Background: Air attenuation of free-air ionization chamber is one of the biggest correction terms and also has an influence on the total uncertainty of correction factor. Purpose: This study aims to measure the air attenuation correction factor for cylindrical free-air ionization chamber

and the correction of attenuation length between the diaphragm and the collecting region of cylindrical free-air ionization chamber in actual for the exact ionization current measurements. Methods: Molybdenum target X-ray vacuum tube test system and cylindrical free-air ionization chamber were used to measure the ionization current by means of vacuum. Electron Gamma Shower (EGSnrc) program was used to simulate constant pressure pipe for X-ray beam of the influence of additional losses. Results: The results showed that the ionization current of free-air ionization chamber was proportional to the pressure of the constant pressure pipe; and the extra loss of constant pressure pipe for X-ray beam was less than 0.5%; the design of the vacuum tube test system conformed to the requirements of experiment. Conclusion: For the air kerma absolute measurement of molybdenum target X-ray

the method of experimental measurement of air attenuation correction factor of cylindrical free air ionization chamber is of scientific reference value. Abstract