title: A new re-entrant ionization chamber for the calibration of iridium-192 high dose rate sources.
abstract: A re-entrant (well-type) ionization chamber has been designed and fabricated at the University of Wisconsin for use with iridium-192 high dose-rate (HDR) remote after-loading brachytherapy devices. The chamber was designed to provide an ionization current of about 10(-8) ampere with a nominal 10 curie iridium-192 source. A narrow opening is provided into the sensitive volume of the chamber to insert a Nucletron MicroSelectron catheter, or catheters with similar diameters from other HDR manufacturers. The chamber exhibits a flat response (+/- 0.1%) for any source position within +/-4 mm of the chamber center. A 300 volt chamber bias yields a 99.96% ion collection efficiency. The chamber is capable of being calibrated directly with an iridium-192 source which has in turn been calibrated with thimble-type ion chambers. Reproducibility for readings in the current mode for 10 consecutive insertions of the MicroSelectron iridium-192 HDR source is within 0.02% or less. Two thimble chambers calibrated by the U.S. National Institute of Standards and Technology provide calibration traceability of iridium-192 HDR sources and re-entrant chambers to a primary national standards laboratory. Results of activity measurements of 6 commercial iridium-192 HDR sources are reported.
authors.affiliation: UCLA Medical Center, Department of Radiation Oncology 90024-6951.
authors.forename: S J
authors.initials: SJ
authors.lastname: Goetsch
authors.forename: F H
authors.initials: FH
authors.lastname: Attix
authors.forename: L A
authors.initials: LA
authors.lastname: DeWerd
authors.forename: B R
authors.initials: BR
authors.lastname: Thomadsen
mesh_terms: D002138:Calibration; D006801:Humans; D007496:Iridium Radioisotopes; D011829:Radiation Dosage
chemical_list: D007496:Iridium Radioisotopes