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:

Q

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.

A

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
 

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 17 March 2009. 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.