AbstractID: 7849 Title: Absorbed Dose Measurements of a Handheld 50-kV X-ray
Source in Water with Ionization Chambers and Plastic Scintillators
Absorbed dose rate measurements of a 50 kV handheld x-ray probe source in a water
phantom are described. The x-ray generator is capable of currents of up to 40 µA and is
designed for cranial brachytherapy and intraoperative applications with applicators. The
measurements were performed in a computer-controlled water phantom in which both the
source and the detectors are mounted. Two small ionization chambers were employed
with volumes 0.02 cm3 and 0.0053cm3, respectively. Both chambers were held in water
tight mounts made of water- equivalent plastic with covering layers of 1.0 mm and 0.5
mm of water-equivalent plastic, respectively. The ionization chambers are calibrated in
terms of air-kerma rate, and conventional protocols are used to convert the measurements
to absorbed dose rate. Two small-volume scintillators were also employed for absorbeddose measurements, both with 1 mm diameters, and 0.4 mm and 1 mm thicknesses,
respectively. The scintillators are calibrated in terms of absorbed dose rate to water using
90
Sr/Y reference beta particle beams. The reference points of the various detectors were
taken as the center of the scintillator volumes and the entrance windows of the ionization
chambers. All detectors were mounted in the water phantom such that the reference
points were at the same location. Measurements were made at distances of 3 mm to 40
mm from the detector reference point to the source isocenter. The results of the
measurements show the expected under-response of the scintillators, and also the volume
averaging effects of the scintillators at very close distances.