Answer to Question #5434 Submitted to "Ask the Experts"

Q

What is radium equivalent activity? What is its importance in building materials?

A

"Radium equivalent" is discussed in Ask the Experts Question 54. By the 1950s, physicians had greatly refined their techniques for employing radium to treat cancer and other disorders. But the development of reactors in the 1940s and 1950s made new radionuclides available (e.g., ¹³⁷CS and ⁶⁰Co) with superior characteristics to radium. A problem with using these new nuclides was that the activities required to treat a disorder differed from the required activities of radium. To take advantage of the physicians' familiarity with the properties of radium, the activities of these new sources were expressed as radium equivalent activities. For example, the activity of ⁶⁰Co would have the same therapeutic effect as one milligram of ²²⁶Ra.

This concept has also been used by some, at least since 1970, to express the activities of naturally occurring radionuclides in ore, building materials, etc. It is certainly not necessary to do this and it can even lead to confusion. I suspect that the radium equivalent concept is often used by an author simply because others have done it.

You might want to refer to a paper in Health Physics (Volume 90, pages 361-370, 2006), "Radioactive Rock Phosphate: The Feed Stock of Phosphate Fertilizers Used in Pakistan," by Tufail et al. for information about the calculation of the radium equivalent and some related indices: the gamma hazard index and the external hazard index.

But to answer your question:

The radium equivalent concept allows a single index or number to describe the gamma output from different mixtures of uranium (i.e., radium), thorium, and ⁴⁰K in a material.The radium equivalent in Bq/kg (becquerels per kilogram) can be calculated as follows:

Ra (eq) = A (Ra) + 1.43 A (Th) + 0.077 A (K)

where

A (Ra) is the activity of ²²⁶Ra (which is the same as that of ²³⁸U) in Bq/kg 
A (Th) is the activity of ²³²Th in Bq/kg, and 
A (K) is the activity of ⁴⁰K in Bq/kg

It then becomes possible for a country to place a single regulatory limit on the radium equivalent activity of natural radionuclides in building materials rather than have to try to limit uranium, thorium, and potassium separately.

Paul Frame, CHP, PhD