Answer to Question #7449 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

Can I use a GM survey meter to measure radiation from a medical x-ray unit? My GM detector is calibrated in µSv h-1 and for my measurements I use 100 kV and a time of three to four seconds.

A

It is possible to use some GM (Geiger-Mueller) instruments to measure stray radiation (scatter and possible leakage radiation) from some x-ray machines. Not all GM detectors are created equal, however, and the detector must have a reasonably thin window or wall for measurements of the x rays. For a machine operating at 100 kV, the effective beam energy is probably between 30 and 40 keV, with photons ranging from energies lower than this up to 100 keV.

Energy-compensated GM detectors that use a wrap of rather heavy metal around the detector provide reasonably uniform energy dependence, but they will normally attenuate the x rays to a greater extent than is desirable. One then uses the thin-window detectors without energy compensation, and such detectors, often calibrated at higher photon energies (e.g., 662 keV from 137Cs), will usually overrespond at the dominant x-ray energies.

In order to interpret dose properly, one would have to know the extent of overresponse, which is often a factor of 2 to 3 for the diagnostic x rays. Some such information is available from the detector manufacturer. Naturally, if your detector was calibrated against x rays comparable to those you wish to measure, you do not have to be concerned about this energy dependence.

A second restriction on the use of the GM detector has to do with the time-dependent output characteristics of the x-ray machine. If the machine is a typical radiographic unit that operates under continuous current, producing a continuous beam, then a GM detector may be acceptable (taking into account any energy dependence as discussed above). If the machine has a pulsed output, however, as is the case with many therapeutic accelerators, and possibly some modern diagnostic pulsed fluoroscopy units, I would not recommend using a GM detector. Depending on the pulse duration and intensity, the GM detector may not interpret actual dose rate but may simply read out the pulse rate of the machine.

Assuming that the detector and machine requirements are met, a third item to keep in mind is the intensity to be measured. You should not attempt measurements in the primary beam as intensities will typically saturate the GM detector. Be aware of the dose rate limitations on the detector and instrument you are using. Because of the high dead time that characterizes GM operation, measurable dose rates are restricted, sometimes to less than 0.5 mSv h-1. Some instruments that include electronic dead-time compensation techniques may extend the measurable dose rates considerably.

Good luck in your measurements.

George Chabot, PhD, CHP
 

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