Answer to Question #9290 Submitted to "Ask the Experts"
Category: Radiation Basics — Alpha Radiation
The following question was answered by an expert in the appropriate field:
I am currently working as a student at a nuclear power generating station (radiation protection unit). One of my tasks consists of establishing area-specific radionuclide distributions for alpha contamination. Notably, loose surface contamination smears (small discs of filter paper) will be used. Usually, for beta contamination, smears are moistened (with water) to pick up as much as possible of the loose contamination that may be present. Since I'll be looking for alpha contamination, should the smear be dry given the fact that alpha particles have low penetration depth? Maybe I could moisten the smears, pick up the loose contamination, and then dry the smears? Any comment(s) and/or suggestion(s) would be greatly appreciated. Thank you so much for taking the time to answer my question.
As you noted, paper smears pick up loose, radioactive contamination from a surface, if any is present. You have two concerns with this method: (1) picking up the contamination efficiently and (2) correctly measuring the radioactivity that is picked up.
Pickup efficiency is often improved by wetting the smear. It's been shown that water-soluble compounds will be collected with greater efficiency if the smear is wet. This was done by testing different smear media and different conditions (wet/dry) (Campbell et al. 1993). I would encourage you to set up such a test yourself.
The second concern is taking a smear that can be measured. A smear that absorbs or "buries" the alpha contamination will defeat this aim. Normally, you want the alpha activity on the smear's surface. However, since you're looking for a radionuclide distribution, you will probably dissolve the smear for the analysis. If so, then there's no need to worry about keeping the activity on the surface. If you perform the tests I suggest, they will also account for the efficiency of the alpha measurement.
Joel Cehn, CHP
Campbell JL, Canterre CR, Farina PC, Muse LA. Wipe testing for surface contamination by tritiated compounds. Hlth Phys 64(5):540–544; 1993.