Answer to Question #12600 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Please define the term "free-in-air" dose. How is this different from either tissue dose or air dose?
As you might suspect, the term "free-in-air" as it applies to radiation measurements of quantities such as dose or air kerma means that the assessment of the quantity of interest, commonly through an appropriate measurement, is made in an air environment free of other perturbing materials, such as dosimetry phantoms or other scattering materials.
This may be different from other dose quantities, such as those that you describe, in that such latter quantities may be determined in situations in which other materials may have dose-influencing impacts. For example, a free-in-air air kerma measurement may be made using an air-equivalent ionization chamber placed at a point of interest in air with no other influencing materials brought into the vicinity. On the other hand, if one is interested in the soft tissue dose near the surface of the body of a person at the same location, an ionization chamber or other possible measurement entity may be placed at that spot on the surface of a tissue-equivalent phantom, commonly constructed of plastic; the response of the measuring device will then be affected by both the direct radiation incident on the device and the radiation scattered back into the device from the phantom.
The quantity known as air dose may be essentially the same as free-in-air kerma if a condition of secondary charged particle equilibrium exists at the point of interest in air and no other influencing materials are present. However, the same quantity in some circumstances may also be applied to air dose evaluated within a material different from air. For example, it is possible to define air dose at a point within a volume of water, lead, or any other medium. Such assessments may be made using theoretical determinations, or they made be made by measurements using an air-equivalent measuring device, such as an air ionization chamber centered at the location of interest in the medium.
I hope this is sufficient to address your concerns.
George Chabot, PhD