Experimental and Theoretical Investigation of the Collection Efficiency of Air-Filled Ionization Chambers in Pulsed Radiation Fields of High Pulse Dose

Ionization chambers are used in physical radiation research and are the most important dosimeters in radiation therapy. In this contribution an investigation of the collection efficiency by experiments and theoretical description is presented. The collection efficiency was measured for a plane-parallel advanced Markus ionization chamber (PTW 34045, 1 mm electrode spacing, 300 V nominal voltage), a chamber often used in clinical practice. The measurements were performed for collection voltages of 100 V and 300 V by irradiation with a pulsed electron beam of varied pulse dose up to approximately 600 mGy (0.8 nC liberated charge). These results are compared to existing descriptions of the collection efficiency and our own model based on a numeric solution by Euler method of a differential equation system modelling the processes within the chamber.
While the existing models accurately describe the collection efficiency at the lower collection voltage (100 V) they fail to reproduce the experimentally observed behavior at the higher collection voltage (300 V) particularly at high pulse doses. In contrast, our own numeric solution reproduces the collection efficiency at all tested voltages and pulse doses. This illustrates the importance of considering additional effects such as electric shielding by the liberated charges and field strength dependent attachment of electrons which are not considered in the existing models. Subsequently, the developed more accurate numeric solution might provide a valuable tool for future investigations.