*3.2.3 Determination of the absorbed dose to air (Dair)*

The determination of the absorbed dose to air (Dair), measured by a standard instrument, is defined as the energy delivered to a mass of air of the well-known sensitive volume of the ionization chamber, defined by the relation:

$$D\_{air} = Q\_{air} \bullet \frac{W\_{air}/e}{m\_{air}} \tag{4}$$

Where.

*Qair* = is the charge produced in the air mass defined as the sensitive volume v of the chamber;

*Wair=e* = average energy needed to produce a pair of ions, its product being equal to the energy given to the air mass *mair* of the reference sensitive volume;

*mair* = equal to the product of the air density *ρair* and the sensitive volume v.

This measurement may require the use of a set of factors necessary to correlate the reading of the measurement system with the final value of the quantity, such as absorbed dose. The measurements must be carried out under the well-standardized reference conditions, that is: radiation field of 10 x 10 cm2 on the surface of the phantom, SSD (source surface distance) equal to 100 cm, with the center of the chamber positioned at 5 cm depth, reference temperature of 22°C (reference in Brazil), atmospheric pressure of 101.3 kPa, and relative humidity between 30 and 70% (**Table 1**).
