Characterization of the new free-air primary standard for low-energy X-rays at CMI

•A new primary standard for low-energy X-ray beam constructed at CMI.•Correction factors calculated by MCNPX simulations.•The chamber performance tested by an informal comparison with BEV (Austria).•The chamber considered ready for key comparison and standardization of X-ray beams.

In 2011 a decision was made by Czech Metrology Institute to build a free-air ionization chamber (FAC) intended to be used as a primary standard of air kerma rate for low-energy X-rays (photon energy below 50 keV, including mammography X-ray qualities) in order to replace the currently used secondary ionization chamber and to decrease the uncertainty of air kerma reference value. In the period 2011–2012, the FAC has been designed, manufactured and put into operation. Its performance was tested using a calibrated secondary chamber and then by an informal comparison with a national primary standard of BEV (Austria). Physical characteristics of the FAC are described and individual correction factors are discussed focusing on computational methods utilized in their estimation. Summary of the correction factors with the uncertainty budget is presented.