A double‐beam interferometer has been designed for detecting changes of refractive index in transparent liquids associated with the absorption of ionizing radiation energy, due to short electron beam pulses from an accelerator. The response time of the interferometer is less than 0.2 &mgr;sec, and refractive index changes of the order of 10−7can be measured, corresponding to a temperature change of ∼10−3 °C and an absorbed dose in water of ∼350 rad. The interferometer can be used as either a real‐time or integrating radiation dosimeter, if the temperature coefficient of the refractive index (dn/dT) is known for the irradiated liquid in the temperature region of interest.