Geant4 simulation study on detection efficiencies of the Compton suppression system at the HL-2A tokamak.

A Compton suppression system (CSS), which plays an important role in γ-ray diagnostics, has been developed to acquire the γ-ray emission spectra generated in the HL-2A tokamak plasma. It takes advantages of the anti-coincidence method of two branches of γ-ray signals, one is from a high purity germanium (HPGe) detector, and the other from several bismuth germinate (BGO) detectors, to output the suppressed spectra. In the present paper, a Geant4 simulation, together with its subsequent data processing which took accidental coincidences into account, was performed to study the detection efficiencies of the CSS for γ-rays with energies between 0.2 and 13 MeV. Calculation results showed that there were small detection efficiency differences between the unsuppressed and suppressed spectra when HPGe count rates were smaller than 105 cps, and the area ratios of additional peaks to their corresponding full energy peaks were not associated with HPGe count rates. Based on the calculation results, the empirical formulas of detection efficiencies of the CSS were put forward. Finally, after the experimental verification of the empirical formulas, a suppressed γ-ray spectrum acquired by the CSS during 2018 HL-2A experiment campaign was unfolded using the detection efficiencies. This work makes it possible for the analyses of the γ-ray emission spectra generated from the HL-2A tokamak.