Answer to Question #10557 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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

My question has to do with losses due to evaporation when collecting swipes for removable tritium in the elemental form. Is any data available showing the amount of loss due to evaporation for wet and dry swipes? Secondly, can evaporation loss be avoided by using the wet method and immediately securing the sample in a 20 ml liquid scintillation counter (LSC) vial?

I am a bit uncertain regarding your concern about “swipes for removable tritium in the elemental form.” Normally the elemental form would imply tritium as the diatomic gas, HT, or possibly T2. I would not expect that you would be taking any swipes of surfaces for gaseous tritium, which would quickly become airborne unless it was being retained in some other matrix.

I shall assume that you are concerned with assessment of tritium surface contamination in less volatile forms such as tritium-labeled organic compounds, metal hydrides (or tritides), or possibly tritiated water. Some nongaseous compounds of tritium, including tritiated water, do exhibit some volatility, and tritium losses from swipes may be a concern in such instances. I know of no studies, however, that have attempted to quantify such losses.

It has been shown that wipe collection efficiency and volatility losses are often reduced by using wipes that have been wetted with appropriate agents. Wetting with water has often been used but common glycerol, which evaporates less readily than water, seems to work quite well in many cases (as noted in NCRP Report No. 47, Tritium Measurement Techniques, 1976, Sections 5.4.2 and 3.6). Glass fiber filter wipes are often a good choice for liquid scintillation counting because the filter is more transparent to the fluorescent light from the scintillator than is paper, and the glass paper is often more effective than paper in removing activity from contaminated surfaces in the wipe process. It often also tends to release the tritium to the cocktail solvent more readily than does paper.

Regardless of which wipe material you use and whether wet or dry, your inference about immediately placing the wipe into the scintillation vial containing the LSC cocktail mix is appropriate. You can find a bit more confirmatory information regarding wipe considerations in a paper that is available through the RSO section of the Health Physics Society. The paper is titled Use of Smears for Assessing Removable Contamination and is by Paul W. Frame and Eric W. Abelquist.

We should keep in mind that the process of taking wipes of surfaces to estimate removable activity via LSC is a very inexact process, subject to many variables and a great deal of uncertainty. When the very low energy beta emitter, tritium, is the radionuclide of interest, the factors leading to such uncertainty are exaggerated. As such, the significance of small losses from volatility is often insignificant compared to the normal variability of results even when no losses occur. This is not to say that we should not try to minimize such losses with reasonable care and countermeasures but only that minimization of small losses may not warrant an excessive expenditure of effort and/or resources.

Best wishes in carrying out your program.

George Chabot, PhD
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