中文摘要：

为利用Geant4工具包对湮灭γ射线进行符合探测，模拟计算了NaI（Tl）探测器对正电子湮灭产生的0．511MeV光子的符合探测效率。计算结果表明，符合探测比未符合时的探测效率降低一个量级，但是由于符合探测能使本底比常规方法降低3—4个量级，因此在实际监测中，符合探测技术能非常有效地解决现有仪器的探测下限较高的问题，同时对能谱稳定的要求大大降低，对监测反应堆安全意义重大。

英文摘要：

Using the Ceant4 toolkit to coincidently detect the annihilation Gamma, the coincidence detection efficiency of 0.511 MeV gamma photons from the positron annihilation in the NaI（T1） detector is simulated. The calculated results reveal that, the efficiency of coincidence detection is lower than the detection efficiency when not in coincidence detection by an order of magnitude, but the background count of the coincidence detection is 3 - 4 orders of magnitude lower than that with normal γ spectrometer method, so in the actual monitoring, the coincidence detection technology can be very effective to solve the higher detection limit of the existing instru- ments, at the same time, the requirements of stability of energy spectrum is greatly reduced which is of great significance for the monitoring of reactor safety.