- Removable vs. Fixed Contamination
- Types of Surveys
- What Type of Instrumentation Should I Use?
- How To Perform a Meter Survey
- How to Perform a Wipe Survey
- Definition of Contamination
- When and Where to Survey
- Documenting Surveys
- Purchase, Repair and Calibration of Survey Meters
Frequent surveys performed by knowledgeable laboratory personnel are the main line of defense to detect spills and to prevent the spread of contamination within and beyond the laboratory. EHS performs monthly contamination surveys in active radioisotope-using labs, but the EHS surveys are intended to verify that no contamination exists. EHS rarely finds contamination, because researchers do a very good job of performing postoperational surveys and cleaning up any contamination that occurs.
This section summarizes the requirements for performing surveys and provides some guidance about how to perform a survey. EHS provides hands-on training in how to use a survey meter to people who attend the Radioactive Materials Safety Class.
Removable contamination can be readily removed without using destructive 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. For instance, contamination that has chemically bonded to a surface is fixed. 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.
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 radioisotopes.
Wipe surveys, using “wipes” counted on a liquid scintillation counter or a gamma counter, can identify removable contamination only but will detect most radioisotopes used at Princeton. Wipe tests are the most versatile and most sensitive method of detecting low-level contamination in the laboratory.
The type of survey you perform to detect contamination depends on the radioisotope you are using.
- G-M Survey Meter: Use a Geiger-Muller (G-M) survey meter to survey 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 moderate energy betas, such as C-14 or S-35. However, the G-M detector has a low efficiency for detecting these moderate energy betas, so it won't detect C-14, S-35, or P-33 in amounts less than about 100,000 dpm. G-M meters should also 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.
- NaI Scintillation Survey Meter: Use a portable thin crystal NaI scintillation survey meter to survey for I-125 contamination and to conduct surveys around low-energy x-ray sources such as x-ray diffractometers and electron microscopes.
- Liquid Scintillation Counter: 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.
The following table summarizes the instrumentation and method of choice for the isotopes most commonly used at Princeton University.
|Radioisotope||Acceptable Survey Method||Comments|
|H-3||LSC||There are no other acceptable survey methods|
|C-14||G-M or LSC||LSC is most sensitive; G-M detects moderate to high levels of contamination; do not cover G-M with parafilm|
|P-32||G-M or LSC||G-M detects low levels of contamination|
|P-33||G-M or LSC||LSC is most sensitive; G-M detects moderate to high levels of contamination; do not cover G-M with parafilm|
|S-35||G-M or LSC||LSC is most sensitive; G-M detects moderate to high levels of contamination; do not cover G-M with parafilm.|
|Zn-65||G-M or g|
|I-125||NaI, g, or LSC|
|U-238||G-M or LSC|
G-M = Survey meter with a Geiger-Muller detector
LSC = wipe survey with liquid scintillation counting
NaI = survey meter with a thin crystal sodium iodide detector
g = wipe survey with gamma counter
Always perform an operational check the first time you use the meter each day or when you suspect it may have been misused, damaged or contaminated.
- Battery Check: 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 (usually two D-cell batteries).
- Background Measurement: 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 EHS to report a contaminated meter.
- Source Check: 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. Call EHS to arrange for repair.
Go to the Resources box at the top of this page to view the Preoperational Check Slideshow.
To Perform the Survey
- No Parafilm: 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.
- Survey Slowly: Slowly move the probe about 1 centimeter above the area of interest. If you move too quickly, you may miss a small area of contamination.
- Surveying by Sound: The pitch of the sound (the number of clicks) emitted by the detector increases as the count rate increases. It is important to know that the pitch is independent of the scale on which the meter is set. This means that you hear the same amount of clicks whether you are on the X0.1 or X1 or X10 or X100 scale. If you survey by sound, you can look at the surface as you survey rather than constantly watching the meter face.
- Finding Contamination: If an item or area with a sustained count rate more than three times background is found, the item or area should be considered to be contaminated.
- Labeling: 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.
- Ambiguous Readings: 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.
- Recording the Survey: 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.
Perform wipe surveys when:
- H-3 or Fe-55 are used
- C-14, P-33, and S-35 are used, to confirm whether removable low-level contamination exists
- a meter survey suggests that low level contamination may be present. The wipe survey will confirm whether contamination is present or not.
The procedure for performing the survey:
- Using a piece of filter paper (about 1” in diameter), Q-tip or other swab, or any other absorbent material, 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 absorbent surfaces, including skin and clothing, moisten the wipe with water to increase the possibility of picking up contamination.
- 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.
- 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.
Call EHS with questions about liquid scintillation and gamma counter use.
If an item or area with a sustained count rate of three times background or greater is found, the item or area should be considered to be contaminated.
When to Survey
Laboratory personnel must conduct individual work area surveys (surveys of floors, workbenches, handles, experimental equipment, etc.) and personal surveys (surveys of one’s person and clothing) under the following conditions:
- at the end of an experimental procedure;
- at the end of each day for multi-day procedures;
- frequently during the manipulation of millicurie quantities of open sources;
- following the opening of radioactive material packages;
- following withdrawals from stock vials containing more than 1 mCi;
- Prior to exiting the laboratory (for personal surveys)
Where to Survey
Survey areas where splashes or spills may have occurred and areas where a person could unknowingly transfer contamination. Typical survey locations include:
- Bench tops, including the edges
- Fume hoods (aprons, sashes, sash handles)
- Beta shields
- Refrigerator and freezer door handles
- Sinks designated for radioactive material disposal (sink basin, surrounding bench, faucet handles)
- Floors: at working areas, laboratory entrances, waste containers, fume hoods
- Communal equipment, such as pipettors, timers, incubators, centrifuges, water baths, etc.
- Non-radioactive trash (to ensure that contaminated waste is not disposed of as regular trash)
- Clean areas (offices, desks, doorknobs, phones, computers)
When to Document Surveys
Document the survey results:
- when radioactivity in amounts of 250 µCi or more have been handled,
- whenever contamination is discovered, regardless of the amount used, and
- to show follow-up actions, whenever contamination has been cleaned up.ow to Document Surveys
How to Document Surveys
Record survey results in the Laboratory Contamination Survey Log. Each log entry should contain the following information:
- Name of person performing the survey
- Date of survey
- Brief description of the area surveyed
- Survey meter results (in cpm), even for background count rates
- Meter identification (model, serial number)
- Follow-up action taken when contamination is found.
- Call EHS for recommendations and information about purchasing a radiation survey meter. After a new meter arrives, call EHS to register the meter. Every survey meter must have a low-activity radioactive check source attached to it. Check sources are available from EHS.
- EHS calibrates laboratory survey meters annually. When it is time for the annual calibration, EHS will notify each lab about the time and location of the calibration.
- Call EHS whenever a survey meter is not functioning properly or needs to be repaired for any reason. EHS maintains repair and calibration records for each survey meter and can offer limited diagnostic and repair services.