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-p late ionization chambers for dosimetry in high-energy photon and elect ron beams in radiation oncology. This investigation details the perfor mance characteristics of this new, commercially available plane-parall el chamber. The magnitude of the polarity effect in high-energy electr on beams is shown to be less than 1% while the polarity effect in high -energy photon beams is lower than several other plane-parallel ioniza tion chambers. The over response of the chamber in the buildup region of normally incident high-energy photon beams is less than 1% for 6- a nd 24-MV x rays while the response of the new chamber to obliquely inc ident x-ray beams was affected much less by the angle of beam incidenc e than the other chambers tested. These superior response characterist ics are primarily due to the construction characteristics of the colle cting 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 greater-than-or-equal-to 25 required to properly measure surface a nd buildup dose in either conventional therapy beams or in beams that are highly contaminated.