Radiation Monitoring Procedures
Three fundamental types of radiation monitoring procedures are conducted at the NIH:
- Hand-held contamination survey meters are used to locate fixed and removable contamination on surfaces, equipment, personnel, and clothing, with the results usually expressed in counts per minute (cpm).
- Swipe or smear sampling is used to locate and quantify removable contamination on surfaces and equipment with results usually expressed in counts per minute (cpm) or disintegrations per minute (dpm).
- Exposure-rate metering is used to measure the exposure rate in an area from contamination and source material, and is usually expressed in milliroentgen per hour (mR/hr).
Routine contamination monitoring at the NIH is recorded and documented on a variety of
radioactive contamination surveys, which are indicated below:
- Daily Surveys
- Monthly Surveys
- Comprehensive Surveys
CONTACT DRS IMMEDIATELY IF ANY AMOUNT OF BARE SKIN HAS BEEN CONTAMINATED
For emergency contact information, see Emergency Contacts
Surveying for Removable Contamination
Removable contamination is defined as any radioactivity that can be transferred from a surface by swiping with moderate pressure. The swipe or smear test is used to assess the level of removable contamination and is the required method for monitoring radionuclides.
A small piece of cloth, filter paper, or fiberglass disk is used to wipe an object or sample location to determine the amount of removable contamination present. Fiberglass disks are preferable for low-energy beta-emitting radionuclides since they dissolve or become transparent in liquid scintillation fluid.
The allowable limits for removable contamination for restricted (posted for unsealed radionuclide work) and unrestricted (publicly-accessible) areas, are given below.
2,200 dpm/100 cm2 beta/gamma contamination
220 dpm/100 cm2 alpha contamination
| 220 dpm/100 cm2 beta/gamma contamination
22 dpm/100 cm2 alpha contamination
|
If a survey meter indicates a count rate greater than twice background, but a swipe analysis indicates that there is no removable contamination, contact the
area health physicist for further instructions. Do not attempt to continue to decontaminate an object or lab location when contamination levels measured with a survey meter cannot be further reduced. The AHP can consider regulatory limits for fixed contamination and advise the staff on what actions, if any, need to be taken to reduce radiation exposures for the fixed contamination.
Counting Methods for Swipes or Smears
Swipes or smears used to assess beta-emitting radionuclide contamination must be analyzed with a liquid scintillation counter (LSC). Swipes or smears to assess gamma-emitting radionuclide contamination can be analyzed with a gamma counter.
While an LSC may be used to assess gamma contamination, close attention must be given to the efficiency of the counter for the radionuclide in question and the techniques used. The manufacturer or qualified service representative should calibrate counters on an annual basis. This calibration determines the counting efficiency for common radionuclides used at NIH, which must be used to convert between counts per minute (cpm) and disintegrations per minute (dpm) as indicated in the Surveying for Removable Contamination section above.
When possible, liquid scintillation samples should be dark-adapted before analysis to reduce the effect of chemiluminescence, the emission of light due to a chemical reaction involving the scintillation fluid. This can be accomplished by simply leaving the rack in the counter with the cover closed for 30 minutes or more.
In addition, a blank sample (vial containing an unused swipe or smear) should be counted for background measurements along with a calibration standard. For good counting statistics, vials should be counted for at least two minutes each.
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