. A neutron attenuator is used to stop possible neutrons before they reach the detector. The neutron attenuator is made of polythene and located inside the collimator. The system is calibrated using gamma radioactive sources (137Cs and 152Eu) and gamma neutron activation (activation of natural copper and ferrum materials by neutrons from a deuterium-deuterium neutron generator). The calibration results are shown in Fig. 1(a). The gamma energy used in the calibration is from approximately 0.5 MeV to 9 MeV.