AbstractBackgroundA 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.PurposeWe 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.MethodsA 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.ResultsCorrection 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.ConclusionsThe 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.