Surveys are important to prevent spreading of contamination. Surveys must be performed by Saturday of the week following the week in which open sources of radioactive materials have been used (unless your permit specifies a different frequency). The surveys should be sufficiently extensive to allow confidence that there is no contamination outside of marked radiation areas. Common places to check for contamination are bench-tops, tools and equipment, floors, door handles and drawer pulls, and computer keyboards.
Removable contamination can be readily transferred from one surface to another. Removable contamination presents an internal and external hazard because it can be picked up on the skin and ingested. Fixed contamination cannot be readily removed and generally poses a lower hazard unless the material is brittle or the contamination is large enough to present an external hazard.
Two types of survey methods are used: 1) a direct or meter survey; and 2) a wipe or smear survey.
Direct surveys, using a Geiger-Mueller (GM) meter or scintillation probe, can identify gross contamination (total contamination consisting of both fixed and removable contamination) but can detect only certain radioisotopes.
Wipe surveys, using “wipes” such as cotton swabs or filter papers counted on a liquid scintillation counter or gamma counter can identify removable contamination only but will detect most radioisotopes used at the university. Wipe surveys are the most versatile and sensitive method of detecting low-level removable contamination in the laboratory.
The portable Geiger-Mueller (GM) 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 GM meter can also be used to identify areas heavily contaminated with lower-energy beta emitters, such as C-14 or S-35, for which the GM meter has a relatively low efficiency. GM meters should not be used for standard surveys for H-3, C-14, S-35, or I-125 because they detect only high levels of contamination.
The portable thin crystal NaI scintillation survey meter should be used to check for 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 (LSC), used for wipe tests, is the most versatile counting instrument because it has a high counting efficiency for a wide range of radionuclides. Most LSCs provide a printout of sample results that may be used as survey record.
Gamma counters are not portable and are used to count swipes of gamma emitters such as Cr-51 or I-125.
Prior to performing any survey, clean gloves should be worn. This prevents the possibility of personal contamination or cross-contamination.
Perform an instrument check. To check the operation of a survey instrument, do the following:
Check the calibration label on the instrument and ensure that the instrument is within the calibration period. If the calibration due date has passed, contact DRS to have the instrument re-calibrated and find another instrument to use.
Turn the switch on the survey meter to "BATTERY," or flip the battery switch to "ON." The needle on the meter face should move to a position within or beyond the indicated area on the meter face scale. Replace batteries if needed before using the survey meter.
If there is an audio switch on the survey meter, turn it to "ON." Set the survey meter to a scale of "X1." The survey meter should chirp or click. If the speaker does not function, the survey meter can be used, but the surveyor will need to check the reading on the survey meter face frequently.
Go to an area with an expected low background rate. Note the count rate when the survey meter is switched to the "X1" scale. The background rate for a GM meter should be less than 100 counts per minute; the background reading for a NaI meter should be less than 400 counts per minute. If background readings exceed these levels, investigate the area for unknown sources of radiation or detector contamination. Do not use the survey meter if it does not register a background rate.
Instrument response check:
Hold the supplied check source (often a thorium lantern mantle) up to the probe window. Note the counting rate. The survey meter should respond to the check source, providing positive indication that the instrument is functioning properly.
Do not cover the probe surface with parafilm or other protective coating. Parafilm and similar materials will shield low-energy betas and may prevent their detection.
Hold the probe window approximately 1 cm from the surface to be surveyed and move the probe over the surface at about 1 cm/second. A faster movement rate may result in missing contamination.
Check the most common sites for contamination, such as the survey meter handle, soap/towel dispensers, drawer handles, refrigerator/freezer handles, chair edges, writing utensils, survey record books, floors, radio dials, microwave oven touch pads/handles, doorknobs, light switches, and non-radioactive trash containers.
Record survey results in a survey log. Obtain several background readings and record the highest result. Next, complete the survey. If survey results are equivalent to the background, log the result as ‘≤ BKG’. A surface may be considered contaminated if the result is greater than the background count rate. If contamination is found, record the result and indicate the action taken. Once corrective actions have been taken, perform another survey of the area until the contamination is within the range of the highest background results.
Prior to performing any survey, clean gloves should be worn. This reduces the likelihood of personal contamination or cross-contamination.
Removable contamination is best identified by a wipe survey, which is performed by rubbing a filter paper (approximately 45 mm in diameter) or cotton swab over the survey area with moderate pressure. The paper or swab may be wetted with ethanol or water to increase the collection efficiency. Usually an area of 100 square centimeters (4 in X 4 in) is surveyed. To monitor a larger area, take additional swipes.
When surveying for low-energy beta emitting isotopes such as H-3, C-14, P-33, and S-35, analyze the wipe using liquid scintillation counting.
When surveying for high-energy beta emitters (such as P-32), wipe samples may be counted using either liquid scintillation counting or a GM meter.
When monitoring for low-energy gamma emitters (such I-125), wipe samples should be counted with a thin crystal NaI scintillation meter.
The net sample count rate is determined by subtracting the background count rate from the gross count rate.
Sample activity is determined by dividing the net sample count rate by the instrument’s efficiency for the isotope in question.
Survey results must be documented on a survey log or similar form. Results may be reported as gross count rate, net count rate, or in units of activity (usually disintegrations per minute). Ensure that the survey log accurately reflects how results are being reported. Similar to a meter survey, if the results are above the highest background sample, then the contamination will need to be removed. Re-survey to confirm effectiveness of the removal. If the contamination cannot be effectively removed contact DRS.
After finishing work with radioactive materials you should always survey yourself. Check your hands, torso, and the bottom of your shoes. It is good practice to have the meter on during work with the probe facing up. This allows you to check your hands frequently and avoid spreading contamination.