Introduction. To be better suited for measurements
at air kerma rates which are common for the calibration of detectors
for dosimetry in brachytherapy and radiation protection, two
graphite-wall, air-filled cavity ionization chambers (PS-10 and
PS-50 built by PTW of Freiburg, Germany) were characterized for use
as primary standards at the Physikalisch-Technische Bundesanstalt
(PTB) for 60Co, 137Cs and 192Ir gamma ray sources.
Methods. Applying experimental or Monte Carlo methods,
corrections were determined for wall effects, stem scattering
effects, point-source non-uniformity, saturation effects, and
deviations from Spencer-Attix cavity theory. The geometrical cavity
volume of the ionization chambers was determined by a special
experimental method. Non-charge-collecting parts of the volume were
calculated by finite-element simulations. In addition, mass
electronic stopping powers and mass-energy absorption coefficients
were reevaluated in close agreement with ICRU 90. To compare the
results to an already established air kerma standard, measurements
of air kerma rates were taken for gamma rays from all three sources.
Results. In contrast to the standards currently in use, the
newly characterized ionization chambers show significant wall
effects up to 4.17% (192Ir). For the PS-10 ionization
chamber, the geometric volume differs by 0.42% from the charge
collecting volume. For the ^192Ir source, Spencer-Attix
corrections were applied exceeding 0.30% for both ionization
chambers. Applying the derived corrections factors, air kerma rates
measured with the two ionization chambers at the three sources did
not differ by more than 0.13%. Comparing the results to an
already established air kerma standard, deviations ranged up to
0.61%. The uncertainty budget for the determination of air kerma
rates resulted in a relative combined standard uncertainty of
0.35%.
Conclusion. Both ionization chambers can be used as primary
air kerma standards for 60Co, 137Cs and 192Ir gamma
ray sources using the values of corresponding physical quantities
and correction factors derived in this work.