The bulk damage (namely the introduction rate of the arsenic antisite As G a and its ionization ratio) was determined as a function of the non-ionizing energy loss (NIEL) of hadrons in semi-insulating GaAs. The study was performed using near-infrared absorption on 23 GeV proton and 192 MeV pion irradiated, Liquid Encapsulated Czochralski (LEC) grown GaAs. Together with the detector performance as a function of the radiation level, the results are used to explain the radiation damage in GaAs particle detectors.