SEMICONDUCTOR-DETECTORS IN OUTPUT FACTOR MEASUREMENTS

Background and purpose: Output factors are generally measured with cyl indrical ionization chambers. It was investigated if Si-diodes of p-ty pe instead could be used. The advantage would be the small detector si ze and the robust construction of the detector. Materials and methods: Two types of diodes were studied, one with a shielding layer of tungs ten specially made to reduce the excess response for scattered photons and one standard diode without any extra shielding. The measurements were performed at accelerating potentials between 4 and 50 MV and beam sizes between 4 cm x 4 cm and 40 cm x 40 cm. Results: The results sho wed that both types of diodes are suitable for measurements of head sc atter factors in mini-phantoms. However, the diodes were found inappro priate for measurement of output factors for large fields in extended water phantoms. For small fields (<10 cm x 10 cm) a small detector is advantageous and no errors due to the scatter contribution were seen. Conclusions: An cylindrical ionization chamber is the best choice for output factor measurements in extended water phantoms for large field sizes while diodes are an alternative in small fields. There were negl igible differences between the detectors in head scatter measurements in mini phantoms. (C) 1997 Elsevier Science Ireland Ltd.