For the purpose of obtaining the lower absolute measurement limit of the air kerma in ¹³⁷Cs γ-ray reference fields, a cavity chamber with a volume of 301.936(19) cm³ was designed and fabricated. The absolute measurement of the air kerma was performed at the minimum effective dose rate of the large-size cavity chamber in our ¹³⁷Cs γ-ray reference field. With the help of the detailed Monte-Carlo (MC) simulations of the large-size cavity chamber and our ¹³⁷Cs irradiation facility, the conversion coefficient from the cavity dose to the reference air kerma was calculated. And the obtained air kerma rate with the MC simulation method was 1.429(13) mGy/h. At the same condition, the measured air kerma rates with the cavity theory method and the substitute method were 1.428(13) mGy/h and 1.419(53) mGy/h, respectively. The measurement result with the full MC simulation method agrees with the other two measurement results within 0.07% and 0.70%, respectively. The combination of the large-size chamber and the full MC simulation method can help calibration laboratories achieve the absolute measurement of the low air kerma rate (such as 1.5 mGy/h) in the ¹³⁷Cs γ-ray reference field.

为了获得在¹³⁷ Csγ射线参考场中空气比释动能的绝对测量下限，设计并制造了一个容积为301.936（19）cm ³的腔室。空气比释动能的绝对测量是在我们的¹³⁷ Csγ射线参考场中，以大腔室的最小有效剂量率进行的。借助大型腔室和¹³⁷的详细蒙特卡洛（MC）模拟在Cs照射设备中，计算了从腔剂量到参考空气比释动能的转换系数。用MC模拟法得到的空气比释动能率为1.429（13）mGy / h。在相同条件下，用腔理论方法和替代方法测得的空气比释动能速率分别为1.428（13）mGy / h和1.419（53）mGy / h。完全MC模拟方法的测量结果与其他两个测量结果相吻合，分别在0.07％和0.70％之内。大型腔室和完整的MC模拟方法的结合可以帮助校准实验室在¹³⁷ Csγ射线参考场中实现对低空气比释动能（例如1.5 mG​​y / h）的绝对测量。