For the quantitative investigation of MeV-photon-induced changes in the refractive indices of bulk semiconductors, a model was established to describe the evolution of the excess carrier density, including the generation and recombination processes. The two key parameters of the evolution model, namely, the summed injection intensity and the gamma intensity curve, were obtained via dose measurements and gamma pulse monitoring, respectively. An interferometric method of measuring instantaneous changes in the refractive index and obtaining real-time measurements of the excess carrier density in bulk materials was successfully implemented. The probe beam was transmitted through a single-mode fiber to form double-beam interference in a slab geometry. Two bulk samples, one consisting of intrinsic GaAs and one of intrinsic ZnO, were tested. The recombination time constant of the intrinsic GaAs sample was found to be approximately 0.6 ns and did not vary distinctly with the photon energy, whereas the ZnO sample's recombination behavior consisted of two components. The short component was evident when short and intense pulses were incident, whereas the long component dominated under long and relatively weak pulses. The method reported in this work can be used to study the excess carrier dynamics induced by pulsed gamma radiation and to investigate the mechanisms of refractive index modulation under pulsed gamma conditions; thus, it is expected to be beneficial for guiding the development of RadOptic systems based on bulk materials.