Answer to Question #9194 Submitted to "Ask the Experts"

Please answer whether my interpretation of radiation measuring devices is correct:

Several answers in your Ask the Experts feature define/calculate dose in terms of activity of a source and the energy(s) of a source. However, I have never seen it explicitly stated that an event detector, e.g., a Geiger counter or a scintillation meter, cannot measure, or even estimate, dose from a source, or combination of sources, emitting photons with multiple energies. Even so, many Geiger and scintillation instruments have readouts in dose units, not count units. These readouts can only be accurate when the meter is measuring a single source at the energy that coincidentally served as the calibration energy for that meter. All dose readouts measured at other energies should be erroneous. Although energy-compensated meters "flatten" the meter's response curve, they in no way compensate for the difference between dose and event counting. For field purposes, only an ionization chamber can measure radiation dose.
 

I believe your statement is too severe in its declaration that "a Geiger counter or a scintillation meter, cannot measure, or even estimate, dose from a source, or combination of sources, emitting photons with multiple energies." There are numerous GM detectors that use simple energy compensation to provide an acceptably flat dose response (+15 to 20%) to photons from less than 100 keV to more than 2 MeV with sufficient accuracy for most routine health physics measurements.

It is also true that a number of successful, relatively energy-independent scintillation detectors have been designed and are available for dose measurements. Perhaps most notable in this regard are plastic scintillators that provide a near soft-tissue response over a wide range of photon energies. For these detector types, the calibration typically uses one or more photon energies that fall within the acceptable range of the detector response. The calibration process requires adjusting the output to yield the correct dose rates when exposed to the source(s).

As you have pointed out, these detector types are event detectors and the calibration process is actually correlating a count rate with a dose rate (or exposure rate). The reason the calibration works for an energy-compensated detector, such as a GM that often uses a metal wrap around the tube to provide the compensation, is that the energy compensation largely eliminates responses to lower-energy photons that in the detector elements would interact by the photoelectric effect, although they might have interacted differently (Compton scatter) in low atomic-number material such as soft tissue or air. The energy compensation ensures that most all of the interactions that occur in the detector from somewhat less than 100 keV to about 2 MeV are Compton interactions, just as they would be in lower atomic-number material. The penalty paid is that responses to low-energy photons is lost or markedly degraded.

We should also note that when photon energies exceed several MeV, energy-compensated GM detectors will often overrespond with respect to exposure or tissue dose measurements because of an excess of pair production events occurring in the detectors, compared to what would occur in lower atomic-number air or tissue. Thus, such a detector is not a good choice to make measurements in containment of a light water reactor under power when photon fields are dominated by the 6⁺ MeV photons from ¹⁶N.

As you have noted, some instruments include count-rate scales as well as dose- or exposure-rate scales. The count-rate scales are often used when the detector might be intended for assessing surface contamination (many GM detectors are fabricated with a thin window that may be exposed for purposes of measuring particulate radiations, such as beta and sometimes alpha, from surface contamination). When measurements of such surface contamination are made, the calibration obtained with the photon source(s) for dose or exposure measurements naturally does not apply. In order to interpret surface-contamination measurements, calibrations should have been made with appropriate particulate radiation sources.

Given the above, your statements—"Although energy-compensated meters 'flatten' the meter's response curve, they in no way compensate for the difference between dose and event counting. For field purposes, only an ionization chamber can measure radiation dose"—are also somewhat inaccurate. It may be true that an ionization chamber is often the best choice when measuring a photon radiation field of unknown energy characteristics or when high field intensities might be encountered (for which saturation effects may occur in an event-type detector), but it is also true that the overall energy characteristics and ranges of dose rates of photon fields encountered in occupational environments are often known sufficiently that the instrument user can make an informed judgment that a compensated GM detector or a scintillation detector would suffice.

I hope this is helpful.

George Chabot, PhD, CHP