Answer to Question #1899 Submitted to "Ask the Experts"

Q

What is the half-life of radium? What is an acceptable background level?

A

The half-life of radium-226 (²²⁶Ra) is 1,600 years (plus or minus five years or so). By background level, I assume you are referring to the concentration of ²²⁶Ra in soil. Typical background concentrations range from 0.5 to 2.0 picocuries (pCi) per gram of soil (one picocurie is approximately 2.2 decays per minute). Of course, background can be higher or lower than that, it just depends on the local geology. In addition, each of the decay products of ²²⁶Ra can be expected to be present at roughly the same concentration as the radium. These decay products are ²²²Rn, ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, ²¹⁴Po, ²¹⁰Pb, ²¹⁰Bi, and ²¹⁰Po. Any background level of a radionuclide (one unaffected by man) is usually assumed to be acceptable. What might be "unacceptable" is an increased concentration above background that can be attributed to man's activities. For information about this issue, I would recommend that you contact the Radiologic Health Branch of the State of California. Measuring the concentration of radium in soil is not something that can be done with a typical handheld survey instrument. It usually involves a laboratory analysis of multiple soil samples (often one kilogram each). In many situations, the primary concern associated with ²²⁶Ra contamination is the elevated indoor concentrations of radon that result from the radium near the building foundations. Measuring radon is a fairly straightforward operation. Once again, the state radiation control program can provide advice. In general, the idea is to have annual average radon concentrations in the home to be less than the Environmental Protection Agency's guideline of 4 pCi per liter of air. Considerable information about radon can be found at EPA Indoor Radon Web page.

Paul Frame, CHP, PhD