Attix free-air chamber correction factors computed using EGSnrc

A cylindrical free-air chamber, the Attix FAC, is used for absolute air-kerma measurements of low-energy photon beams at the University of Wisconsin Medical Radiation Research Center. Correction factors for air-kerma measurements of specific beams were determined in the 1990s. In order to measure air-kerma rates of beams in development, new correction factors must be computed.

We aimed to compute monoenergetic correction factors for air-kerma measurements with the Attix FAC that could be used to determine corrections for arbitrary polyenergetic beams.

A model of the Attix FAC was created in the Monte Carlo code, EGSnrc. The EGSnrc user codes, egs_fac, and egs_chamber, were utilized to calculate aperture transmission, scatter, collecting rod electron loss, and wall electron loss correction factors for incident monoenergetic photon beams with energies between 5 and 50 keV. Beam-specific correction factors were then derived from the monoenergetic correction factors and compared with the currently accepted values.

Correction factors were computed in 0.5 keV intervals. The newly calculated beam-specific correction factors and the old conventional values agreed within 0.1% for all beams investigated.

The process for determining monoenergetic correction factors for air-kerma measurements with a free-air chamber is detailed in this work. Beam-specific correction factors can then be calculated if photon spectra are known. This process can be carried out for any free-air chamber, given specific materials and dimensions for modeling.