Answer to Question #2834 Submitted to "Ask the Experts"

I have an Eberline ASP2e meter with a HP360 probe. It reads cpm. I have an item suspected to be contaminated with ²³²Th Fluoride. Readings are approximately 7,000 cpm. How can this number be interpreted to know if it is safe for people to be working with these items? How do I convert the reading if the probe's efficiency to ²³²Th is not known?

You should look at some of the other Q&A in this topical area of ATE. In particular, see Q2429 relating to calibration of alpha probes. With regard to your questions, you need to calibrate this probe with a traceable ²³²Th source for its respective 4.0 MeV alphas. Alternately, you might use a traceable depleted ²³⁸U source, where most of the alphas will be from ²³⁸U, and a lesser degree from ²³⁴U (which has a slightly higher alpha energy). Next you need to know the active area of the probe in square centimeters (cm²). With a calibration efficiency factor, you'll be able to convert from observed cpm to dpm and then scale to "expected" dpm per 100 cm² from the known area of the probe. This can then be compared to recommended levels of ²³²Th contamination. The U.S. Nuclear Regulatory Commission Reg. Guide 1.86, for release of surface-contaminated materials, gives a limit of 1,000 dpm/100 cm² for ²³²Th. Given the intake concentrations for ²³²Th are quite low, and ²³²Th fluoride may be ingested or inhaled in a work setting, I would seek assistance from a professional health physicist.

David J. Allard, CHP