Answer to Question #8162 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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


We are buying a lot of sheet metal (iron) from various sources, which we use to make mechanical boxes. Recently, we learned that sheet metal (in our country) might contain radioactive material, which could have entered during the reprocessing of scrap metal. We identified a radiation meter with the following specifications. The meter has an external probe to detect alpha, beta, and gamma radiation. It detects gamma rays greater than 30 keV, beta particle energies greater than 100 keV, and alpha particles greater than 2 MeV. The detector probe is a halogen quenched GM tube with a measurement range of 0 to 10,000 counts per second (cps), with an end window active area of 6.1 cm2, and a window thickness of 1.5-2 mg cm-2. The meter's sensitivity should be about 1.0 cps for a 90Sr/90Y source of 37 Bq (dps) per 100 cm2. We would like to know if this meter is capable of measuring low levels of radioactive contamination from sheet metal.


The meter and probe you describe, with the probe's "thin" window and a 6.1 cm2 active area, is a good radiation detector to detect alpha, beta, and gamma radiation. I prefer one with a larger window of about 12 to 15 cm2 active area, but the one you have should work okay for screening the sheet metal for radioactive contamination. You should hold the probe as close to the sheet metal as possible with the window facing the metal and scan for radiation by moving the probe only about 2 cm per second. Spot checks on each sheet should easily determine if the metal is contaminated above the naturally occurring background count rate of about 0.4 to 1.7 counts per second, which is typical of many areas.

John P. Hageman, MS, CHP

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