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

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 medium-energy X-rays (photon energy from 40 to 300 keV, including mammography X-ray qualities) in order to replace currently used secondary ionization chamber and to decrease the uncertainty of air kerma reference value. In the period of 2013–2017, the FAC has been designed, manufactured and put into operation. Correction factors were measured or calculated by a Monte Carlo method. FAC performance was preliminary tested using a calibrated secondary chamber. Physical characteristics of the FAC are described and a summary of the correction factors with the uncertainty budget is presented.