Instrument Surveys & Wipe Tests
Geiger counters (GM) are portable instruments used to detect ionizing radiation and can also be used to survey areas for ambient radiation dose rates (“area surveys”), providing the correct detector is used. The Geiger counter is the least expensive, fastest and generally the most reliable means of detecting and measuring radioactive contamination. The beta pancake detector is used with the Geiger counter for finding and measuring beta radiation, and will detect all beta radioisotopes used at Emory except H-3 and Ni-63. It does not detect those nuclides because their betas are too low in energy to penetrate the window of the detector. Radioisotopes which may be detected with the beta pancake are C-14, S-35, P-33, P-32, Ca-45, Cl-36 and other beta emitting nuclides.
The low energy gamma probe is used with the Geiger counter to detect and measure gamma radioisotopes of various energies. It is most efficient for I-125, but will perform adequately for Cr-51, In-111, Co-57 and other gamma emitting nuclides. These detectors will also detect low energy x-rays, such as those emitted by beta emitters producing Bremsstrahlung radiation.
When to Perform
Dose-rate surveys, at a minimum, must be performed in locations where workers are exposed to radiation levels that might result in radiation doses in excess of 2 mrem/hr. Contamination surveys must be performed:
- To evaluate radioactive contamination that could be present on surfaces of floors, walls, laboratory furniture, and equipment;
- After any spill or contamination event;
- When procedures or processes have changed;
- To evaluate contamination of users and the immediate work area, at the end of the day, when radioactive material is used;
- In areas adjacent to restricted areas and in all areas through which radioactive materials are transferred and temporarily stored before shipment.
How to Record Survey Results
Record Geiger Counter survey results in mR/hr, or include the instruments' sensitivity on your report (please see Area Survey Record (linked below). If you use gamma or high energy beta emitters you must record an instrument survey each week, usually recorded on the same sheet as the wipe test. If you must use CPM instead of mR/h, also record the sensitivity which is stated on the meter's calibration sticker. Typical sensitivities are 1000 to 3000 CPM per mR/hr.
For more information, please consult the Area Survey Record, Example Map, and Radiation Safety Record Keeping Guidelines (all linked below).
In conjunction with Instrument Surveys, wipe tests are performed to detect and quantify radioactive contamination on surfaces of work areas and/or equipment. Removable contamination can be detected and measured through a wipe test of the surface, which is counted in an appropriate counting instrument, such as a liquid scintillation counter, a sodium iodide or germanium gamma counter, or a proportional alpha/beta counter.
How to Record Wipe Test Results
Record wipe tests results in DPM, or include the counter efficiency on your report. It is required that labs record a wipe test each week radioactivity is used to check for contamination. Many labs record the results of their survey in CPM, or counts per minute when the regulations require that the results be recorded in units of activity, such as uCi or more commonly, DPM or disintegrations per minute. Using CPM underestimates the amount of contamination due to counting losses. Many modern scintillation counters can display DPM, and if so, you should use those display options. Alternatively you can write the counter efficiency somewhere on the report, remembering that you would have slightly more contamination present, decontaminate at slightly lower CPM trigger levels. Efficiencies should be checked at least annually, and is often done during the annual preventative maintenance service. Typical efficiencies for counting in liquid scintillation cocktail are:
55% - 65%
90% - 95% (also S35 & Ca45)
95% - 99%
and are determined primarily by comparing the activity of a known standard sample with the count rate observed from that sample:
Efficiency = Observed CPM/Known (true) DPM
from which we can calculate the true activity of a wipe or sample, once the efficiency has been determined:
Known (true) DPM = Observed CPM/Efficiency
mCerenkov and gamma counting efficiencies vary widely and should be determined for each situation.