Answer to Question #2352 Submitted to "Ask the Experts"

Q

I am doing some experimental work with uranium (U) and thorium (Th) minerals. I have several questions regarding measuring the radiation levels and using the results to estimate the age of the mineral (assuming no daughters were simultaneously deposited). What is the simplest and best method for determining the specific activity of a sample (Bq/g)? At how many diameters distance can a radiation source be considered a point source (assuming a spherical sample)? This question also has implications for protective measures - distance. What is the best method to determine background radiation levels? Does time of day/time of year have any effect on this measurement (cosmic/solar radiation effects)?

A

From your questions, it appears that you're familiar with "radiometric dating" of rock or mineral samples. I just did a quick search of the subject "geologic dating," with a send key word "uranium," and found the Department of Earth Sciences, University of Rochester, Web site of interest. This is an interesting subject, a science in itself with obvious challenges. I would look at this site and its links.

But to answer your question, the easiest way to measure the "specific activity" of a sample would be by nondestructive gamma spectroscopy. To minimize attenuation error, you want to use a medium- to high-energy gamma ray. Also, looking at the U and Th decay series, and to minimize equilibrium disruption for dating, you would want to focus on a radionuclide in the series before radon. This doesn't leave a lot of options. To get to the parent specific activity for uranium-238 and thorium-232, you may try counting for the protactinium-234m (with one good gamma at 1001 keV and 0.589 percent yield) and actinium-228 (with two gammas at 911/969 keV and 27.7/16.6 percent yields) respectively.

At four to five diameters of the sample, it will geometrically behave like a point source for inverse square corrections. Count your background with the same geometry and sample configuration, but with some surrogate blank. However, it should be chemically pure with little or no natural radioactivity. Your terrestrial background is constant and the cosmic-ray background should not vary with time of day or year. The solar cycle does vary over 11 years, but regular checks of background should address that.

As far as radiation protection with U/Th mineral samples, they normally would not present an external hazard from any gamma exposure in small quantities, and they would not present a radon emanation concern. This may not be the case with larger collections such as you might find in a college or university. Here you want to make sure the 100 mrem per year public dose limit is not approached or exceeded.

David J. Allard, CHP