Answer to Question #2429 Submitted to "Ask the Experts"

Q

While using a military detector (AN/PDR77) connected to an alpha probe, I obtain a reading of 500 cpm. I later found out that the source was americium-241. How can I convert the cpm to Bq or Gy or any other measurement dealing with dose?

A

You need to calibrate your alpha probe with a traceable alpha standard source. This is most often done using a common plated source of thorium-230 or plutonium-239, though an americium-241 source may be available. Americium-241 has about a 5.4 MeV alpha, so calibration with the 5.1 MeV plutonium-239 alpha will be conservative. Using the 4.6 MeV thorium-230 alpha may be too conservative. Knowing the 4-Pi alpha emission rate from your standard source, you can experimentally determine the detection efficiency in counts per disintegration (c/d). One disintegration per second (dps) is a becquerel (Bq). You should also know the area of your detector in square centimeters (cm²) so you can compare acceptable contamination standards, e.g., the Nuclear Regulatory Commission Regulatory Guide 1.86 has limits stated in dpm/100 cm².

With alpha contamination you're not concerned about dose from direct contact, as the alpha wouldn't penetrate the dead layer of the skin (~ 40 mg/cm² on palms and 7 mg/cm² for other areas of the body). Here ingestion or inhalation from airborne activity is the concern.

If it were beta activity, you could use the Loevinger semiempirical point source function for a thin source with "infinite" dimensions.

David J. Allard, CHP