The response characteristics of a newly designed plane-parallel ionization chamber in high-energy photon and electron beams.

A new plane-parallel ionization chamber has been designed by Attix to overcome the shortcoming of previous commercially available parallel-plate ionization chambers for dosimetry in high-energy photon and electron beams in radiation oncology. This investigation details the performance characteristics of this new, commercially available plane-parallel chamber. The magnitude of the polarity effect in high-energy electron beams is shown to be less than 1% while the polarity effect in high-energy photon beams is lower than several other plane-parallel ionization chambers. The over response of the chamber in the buildup region of normally incident high-energy photon beams is less than 1% for 6- and 24-MV x rays while the response of the new chamber to obliquely incident x-ray beams was affected much less by the angle of beam incidence than the other chambers tested. These superior response characteristics are primarily due to the construction characteristics of the collecting electrode arrangement. The Attix chamber, with a wall diameter (w) of 40 mm and a plate separation (s) of 1 mm, has an aspect ratio, (w/s), of 40. This exceeds the previously reported design criterion of w/s > or = 25 required to properly measure surface and buildup dose in either conventional therapy beams or in beams that are highly contaminated.