Answer to Question #13085 Submitted to "Ask the Experts"

Can you please define when and why to use an ionization chamber vs. a scintillation detector? How does one determine when to use one vs. the other?

The precise answer to your question depends, at least in part, on the specific characteristics of the two detectors, and what you are trying to measure. I shall assume that your concern is generally with dose-related measurements and that your major concern is with ionizing photons—x rays or gamma rays. I will attempt to make some comparative points that I hope will be helpful in your decision making.

Ionization detectors, especially air-filled chambers, often open to atmosphere, have long been detectors of choice for exposure or dose-related measurements, especially at moderate to high dose rates. Thus, many such ionization chamber-based instruments used for health physics survey type measurements, properly calibrated to interpret the quantity of interest, will have ranges from perhaps 10 microSieverts per hour (µSv h^-1) to perhaps 50–500 mSv h^-1. Small volume ion chambers commonly used in radiation therapy applications are designed to measure much higher dose rates. Properly designed air ionization chambers have a generally favorable energy response to photons over a rather wide range of energies, thus making them suitable for measurements when the photon energy distributions are not well-defined. Ionization chambers operate as mean level devices, meaning that the signal measured is the average current resulting from ionization produced by multiple events in the detector; consequently, the signal provides a direct measure of the actual primary ionization being produced in the detector volume, and this is directly proportional to dose to the gas, which is related to dose in tissue through calibration. Because true ionization chambers do not use any gas multiplication to increase sensitivity, the typical health physics instruments are often not useful for making low level dose measurements. In that respect they would not be a good choice, for example, if one were using an instrument to attempt to find a lost source or if one were trying to measure small deviations from background radiation levels. One modification that some manufacturers have made to enhance sensitivity is to use a pressurized gas in a sealed chamber to enhance the degree of ionization and allow measurements even at background levels.

On the other hand, many scintillation detectors have very high sensitivity to photons, and may provide much greater responses to such radiation than would a typical ionization chamber. This is especially true for inorganic scintillators, the most common of which is NaI(Tl). Survey type instruments using this material are often fabricated with detectors cylindrical in shape with common dimensions of 2.5 cm x 2.5 cm or 5 cm x 5 cm. Having both a large mass and a high atomic number, compared to air, such a detector will have a much higher sensitivity to photons over a wide range of energies. At the same time, because such detectors have an effective atomic number considerably higher than that of soft tissue, they generally exhibit a rather large energy dependence when attempting to measure tissue dose. These detectors almost always operate as event type detectors (unlike the mean level ion chambers), and count single events associated with individual interactions within the detector. The device will then measure the interaction rate of photons in the detector volume; because of the energy dependence the count rate is often not proportional to the tissue dose rate. Some energy compensation may be used by providing some added shielding around the detector to decrease response to photons below a few hundred keV in energy, but this inhibits measurements at lower energies. Additionally, at high dose rates, the larger volume inorganic scintillator instruments may not be able to handle the very high count rates, thus restricting their use to lower dose rates. This can be overcome to some extent by making small volume detectors. There are numerous inorganic scintillators that have been developed (e.g., CsI, CeF₃, LaF₃(Nd)), and some have properties more desirable than others, depending on the intended purpose, but I will not discuss these further.

One class of scintillation detectors that have found success, especially in high sensitivity instruments with a near tissue-equivalent energy dependence, are plastic-based scintillators, which incorporate organic scintillators into the plastic in the form of a solid solution. Such instruments have gained popularity as microdose meters, providing good sensitivity to monitor relatively small deviations from background levels and providing a near soft tissue equivalent response over a wide range of energies.

While there is much more that could be said on this topic, I hope the above is sufficient for your needs.

George Chabot, CHP, PhD