Conducting Wipe Tests

Purpose

Wipe testing is a method used for surveying radioactive contamination on surfaces. All Principal Investigators actively using radioactive material must ensure that routine wipe tests are conducted in their laboratories. Normally these wipe tests are conducted on a monthly basis at a minimum. However, if researchers use large amounts of radioactive material in a laboratory, it may be necessary to conduct more frequent wipe tests. 

Definitions

  • Background Sample (Blank): A clean cotton swab or filter paper used to determine the number of counts corresponding to a contamination free surface.
  • Control Sample: A scintillation vial containing a known amount of radioactive material, typically less than 0.1 uCi, and a clean cotton swab or filter paper. Most liquid scintillation counters at Einstein will have a control for 3H, 14C, and background.
  • Disintegration per minute (dpm): The rate of radioactive decay, indicating how many atoms in a radioactive sample are undergoing transformation (or disintegrating) within one minute. It is used to quantify the activity of a radioactive source. 
    1 becquerel (Bq), the SI unit for radioactivity, equals 60 disintegrations per minute. 
    1 curie (Ci), a traditional unit for radioactivity, equals 2.22 x 1012 disintegrations per minute.
  • Counts per minute (cpm): The number of radiation events detected by a radiation detector in one minute. Cpm does not represent the amount or strength of the radiation. Cpm is used with survey meters such as a Geiger counter to measure contamination.
  • Inactive Status: Principal Investigators who are not using radioactive material for an extended period may choose to place their radioactive materials license on “inactive status”. When the license is on inactive status, the Principal Investigator does not need to perform wipe tests in their laboratory. The laboratory may not order or work with any radioactive material while on inactive status.

Procedure 

A wipe test map of the laboratory should be created which shows the locations of benches, refrigerators, freezers, sinks, fume hoods, and biosafety cabinets. Any equipment used in radioactive research  such as centrifuges, hybridization ovens and water baths should be drawn on the map as well. The map should indicate areas to be checked for contamination using the wipe test method. Some laboratories use numbers to correspond to the various locations throughout the space. The map should be stored in the wipe test section of the laboratory’s radiation safety binder. The number of wipe tests to be conducted depends on the size of the laboratory and the number of laboratory benches used for radioactive research.

  • Here are some common locations that should be monitored for contamination using the wipe test method:
    • Laboratory benches used for radioactive material research
    • Floors in front of benches
    • Equipment used with radioactive material (centrifuges, water baths, hybridization ovens, shakers, etc.) 
    • Door handle to freezer and/or refrigerator used for storing radioactive material
    • Sink used for disposal of radioactive material
    • Exterior of radioactive waste containers (plus the surrounding floor)
    • Fume hoods and/or biosafety cabinets used for radioactive material research
    • Laboratory door handle
    • Safety devices (acrylic or lead shielding, survey meter)
  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 surfaces/areas throughout the laboratory using one cotton swab/filter for each location. Place each cotton swab/filter disk into a scintillation vial. Add scintillation fluid (cocktail) to the vial covering up the wipe test sample (fill ~ 50% of 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. If the laboratory uses numbers to correspond to specific locations, write the number on the top of the wipe test vial. Ensure that the settings on the LSC are set to detect the radioisotopes used in the laboratory (this is based off of the energy). Set the LSC 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). See below for the conversion formula.
  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.  

 

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 cm– alpha emitters

< 1000 dpm 100 cm– beta & gamma emitters

Laboratories: cleanup recommended to as low as practicable levels

> 100 dpm 100 cm– alpha emitters

> 1000 dpm 100 cm– beta & gamma emitters

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

As a rule-of-thumb if any of your sample results are 3 to 4 times the background result, it is considered contaminated. Locate the surface that is contaminated and clean it using a radioactive decontaminate cleaner such as NoCount solution. The cleanup debris should be collected and disposed of in a radioactive dry waste container and labeled accordingly. Continue to wipe test after cleanup until results are satisfactory.  

How to Convert from cpm to dpm:
 

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

Recordkeeping

The results of wipe tests must be kept in the laboratory’s yellow radiation safety binder. The records must contain the laboratory wipe test map and the wipe test results in dpm from the scintillation counter or gamma counter. Monthly wipe tests and any follow up contamination checks should be filed within the binder. 

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