Abstract: A standard device for γ-ray air kerma (radiotherapy) was established using ⁶⁰Co radioactive source with an activity of 1.85×10¹⁴Bq. Meanwhile, a safety interlock system with six subsystems and logic of “and” gate was designed for the standard device. In order to evaluate the performance indicators of the standard device, the air kerma rate range of the radiation field, the radiation field, uniformity and scattering were measured in sequence. The measurement results show that at a distance of 1 m to 5 m, the air kerma rate of the radiation field without lead attenuation ranges from 1.71 Gy/h to 43.5 Gy/h, and the minimum air kerma is after adding the attenuator The kerma rate can reach 1.5×10^-4 Gy/h. Under the condition of no attenuator, the inverse square law of the standard device is established within 1% in the range of 1 m to 5 m from the radiation source. Under the condition of adding the attenuator, the inverse square law of the standard device is established within 3.5% in the range of 1 m to 5 m from the radiation source. At a position of 2 m, the radius of the radiation field where the air kerma rate fluctuates within 5% is 13 cm, and the radius of the radiation field where the air kernal energy rate fluctuates within 1% is 7 cm. The technical performance indicators of the standard device meet the verification/calibration requirements of the radiotherapy dosimeter.

Key words: ⁶⁰Co γ ray, safety interlock system, radiotherapy, air kerma rate