Why is a pancake probe reading on a smear conveyed in dpm while a direct frisk is conveyed in cpm?
Thank you for your question.
When you hold a pancake probe up to a surface and frisk it directly, the measurement is dependent on probe distance, angle, source distribution (localized vs spread), and other geometrical considerations. Because those variables are uncertain and hard to control in the field, the result is typically reported in count rate (cpm) per area surveyed.
A smear sample, on the other hand, is a more controlled measurement. A defined surface area is wiped using a smear, and the smear is placed in a reproducible counting geometry. One can then use the detector efficiency for the nuclide of concern to convert the count rate (cpm) to activity (dpm) per area swiped.
Note that it is fully possible to report a direct frisk in dpm over the area if you carefully control the variables mentioned above.
I hope this answers your question.
Deepesh Poudel, Ph.D., CHP
Answer posted on 6 October 2025. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, 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. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.