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 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.