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Monitoring Instruments

Every laboratory using radioactive materials must possess or have available for immediate use appropriate radiation monitoring equipment. This equipment must be in good working order, and must be calibrated yearly by EHS Health Physics staff. Results of this calibration will be forwarded to the project leader. Equipment that has not passed this annual examination must be removed from service until it is repaired or replaced. If you believe that there is a problem with your equipment, contact EHS and arrange a time when the equipment can be inspected and calibrated.

Radioactive monitoring instruments must be capable of detecting the radioisotope being monitored at or below the contamination limits listed in the section on Radiation Surveys. To calculate the sensitivity of the survey instrument, the following formula may be used.

(Background of instrument x 2) (Efficiency* for the isotope being measured) = Minimum Detectable
Activity in DPM
*Note that efficiencies are given in the EHS calibration results.

There are several types of monitoring instruments commonly used in teaching and research laboratories. The most widely used instrument is the Geiger counter, a portable instrument capable of detecting beta or gamma radiation, providing the appropriate 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 Michigan State University except 3H and 63Ni. 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 reliably with the beta pancake are 14C, 35S, 33P, 32P, 45Ca, 36Cl, and other beta emitting nuclides.

The low energy gamma (LEG) probe is used with the Geiger counter to detect and measure gamma radioisotopes of various energies. It is most efficient for 125I, but will perform adequately for 51Cr, 111In, 60Co and other gamma emitting nuclides. These detectors will also detect low energy x-rays, such as those emitted by beta emitters producing Bremsstrahlung radiation.

Another instrument in common use is the liquid scintillation counter. It is necessary to use it in radiation safety surveys for 3H and 63Ni, since no other instrument will detect these nuclides. Liquid scintillation counters work for both beta and gamma nuclides for quantifying what is in a sample. It is not an adequate primary method of evaluating contamination surveys, however, since samples measured consist of wipes of the areas of suspected contamination. If the contamination is not removable, the wipe will not pick it up, and contamination will not be detected. It is also possible for only part of the contamination present to come up on a wipe, not giving an accurate measurement of the contamination present.

A third instrument which may be used to evaluate contamination is the gamma well counter. Again, this is used to gather data in samples, but for the same reasons as the liquid scintillation counter, it is not a good radiation survey instrument.

Ion chambers are used commonly by the Radiation Safety staff and in locations where frequent and higher flux external radiation hazards are present; they are typically not used for contamination surveys by laboratory staff. These instruments measure the ions produced in air (of one sign) by gamma radiation, and are a good indicator of radiation exposure fields. They are useful for exposure potential screening on shipments, hot parts at the Cyclotron, drums of waste at the Radioactive Waste Building, packages prior to shipment and sources and stocks of radioactive materials.

Other more sophisticated instruments used to detect and quantify radiation are the gamma spectrometer or multi-channel analyzer, neutron detectors, alpha detectors, and a wide array of electronic dosimeters, area monitors, and even portal monitors (which a person walks through to detect any contamination on the body or clothing; these are used at reactors).

For effective and accurate data gathering in radiation, follow a few simple guidelines:

  1. Survey at the proper geometry. Hold the detector about 1 cm. or 1/2 inch above the surfaces monitored. If the detector is too far away, serious underestimation of activity or no detection of activity present may occur. If the detector is too close, contamination of the detector may occur.
  2. Use the correct detector. Do not survey for beta radiation with a gamma probe, or for gamma radiation with a beta probe. NO GEIGER COUNTER WILL DETECT TRITIUM; LIQUID SCINTILLATION TECHNIQUES MUST BE UTILIZED.
  3. Survey slowly; do not race the detector over the surface or wave it like a magic wand; the sensitivity of the detection is inversely proportional to increasing survey speed.
  4. Do not cover the detector while surveying; covers decrease or eliminate detection, since they act as a shield.

Calibrations

Radiation Safety Manual Table of Contents


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