Answer to Question #8724 Submitted to "Ask the Experts"

Category: Radiation Basics

The following question was answered by an expert in the appropriate field:

What is the effective atomic number of glass-fiber filter paper? This is required to discriminate the uranium activity and the radon daughter activity in the work atmosphere by the reverse counting method.

The effective atomic number of a particular compound or mixture is often obtained by summing the products of the individual constituent atomic numbers, Zi, and their respective weight fractions, wi:

Zeff = S wiZi .

The glass commonly used in glass-fiber filters is a borosilicate glass, similar to Pyrex®. These glasses can have components that vary with the manufacturer. The National Institute of Standards and Technology (NIST) gives the following composition of a typical Pyrex® glass:

Element Atomic number Weight fraction
Boron 5 0.040064
Oxygen 8 0.539562
Sodium 11 0.028191
Aluminum 13 0.011644
Silicon 14 0.377220
Potassium 19 0.003321

By using the above formula to calculate the effective atomic number for this material we obtain a value of 10.3. Naturally this value will change if the composition of the glass changes. There have been some glass-fiber materials produced that have incorporated small percentages of higher atomic number elements such as calcium, barium, and zirconium, and these materials have higher effective atomic numbers than the value calculated here. If possible, you should try to obtain from the manufacturer the composition of the glass fibers used in the filters you are using so that you can do the most meaningful calculation.

It is not clear to me how you are using the effective atomic number in your radioactivity analysis, but you should also consider that glass-fiber filters may consist of the pure glass fibers with no additives, but there are also filters that use organic binders, often acrylic resins, to provide better physical integrity. There are also some glass-fiber filters that use synthetic materials such as polyester as backings to increase filter strength. Naturally, the presence of binders or other materials may have some effect on the atomic number calculation.

George Chabot, PhD, CHP

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