University of Vermont

Radiation Detectors

Proper Use of Portable Radiation Detectors

When it comes to detecting the presence of radioactive contamination in the workplace, our normal senses of sight, hearing, smell, taste, and touch are useless. In order to maintain a contamination-free work environment, the proper use of radiation detectors is critical. Fol lowing are some simple techniques to maintain quality control of your instrument and realize its full potential as a survey meter.

1) Calibrate your instrument on an annual basis. This will ensure that the high voltage set tings are at their proper settings, and that the instrument is in good working condition. This is also required by the NRC for instruments used for area and personnel surveys.

2) Before using the instrument for any contamination or personnel surveys you should check the battery level, and the counters response with a check source. The check source can be a calibrated standard, stock vial or any known activity radioactive source. This is important even if the instrument has been calibrated recently, as the instrument may have been physically damaged since calibration. At the Radiation Safety Office we perform quality control using calibrated standards on all our instruments on a daily basis.

3) Make sure you are using the appropriate detector for the radionuclide you are surveying for. For example to detect beta emitters such as P-32, the G.M. tube is most efficient, while for gamma emitters such as I-125, the external probe containing a NaI crystal is most efficient.

4) Before beginning the survey, you should take a reading to determine the background. If the background seems excessively high, check the area to determine if there is a radioactive source causing an artificially high background. While surveying, pass the Geiger counter slowly over the survey area. If you go too fast, the counter will not have adequate time to respond. The counter's detector should pass over the area about 1 - 2 inches from the surface. In order to keep the instrument itself from being contaminated, it is a good idea to place saran wrap or parafilm around the end of the detector. Never use the instrument with the plastic cap on the end. This cap is to protect the instrument from physical damage only. If the cap is left on, some of the radiation you are trying to detect will be shielded, and the instrument will lose efficiency.

5) Remember that any area that exceeds 3 times the background is considered significantly contaminated and must be decontaminated and resurveyed. Also remember that this rule applies only to contaminated surface areas and not to disposable items. In order for possibly radioactive items to be disposed of in the normal trash, they must be equal to or less than background.

If you have any questions about using Geiger counters, calibration information, check sources, or radiation safety in the work environment, please contact the RSO (656-2570). We will be glad to assist you and answer any questions you may have.

-Ron Kimball

Last modified February 25 2005 02:19 PM

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