GEANT4 Simulation of the GTAF

To fulfill the needs of neutron capture reaction cross-section measurement in keV energy scale in the field of nuclear astrophysics and advanced nuclear energy system development, the 4π  BaF2  Gamma-ray Total Absorption Facility (GTAF) developed by the Key Laboratory of Nuclear Data of the China Institute of Atomic Energy (CIAE) has been transplanted and installed at the Back-streaming White Neutron Source (Back-n) of the China Spallation Neutron Source (CSNS) in 2019. A series of results has been achieved and published based on the GTAF since then, and due to which the needs of reducing backgrounds are becoming increasingly urgent. In order to understand the origins of backgrounds and to optimize the facilities, a detailed simulation program using GEANT4 toolkits was established and presented in this paper. To demonstrate the availability of the proved codes, several practical examples of assisting the process of experimental data and helping verify the optimization proposition are also shown in this paper.