Answer to Question #10121 Submitted to "Ask the Experts"

I would like to know the various techniques for the collection efficiency of filter papers used to determine airborne radioactive particles, especially in continuous air monitors (CAM). Reference for available documents would also be appreciated.

Manufacturers often provide the collection efficiencies for filter media that they provide. In some instances, though, such efficiencies may not be available or may be in doubt, and the user may have to determine the collection efficiency of the particular filter of interest.

There are various protocols that are used for filter testing. You may be familiar with the DOP aerosol penetration in which fine droplets of dioctyl phthalate (DOP) are generated and their concentrations are measured upstream and downstream of a filter in an exhaust line. This test has been especially common for in-place filter testing of filters used for air cleanup, but it has also been used for testing of filters used in air sampling. The DOP aerosol has become less popular in recent years and has been replaced in some applications with other aerosols believed to be less of a possible health concern. The most popular particle size for use has been in the 0.2 to 0.5 micrometer diameter range. For many applications this size range is the most difficult to capture on filter media; smaller sizes are more easily collected by diffusion processes, and larger sizes are more easily collected by inertial effects, although variations with linear flow rate do occur. There are many references to the DOP technique on the internet.

A more common technique for testing of air sampling filters by many users involves a two-filter technique in which the test filter in a filter holder is placed first; it is backed up by a filter that is known to have a nearly 100% collection efficiency (such as a high-efficiency glass fiber filter or a membrane filter) in its own holder. A radioactive aerosol is then generated and sampled. The collection efficiency is then estimated from the ratio of activity on the first filter to the combined activities on the two filters. Effects of penetration into the test filter may be evaluated by running a similar test with another high efficiency filter in parallel with the other two filters. This type of testing has been recommended in US NRC NUREG-1400.

Section 6.2 describes the approach. The same approach has been recommended in a standard of the American National Standards Institute and the Health Physics Society, ANSI/HPS 13.1-2011. If you are a member of the HPS you can download this standard free of charge from the Members Only section of the Health Physics Society website.

Generation of an appropriate radioactive aerosol to perform the testing may present some limitations. I have performed some testing in the past using an aerosol consisting of ²¹²Pb and ²¹²Bi generated from the decay of ²²⁰Rn (thoron) produced by an emanation source that used ²²⁸Th as the parent radionuclide. A paper by Yoshida et al. shows the testing procedure and results when a similar aerosol was used.

I hope this is helpful.

George Chabot, PhD, CHP