Comparison of the Measured Stem Leakage Correction Factor for an Ionization Chamber in Air to the Monte Carlo Simulation

Stem leakage arises as a consequence of the direct irradiation of a sensitive volume, and the metal stem of the ionization chamber, and the chamber response is likely to be affected by the photons scattered from the chamber stem. These interactions contribute to the apparent measured exposure. Build-up caps were used in the exposure measurement. The stem leakages of PTW model TM 30013 and TM 31010 (vented to air, sensitive volume: 0.6 and 0.125 cm(3)) ionization chambers were measured. High-energy photons generated by a linear accelerator and the sensitive volume of the ionization chamber were measured to examine the stem leakage of the ionization chamber and were compared with the results of a Monte Carlo simulation. As a result, the electrons ejected from the metal stem could were found to collect at the central electrode. The stem leakage correction factor (k(stem, leak)) decreased with increasing photon energy and sensitive volume. The stern of the ionization chamber that scattered photons made the largest contribution to the exposure dose, showing a dependency on the energy of photons and the sensitive volume of the ionization chamber. The stem leakage correction factors were determined within 0.4% according to the length of the stem exposed in repeated measurements for various radiation fields.