Effect of Three-Dimensional Detector Orientation on Small-Field Output Factors

The IAEA TRS 483 has recommended that the orientation for cylindrical ionization chambers be perpendicular to the beam for small-field output factor (OF) measurements. The recommendation was based on the unavailability of field output correction factor data for measurements using parallel orientation at the time of publication. Two three-dimensional (3D) air ionization chambers were used to perform measurements in parallel and perpendicular orientations and compared to data determined using a PTW 31018. The aim of the study was to establish whether the 3D detectors behaved as spherical or cylindrical devices. From the results, it was confirmed that the PTW 31016 and PTW 31021 detectors are suitable for OF measurements in both orientations for field sizes down to an equivalent square field of 1.8 cm and 0.96 cm, respectively, using the field output correction factor data published in the IAEA TRS 483. The preferred orientation is parallel to the beam to facilitate beam profile measurements and minimize the irradiation of the chamber stem and detector cable and decrease the volume averaging factor.