Large-volume ionization chamber with variable apertures for air-kerma measurements of low-energy radiation sources

This report describes the design and initial measurement results of a large-volume ionization chamber designed to realize the air kerma from therapeutic radiation sources emitting photons with energies up to about 70 keV. The measured ionization current is used to provide absolute source calibrations by the most widely used brachytherapy source strength metric called air-kerma strength. The variable-aperture free-air chamber (VAFAC), named because of its variable aperture stand, will provide insight into the angular dependence of air-kerma strength measurements. Another unique feature of this ionization chamber is its seed holder design that reduces unwanted scatter, ensures vertical positioning, and rotates the seed with a positional accuracy of 0.3°. Benchmark experiments show that the VAFAC agrees well with existing National Institute of Standards and Technology standards and is capable of measuring clinical-strength seeds with repeatabilities of less than 2% at the 1 σ level ( k = 1 ) .