the detector, and other factors.  Moreover, the radionuclide for which an activity determination is desired may not emit one 'ray' per disintegration. For example, approximately 9 gamma rays are emitted per 10 nuclear disintegrations for the radionuclide most commonly used in nuclear medicine (^99mTc).

Roger W. Broseus, CHP, PhD