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18 March 2010

Answer to Question #2881 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Basic dose information, dose quantities, units

The following question was answered by an expert in the appropriate field:

Is dpm the same as cpm? Is the conversion factor for converting Ci to dpm the same as the one for conversion of Ci to cpm?

Disintegrations per minute (dpm) and counts per minute (cpm) are not the same thing. Dpm describes the activity of a source—the rate at which atoms in a source are decaying, in this case the number of atoms per minute. Cpm, on the other hand, describes the rate at which events are being registered by an instrument—the number of counts per minute. With a source in place, the disintegration rate and the count rate are, of course, related. If the counter is 100% efficient and background radiation is negligible, then the two rates are numerically equal. Otherwise, the count rate is less than the disintegration rate. Under a fixed set of conditions, an instrument could be calibrated to determine activity from the measured count rate.

James E. Turner, CHP, PhD

Answer posted on 24 August 2003. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.