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Real-time monitoring of the Bragg peak location of carbon ions is urgently required for the quality control of hadron therapy.In this study,we design an annular detector to monitor the Bragg peak location of carbon ions with Geant4 simulation.This 360° surrounding structure has a high detection efficiency for the small-dose situation.The detector consists of a multilayered collimator system and an NaI scintillator for prompt gamma counting.The multilayered collimator includes a lead layer to prevent unwanted gammas and the paraffin and boron carbide layers to moderate and capture fast neutrons.An inclination of the detector further diminishes the background signal caused by neutrons.The detector,with optimized parameters,is applicable to carbon ions of different energies.In addition,the scintillator is replaced by an improved EJ301 organic liquid scintillator to discriminate gammas and neutrons.Inserting thin Fe slices into the liquid scintillator improves the energy deposition efficiency.The Bragg peak location of 200 MeV/u carbon ions can be monitored by prompt gamma detection with the improved liquid scintillator.