Answer to Question #11680 Submitted to "Ask the Experts"

I am an amateur geologist with an extensive collection in my basement. I recently added a uraninite specimen which sits in a "lead castle" under 5 centimeters (cm) of lead. The house background was 4–8 millisieverts (mSv) before I brought the mineral home and is now 8–16 mSv with the mineral. Given this increase, would it be best for me to remove the specimen from my home or is this increase so small that it is negligible? If it was your home, what would you do?

Cool—I'm jealous! Actually, I have an undergraduate and MS degree in geology so I collect rocks and minerals also, and I'm always on the lookout for radioactive specimens too. I've got a small piece of uraninite myself, but my pride and joy is a beautiful piece of torbiernite, which has a crust of emerald-green platy crystals a few millimeters (mm) on a side. That one reads about 1–2 millisieverts per hour (mSv h^-1), but I take that reading with a grain of salt—for the same reasons you might want to do so with your specimen, depending on what you're using to make your measurements.

My guess would be that you're using a Geiger counter or a sodium iodide detector of some sort. If so, then what you're seeing is the effect of what's called an energy-dependent radiation detector. Here's what's probably happening.

First, radiation exposure is a measure of the amount of energy deposited per unit of mass. Ideally we'd measure this directly, and if you're using an ionization chamber then this is exactly what you're measuring. But most radiation detectors—Geiger counters and scintillation-type detectors in particular—take a shortcut. Specifically, they're calibrated with a radioactive source that emits radiation of a particular energy (cesium-137 [¹³⁷Cs], for example, gives off gamma radiation with an energy of 662,000 electron volts [eV], or 662 kiloelectronvolts [keV]). Since we know the energy of each ¹³⁷Cs gamma, we know how many gammas it takes to give us a dose rate of 1 mSv h^-1. From there, we just "tell" the radiation detector that a certain count rate is equivalent to 1 mSv h^-1.

The problem with this is that you're just counting gammas, and if the gammas have an energy that's lower than 662 keV, then your meter will show a radiation dose that's higher than what the meter is exposed to. It turns out that the uranium in your uraninite gives off some fairly low-energy gammas. Not only that, but uranium decays through over a dozen steps (all radioactive) until it reaches stable lead, and a number of those steps give off gamma radiation as well. So your piece of uraninite is emitting radiation from uranium—and also from radium, polonium, thorium, bismuth, and much more—and most of these gammas are less energetic than 662 keV. This means that your radiation instrument is going to read a higher dose from your mineral specimen than actually exists. But it gets even a little more interesting when you put the lead in place!

There are some fairly high-energy gammas that are also emitted by the uranium decay series, and when these pass through the lead they undergo all sorts of interesting interactions (too much to get into here). Many of these interactions result in the gammas from the uranium scattering and undergoing other interactions that produce a flurry of low-energy gammas. So these are also entering your detector and are also going to cause your detector to read too high.

The bottom line is that any shielding you put in front of your uraninite is going to cause the radiation dose rate to go down—if your measurements show otherwise, then the only plausible conclusion is that your meter is being fooled. And the most likely way for it to be fooled is that it's reading a bunch of low-energy gammas, thinking that they're actually more energetic.

The last thing that might be happening here depends on how far away you're making your measurements. If you're at a distance of, say, 1 cm from the uraninite, then the dose rate at a distance of 30 cm (a fairly standard distance for making such measurements) will be only about 0.1% as high, and the dose rate at a distance of 1 meter (m) will be only about 0.01% as high (reductions of a factor of 1,000 and 10,000, respectively).

One quick example, and then I'll stop! I was involved in characterizing a site contaminated with thorium. A consultant went through and made measurements, finding that radiation dose rates were high enough to cause concern that workers at the site might be receiving excessive dose. We found out that the consultant had used a scintillation-type detector so we went back and made measurements using an ion chamber. We consistently found that dose rates with the ion chamber (the right instrument to use) were only a fraction of what was measured with the scintillation counter. When we used the ion chamber readings, we found that nobody was receiving a dose that was too high.

I hope this helps. And if you're still concerned let me know—I'll be happy to take the rock off your hands and add it to my collection!

P. Andrew Karam, PhD, CHP