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23 July 2008

Answer to Question #573 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Naturally Occurring Radioactive Material (NORM)

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

For the past few years I have been collecting and selling various minerals. Recently I have discovered a very large deposit of Autunite. I have a CDV 700 geiger counter which peaks out with the probe up to 1 inch away from a specimen and the shield open. That's over 30,000 counts/minute. With the shield closed the readings are 2 to 5 mR/Hr. How dangerous is this amount of radioactivity? What would be safe exposure time to this mineral? Thanks, Ron

To answer this question, I must address a number of somewhat independent issues. The bottom line will be that you really have nothing to concern yourself about as long as you are not accumulating large quantities of the ore.

Measurements made with an old instrument that hasn't been calibrated will be of questionable accuracy. Even with a properly working instrument, measurements made close to a source (e.g., at one inch) can be misleading. This is because accurate readings require that the strength of the radiation field be uniform throughout the volume of the detector (the GM probe) and this will not be the case close to the source.
The portion of your body receiving the highest dose will be your hands and the hands are relatively insensitive to radiation. The portion of the body of most concern, the trunk, will receive a much lower dose because the intensity of radiation from a small source decreases dramatically with distance.
Even if your entire body received 2 mR/hr (which is impossible from such a sample) for eight hours a day, five days a week, and fifty weeks a year, your exposure would be less than the 5000 millirem annual limit which is considered "safe" by the regulatory community for radiation workers. There is no relevant limit that precisely fits your situation, but members of the public have sometimes been "limited" to 100 or 500 millirem per year.
I am unaware of any radiation injuries or other biological effects that have been attributed to the mere handling of radioactive ore. The increased risk of cancer observed in the underground uranium miners was due to the high levels of radon in the mines—not the direct radiation from the ore.
Having said all this, the guiding philosophy in the radiation safety community is that we should keep exposures to radiation as low as reasonably achievable. This is what we refer to as the ALARA principle.

The simple ways for you to achieve this would be to reduce the amount of time you handle the ore, keep the ore stored at some distance from occupied areas, and store the ore in a shielded container or room (e.g., a room with brick walls). These actions are really "overkill" for the small sample you have described however. Their main benefit might be to provide "peace of mind." Paul Frame, CHP, Ph.D.

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