backAn almost parallel X-ray beam enters an air-filled, open parallel-plate chamber through an aperture of cross-sectional area A and leaves it through an exit opening. The axis of the beam runs through the centre of the pair of plates, to the upper plate (HV) of which an electric potential is applied which is higher than that applied to the lower plates (S-M-S). The electric field between the plates serves to remove the ion pairs generated by the X-radiation via secondary electrons.
The charges are dissipated via the measuring electrode (M) whose length in beam direction defines the ion collection volume. The guard electrodes (S) are to ensure the homogeneity of the electric field of the collection volume. The chamber dimensions must be chosen such that secondary electron equilibrium (SEE) exists for the reference volume VB (hatched area). Under SEE conditions, the absorbed dose to air is equal to the air kerma, except for the bremsstrahlung losses. The ion charge measured with a free-air chamber under SEE conditions can be converted into the absorbed dose to air using the ionization constant of air (W/e = 33.97 J/C). The following relationship between the air kerma, Ka, and the charge, Q/m, measured per unit of mass is obtained:
The deviations of a real free-air ionization chamber from an ideal one are taken into account by the product of correction factors ki.