National Physical Laboratory

Indirect measurement of surface contamination (wipe testing)

Wipe tests can be either ‘Dry wipe’ or ‘Wet wipe’.

In general it will be a senior health physics professional who will make the decision on which to use. Purely fixed contamination will not be detected using a wipe method. 

How to do a wipe test

A typical wipe test procedure uses a filter paper firmly wiped over an area of 100 to 1000 cm2 of a surface that may be contaminated with radioactivity.  The filter paper can either be placed in a lab counter drawer to assess the level and type of activity, or sent to a radio-chemistry laboratory for a full assessment of nuclide type and activity.  In both instances all measurements should be traceable to national standards of radioactivity.

The uncertainties associated with assessing the levels of contamination from wipe tests are great.

The wipe efficiency, or pick up factor, i.e. the percentage of contamination picked up by a single swipe of the surface, is affected by many factors including, but not limited to:

  • type of wipe used
  • pressure applied by the person when making the wipe
  • area wiped
  • contamination distribution
  • porosity, chemical composition, texture and cleanliness of the surface

‘Pick-up’ factors can vary enormously and are almost impossible to assess accurately. The uncertainties in the ‘pick-up’ factor are an order of magnitude larger than other uncertainties in the measurement.

Consequently it has been common practice to allocate a value of 10% to the ‘Pick-up’ factor, as essentially a ‘Best Guess’.

More detail on estimating surface activity, identification of significant emissions, estimating instrument response, estimating surface effects, example of decay scheme correction can be found in the module ‘Estimating surface activity’.

 

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