Establishment of air kerma reference standard for low dose rate Cs‐137 brachytherapy sources

A guarded cylindrical graphite ionization chamber of nominal volume 1000   cm 3was designed and fabricated for use as a reference standard for low‐dose rate  137 Cs brachytherapy sources. The air kerma calibration coefficient ( N K ) of this ionization chamber was estimated analytically using Burlin's general cavity theory, as well as by the Monte Carlo simulation and validated experimentally using Amersham CDCS‐J‐type  137 Cs reference source. In the analytical method, theN Kwas calculated for 662 keV gamma rays of  137 Cs brachytherapy source. In the Monte Carlo method, the geometry of the measurement setup and physics‐related input data of the  137 Cs source and the surrounding material were simulated using the Monte Carlo N‐Particle code. The photon energy fluence was used to arrive at the reference air kerma rate (RAKR) using mass energy absorption coefficient. The energy deposition rates were used to simulate the value of charge rate in the ionization chamber, and theN Kwas determined. The analytical and Monte Carlo values ofN Kof the cylindrical graphite ionization chamber for  137 Cs brachytherapy source are in agreement within 1.07%. The deviation of analytical and Monte Carlo values from experimental values ofN Kis 0.36% and 0.72%, respectively. This agreement validates the analytical value, and establishes this chamber as a reference standard for RAKR or AKS measurement of  137 Cs brachytherapy sources. PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.bg, 87.55.Qr