The optical index modulation was theoretically estimated and demonstrated under short X-ray excitation in low-temperature-grown GaAs(LT_GaAs). Hot-electron thermalization time <1 ps， carrier recombination time <2 ps and the duration of the index perturbation was determined by the carrier recombination time which was of order -2 ps in LT_GaAs with a high density of recombination defects. Predictions of radiation-induced changed in the optical refractive index were in reasonably good agreement with the limited experimental data available, suggesting that LT_GaAs was a highly promising material for high speed single transient ionizing radiation detector.