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A Bonner sphere spectrometer (BSS) was developed compensating for the lack of active BSSs for intense neutron field characterization. The spectrometer combines the merits of present active and passive BSSs, namely, online data acquisition capability and intense neutron field resistance, respectively. The key elements of the development are the utilization of diamond detectors as thermal neutron sensors of BSSs and the incorporation of the air gap into the design of the diamond detector for optimizing the pulse height spectrum in order to enhance the rejection capability to γ ray backgrounds and to decrease the impacts of spectrometer instabilities. A two-step method capable of >100 times of calculation time saving compared to the whole geometry model was suggested to establish the response function for neutrons below 20 MeV whose reliability was verified by the two other models. The applicability of the BSS to intense neutron field characterization was demonstrated by the good performance in the Experimental Advanced Superconducting Tokamak (EAST) neutron field with an emission rate of ∼1013–1014 neutrons/s. The spectrometer is dedicated to the characterization of intense neutron fields around tokamaks. These devices may find an application in future tokamaks, such as the International Thermonuclear Experimental Reactor, the Demonstration Power Station, and the China Fusion Engineering Test Reactor, whose neutron emission rates will be >104 times higher than those of current tokamaks. [ABSTRACT FROM AUTHOR]