Answer to Question #1509 Submitted to "Ask the Experts"

Q

How do you convert experimentally measured activity in cpm to curies?

A

Regardless of the radiation detector you are using—for example, Geiger-Mueller tube, sodium iodide crystal, plastic scintillator, etc.—the interaction of the radiation with the detector element will cause an electronic pulse. This in turn is recorded by a meter or scaler and displayed as a "counts" or a counts per unit time, for example, counts per minute or cpm. Because of geometric coverage and interaction probabilities, the detector will not be 100% efficient at detecting all the events from the radioactive source being monitored. Thus, the decays or "disintegrations" per unit time are not fully represented. To obtain the true disintegrations per unit time (for example, per minute) or dpm one must apply an efficiency factor. As noted, this must be determined experimentally by using a radioactive source similar in radiation type, energy, geometry, etc., to what is being measured. Traceability to a national standards organization is a must with the reference source. Once the fractional detection efficiency (E) is determined, then you can divide the counts to obtain disintegrations. Once you have the disintegrations per unit time, a conversion factor is applied to obtain the number of curies (Ci) or becquerel (Bq). A Bq is one disintegration per second. A Ci is 3.7×10¹⁰ disintegrations per second (dps). Thus, for your problem you would follow these calculational steps:

1. dpm = (cpm / E),
    
2. dps = (dpm / 60 seconds per minute),
    
3. Ci = (dps / 3.7×10¹⁰)

David J. Allard, CHP