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 done using a survey meter or by wipe testing the area. 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 can be found below on this webpage. The entire laboratory where radioactive material is used must be wipe tested on a monthly basis at minimum. The results of these wipe tests must be in disintegrations per minute (dpm) and be kept on file in the laboratory’s yellow Radiation Safety Binder. The Radiation Safety Officer may recommend different wipe test frequencies depending on the radionuclide and amounts used.

How to perform a wipe test 

  1. Pick an area to wipe test (lab bench, fume hood, centrifuge, etc.) and use a cotton swab or filter disk to wipe over a 100 cm2 area. Wipe test various work surfaces as well as other areas of the room which should not, but could, come in contact with radionuclides, such as doorknobs, refrigerator handles, sink parts, floor areas, etc., and place each cotton swab/filter disk into a vial. Add scintillation fluid to the vial.
  2. Use unquenched standards of known activity to ensure the machine is operating efficiently. Most liquid scintillation counters (LSC’s) will have a tritium (3H), carbon-14 (14C), and background standard included. If standards are not included, or the laboratory needs to create their own standards, they can easily be created using a known amount of activity from a specific radionuclide along with scintillation fluid in a vial. A background standard can be created by simply adding a cotton swab/filter disc to a vial with scintillation fluid. The standards are used to obtain the counting efficiency. If converting from cpm to dpm, the counting efficiency must be known. Please refer to the Wipe Test Calibration Worksheet for more information.  
  3. Add the wipe test vials to a liquid scintillation counter or gamma counter. Ensure that the settings on the LSC are set to detect the radioisotopes used in the laboratory. Set the count time for at least 1-2 minutes per vial.
  4. Print out the results. If the results are in counts per minute (cpm), they must be converted to disintegrations per minute (dpm).
  5. Records of all wipe test results must be maintained on file in each licensed laboratory. The printout from the liquid scintillation counter or gamma counter must be stored within the labs yellow Radiation Safety Binder. 

How to Convert from cpm to dpm:
 

Formula: Wipe Test Activity (dpm) =
Wipe Test Results (cpm) - background results (cpm) Efficiency

Wipe Test Contamination Limits:

Contamination Levels

Location

<20 dpm/100 cm2 - alpha emitters 

<200 dpm/100 cm2 – beta & gamma emitters 

Unrestricted areas (hallways, offices)

< 100 dpm 100 cm2 – alpha emitters 

< 1000 dpm 100 cm2 – beta & gamma emitters 

Laboratories: cleanup recommended to as low as practicable levels

> 100 dpm 100 cm2 – alpha emitters 

> 1000 dpm 100 cm2 – beta & gamma emitters 

 

Clean up to less than 1000 dpm/100 cm2 for beta & gamma  emitters or 100 dpm/100 cm2 for alpha emitters. Cleanup recommended  to as low as practicable levels. 

Checking for contamination with a survey meter:

Most laboratories that work with radioactive materials are required to have a survey meter such as a Geiger counter present within the laboratory. As when using any radiation survey meter, the researcher must first ensure that the meter has been calibrated within the last 12 months, conduct a battery test, and test that the meter is working properly with a known source of radiation. Contact EH&S regarding calibration and repairs. 

Use a survey meter that works with the radioisotopes used in the laboratory. A Geiger counter should be used for beta emitters while a scintillation meter with a Sodium Iodide (NaI) probe should be used for gamma and x-ray emitters. 

After the meter function test, begin your survey on the lowest meter scale multiplier option (X0.1 or X1). When using the survey meter, monitor the surface area slowly at about a ½ inch per second and hold the meter probe roughly a ½ inch above the surface. Be sure not to contaminate the meter probe by getting contamination on it. 

Contamination should be checked with meters calibrated in counts per minute (cpm), as milliroentgens per hour (mR/hr) is used for exposure rate. Many meters will have a combination scale which shows both cpm and mR/hr. Typical background levels are between 5 – 100 cpm for a GM meter probe such as the 44-9, and below 400 cpm when using a scintillation probe (NaI). Exposure rate background is typically between 0.01 – 0.03 mR/hr.     

Note: The rapid clicking sound you hear when using a Geiger counter is the generation of ion pairs within the meter. The more rapid the clicking sound means the higher the level of radiation detected. Keep in mind that the meter will still generate a clicking sound when detecting background radiation. 

For assistance with using a liquid scintillation counter for wipe testing, please refer to the Guide to Liquid Scintillation Counting.