Abstract: In order to carry out the work of traceability and transmission of X-rays at the dose rate of radiotherapy, according to IEC 60731—1997 standard, a standard device of X-rays air kerma (radiotherapy) with tube voltage of 10-250 kV and dose rate range of 1.0×10^-3 -10 Gy/h was established. Of which 10-60 kV X-ray air kerma (radiotherapy) standard device at 1.0 m heterogeneity is less than 1% of the radiation field for ø60 mm, scattering contribution to radiation field is less than 1.2%, from the source 1 m-5 m within the inverse square law at 2.5%, within the scope of using standard ionization chamber stability of the measurement device is 1.8%, repeatability is 0.1%. 60-250 kV X-ray air kerma (radiotherapy) standard device at 1.0 m heterogeneity is less than 1% of the radiation field for ø80 mm, scattering contribution to radiation field is less than 1.2%, from the source 1 m~5 m within the inverse square law at 1.5%, within the scope of using standard ionization chamber stability of the measurement device is 1.7%, repeatability is 0.03%. The relative extended uncertainty of the radiation field air kerma (rare) of the standard device was 3.0% (k=2). The performance indexes of the measured device all met the verification/calibration requirements of the therapeutic level dose detection instrument.

Key words: X-ray, half-value layer, radiotherapy, air kerma (rare)