Environmental Health & Safety

Radioactive Contamination Checks

In order to know if the working surfaces in your laboratory are free of radioactive contamination, a contamination check should be performed after each experiment. This check can be with a survey meter or a wipe test. Contamination can be removable or non-removable (fixed). Contamination is removable when it can be detected by a wipe test. Areas with removable contamination should be cleaned and those with non-removable contamination should be covered with the appropriate shielding material, if the radiation levels warrant it, to avoid unnecessary personnel exposure. A guide to radioactive surface contamination limits will be found at the end of this document. The entire laboratory where radioactive material is used must be wipe tested at least once per month. The result of these wipe tests should be calculated in disintegrations per minute (dpm) and kept on file in the laboratory. The RSO may recommend different wipe test frequencies depending on the radionuclides and amounts used.

Guide for performing a wipe test

  1. For each room or area where radionuclides are used or stored, use a filter disk or glass fiber dampened with water, or other appropriate solvent and wipe over a 100 cm2 area. Wipe test the various work surfaces as well as other areas of the room which should not, but could, come in contact with radionuclides, such as door knobs, refrigerator handles, sink parts, floor areas, etc., and place each filter disk in a vial. Add scintillation fluid to count beta emitters.
  2. As a background control, dampen a filter disk, and count in a vial as above; then add a known amount of the appropriate radionuclide to filters as internal standards to obtain the counting efficiency. Also, include control vials containing appropriate standards (H-3, C-14, I-125, etc.), so that counter efficiency can be verified.
  3. Count vials long enough to attain a 95% confidence level with settings appropriate to the isotopes used in the laboratory.
  4. Determine the activity of each vial in disintegrations per minute (dpm) per 100 cm2 as follows:
    • Net cpm = gross cpm- background cpm
    • Where cpm = counts per minute
    • Efficiency of counting (E) = net cpm of standard/dpm of standard
    • Where dpm of standard = uCi x2.22x106 dpm/uCi = Bq x60
    • Then dpm/100cm2 = (net cpm/100 cm2)/E
  5. Records of all wipe test results must be maintained on file in each laboratory.
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