  The sensitivity of the dark photon search through invisible decay final
states in low background experiments significantly relies on the neutron and
muon veto efficiency, which depends on the amount of material used and the
design of detector geometry. This paper presents an optimized design of a
hadronic calorimeter (HCAL) used for the DarkSHINE experiment, which is studied
using a GEANT4-based simulation framework. The geometry is optimized by
comparing a traditional design with uniform absorbers to one that uses
different thicknesses at different locations of the detector, which enhances
the efficiency of vetoing low-energy neutrons at the sub-GeV level. The overall
size and total amount of material used in HCAL are optimized to be lower due to
the load and budget requirements, while the overall performance is studied to
meet the physical objectives.
