General ion recombination for ionization chambers used under irradiation conditions relevant for diagnostic radiology.

General ion recombination has been studied under irradiation conditions relevant for diagnostic radiology and with four different ionization chambers. When the exposure time is appreciably shorter than the ion transit time, the exposure can be designated as pulsed irradiation. On the contrary, for relatively long irradiation times, the term continuous irradiation can be applied. Recombination was estimated by measuring the collected charge at various collecting potentials of the ionization chamber. This is a well-known method in radiotherapy, but unfortunately it cannot be used in diagnostic radiology with typical exposure meters, since they do not offer the option of varying the collecting potential. For exposures with diagnostic x-ray units, an alternative approach is to vary the exposure or exposure rate over a wide range at a constant collecting potential. Experimental and theoretical estimates of ion recombination did not yield similar values. This might be due to several causes, such as differences between the actual and the nominal dimensions and volumes of the ionization chambers, due to errors and uncertainties in the physical parameters used in the theoretical models or due to deviations of the shape of the ionization chambers from the perfect cylindrical or parallel plate geometry. For better accuracy, corrections for recombination losses should therefore be based on experimental verification rather than on theoretical models.