Answer to Question #13610 Submitted to "Ask the Experts"

Can a pressurized ion chamber survey meter be replaced by a CsI(TI)-based survey meter of range up to 6 MeV. Are pressurized ion chambers best for x-ray scanner leak detection? Is there an alternative for pressurized ion chambers?

I cannot provide a definitive answer to your questions without knowing all the details of expected use, but we can provide some information that I hope will be useful. During the discussion I will provide some representative examples of specific detector types; these are intended to be examples, and their inclusion here does not represent an endorsement of any specific instrument.

Regarding your first question as to whether a pressurized ion chamber can be replaced by a CsI(Tl) detector, the answer depends on what the specific application is and what the detector designs look like. If you are trying to make dose-related measurements it is generally true that the photon energy dependent response is significantly greater for the CsI because this detector has a very high effective atomic number compared to the typical gas-filled ion chamber, especially if the latter is an air-filled chamber. The high atomic number tends to produce an overresponse in tissue or air at lower energies (less than a few hundred keV) because of enhanced photoelectric interactions and possible overresponse at higher energies (greater than 2 MeV) because of enhanced pair production interactions. It is possible to provide some energy compensation for the CsI detector, but this often results in excess reduction of low energy dose estimates. The CsI(Tl) detector has found favored use in some instruments intended primarily for energy spectroscopic assessment, but it is not a favored detector for dose-related measurements.

Concerning your second question, some pressurized ion chambers are used for measurements of x-ray leakage, but they are not always the "best" choice, depending on your specific needs and interests. Because these ion chambers are pressurized, commonly to about five to ten atmospheres, the chamber wall thicknesses must be sufficient to sustain the pressure without leakage; considering this together with the need for the instrument's casing, the low energy photon response (less than about 50 keV) may be somewhat sacrificed compared to some other thinner wall/window detectors, leading to possible underestimates of doses at the lower energies. Here is a link to one such instrument.

This brings us to your third question regarding availability of alternative detector types. Other instrument options may include Geiger-Mueller based detectors with thin windows and some possible energy compensation that allow for acceptably accurate dose measurements down to about 20 keV (here is a link to one GM-based instrument). Another option is a plastic scintillator-based instrument. These solid plastic detectors have high sensitivity and, because the plastic is fairly similar in effective atomic number to soft tissue, they provide good energy response relative to tissue. Some also allow acceptably accurate measurements to photon energies as low as 15 to 20 keV The model designated MICROREMAOLEED extends the low energy limit to less than 20 keV.

George Chabot, PhD, CHP