APPENDIX A: Contamination Surveys

• Types of Contamination
• Types of Surveys
• Survey Instrumentation
• How to Perform a Meter Survey
• How to Perform a Wipe Test

Types of Contamination (top)

Removable contamination can be readily removed using proper decontamination procedures. Removable contamination in any amount may present both an external and internal hazard because it can be picked up on skin and possibly ingested.

Fixed contamination cannot be readily decontaminated. Fixed contamination generally does not present a significant hazard unless the material comes loose or is present in such large amounts that it presents an external radiation hazard.

Types of Surveys (top)

Meter surveys, using Geiger detectors or scintillation probes, can identify gross contamination (total contamination consisting of both fixed and removable contamination) but will detect only certain isotopes.

Wipe surveys, using “wipes” counted on a liquid scintillation counter or a gamma counter, can identify removable contamination only but will detect most isotopes used at Princeton. Wipe tests are the most versatile and most sensitive method of detecting low-level contamination in the laboratory.

Survey Instrumentation (top)

The portable Geiger-Muller (G-M) survey meter is best used for P-32, a high energy beta emitter, and other high energy beta and gamma emitters, such as Co-60, Zn-65, Cs-137, and U-238. A G-M meter can also be used to identify areas heavily contaminated with lower energy betas, such as C-14 or S-35, for which the G-M meter has a relatively low efficiency. G-M meters should not be used to survey for I-125 contamination, since G-M meters will detect I-125 only when there are very high levels of contamination.

The portable thin crystal NaI scintillation survey meter is used to locate I-125 contamination and to conduct surveys around low-energy x-ray sources such as x-ray diffractometers and electron microscopes.

The liquid scintillation counter, used for counting wipe tests, is not portable but is the most versatile counting instrument because it has a high counting efficiency for a wide range of radionuclides.

Gamma counters are not portable and are used to count wipe tests for photon emitters, such as Cr-51 or I-125.

How to Perform a Meter Survey (top)

1. Check the survey meter’s battery by turning the meter knob to the battery test position. If the battery is adequately charged, the meter needle will swing to the battery test position on the meter face. Replace the batteries if the batteries are low.

2. Perform an operational check the first time you use the meter each day or when you suspect it may have been misused or damaged. Look at the calibration sticker on the side of the meter and note what the expected reading for the operational check source should be. Turn the meter on and turn the meter’s multiplier switch to a setting that will measure the check source and will provide a mid-scale reading but will not cause the needle to swing beyond full scale. For a Ludlum G-M survey meter the multiplier knob should generally be set to the X1 position. Place the probe firmly against the check source on the side of the meter and note the meter response. If the observed meter response differs from the expected response by more than 20%, the meter should be considered nonfunctional and should be taken out of service.

3. Take the meter to an area away from sources of radiation and note the meter background reading. Typically, the background for a G-M meter with a pancake survey probe should be less than 100 counts per minute (cpm) while the background reading for a meter with a NaI scintillation crystal should be less than 300 cpm. If the meter’s background reading is substantially greater than expected, confirm that there are no unexpected sources of radiation or radioactive materials in the vicinity, and then call Environmental Health & Safety (EHS) to report a contaminated meter.

4. Do not cover the probe surface with parafilm or other protective covering. Parafilm and similar materials will shield the low energy betas from C-14, P-33 and S-35 and will prevent the meter from detecting contamination.

5. Slowly move the probe about 1 centimeter above the area of interest.

6. If an item or area with a sustained count rate more three times background is found, the item or area should be considered to be contaminated.

7. Immediately label the area or item and promptly decontaminate it. Decontamination procedures are provided in Section 1. If an area cannot be decontaminated, the contaminated area should be marked and labeled to indicate the isotope, date and level of contamination.

8. Sometimes, especially in the presence of other radioactive materials, the meter survey may be equivocal. When the meter survey indicates that low level contamination may be present, a wipe survey should be performed to confirm or disprove the presence of contamination.

9. Document the survey results whenever contamination is discovered or if 250 µCi or more have been handled. Record survey results in the laboratory survey log. This is a University requirement.

How to Perform a Wipe Test (top)

Wipe surveys must be performed when H-3 is used and is the survey method of choice to detect the presence of low levels of removable C-14, P-33 and S-35 contamination. Wipe surveys should also be performed to confirm the presence of contamination when a meter survey suggests that low level contamination may be present.

1. Using a piece of filter paper (about 1” in diameter), Q-tip or other swab, wipe the area being surveyed. If the area is very large, subdivide it into smaller areas and use several wipes to better pinpoint the location of contamination. For some surfaces, including skin and clothing, the wipe media should be moistened with water or other appropriate solvent.

2. Prepare the sample for counting as suggested in the counter’s operating manual. Analyze the wipe samples in a liquid scintillation counter for H-3 and other beta emitters and preferably in a gamma counter for Cr-51 and I-125.

3. Sample activity is determined by dividing the sample count by the counter’s efficiency for the isotope in question. The counter’s operating manual should provide information about efficiencies and activity determination.

4. Call EHS with questions about liquid scintillation and gamma counter use.

A copy of the official Radioisotope Laboratory Survey Log can be found here