Answer to Question #4182 Submitted to "Ask the Experts"

I would like to know how thick the layer of lead has to be to stop the radiation of uranium ore, and what is the average radiation level?

I cannot provide a very precise answer to your question regarding shielding of radiation from uranium ore because there are a number of variables that you have not specified that influence the answer. These variables include the uranium concentration in the ore, the amount of ore present, the geometry of the ore, and whether or not the ore is covered by other materials, as it might be if it is in the earth. I will attempt to provide some information that I hope will be useful to you.

Uranium and its radioactive decay products, referred to as progeny, emit alpha, beta, and gamma radiations. In most practical measurement situations the alpha radiation from ore would not be detected because it is easily stopped by small thicknesses of materials, including a few cm of air. Some beta radiation, especially the high-energy beta radiation from metastable ²³⁴Pa, one of the radioactive progeny produced from ²³⁸U decay, can be detected if the ore is not shielded appreciably. This beta radiation may contribute most of the dose rate when a detector with a fairly thin entrance window is held close to the uranium ore. The beta radiation can be eliminated by shielding the ore with about 1.5 cm of plastic. The beta radiation may also be stopped by about 2 mm of lead, although the high atomic number lead will lead to the production of some secondary penetrating photon radiation, called bremsstrahlung. The most significant gamma radiation from uranium ore comes from some of the radioactive progeny, especially ²¹⁴Pb and ²¹⁴Bi, that are produced as part of the decay series of ²³⁸U. The gamma ray energies of interest range from about 0.3 MeV (million electron volts) to almost 2 MeV and are sufficiently penetrating that some gamma radiation can be detected as it is emitted from ore somewhat below the surface of the earth. Since gamma radiation does not have a finite range and is attenuated in a generally exponential fashion with increasing thickness of materials, we cannot specify a fixed thickness of lead that will stop all the gamma radiation. We can give approximate thicknesses that will reduce the gamma radiation intensity and dose rate by a particular amount. For example, about 5 cm of lead will reduce the gamma-dose rate from a small geometry source of ore by somewhat more than a factor of 10; 10 cm of lead will provide attenuation by about a factor of 250, and 15 cm will reduce the gamma dose rate by a factor of roughly 3,000.

Ores can vary widely in uranium content. The average concentration of uranium in ores in the United States that might be useful, from a uranium-mining point of view is about 0.2%. This concentration translates to a radioactivity concentration of ²³⁸U of about 600 picocuries per gram of ore. Such a concentration, along with the distribution of radioactive progeny produced by the uranium decay, if it were uniformly distributed throughout a volume near the surface of the earth and extending over several meters in lateral dimensions and at least a foot deep might yield gamma dose rates of close to 1 mrad/hour within a few feet of the earth's surface, a number that is more than 100 times the usual background rate in nonuranium areas. A small sample of high uranium content ore held in proximity to a thin window detector may produce dose rates about 100 times the rate from the distributed uranium noted above. In the latter case the beta radiation may contribute a large part of the detector reading.

George Chabot, CHP, PhD