Аннотация:We examine the efficiency of the spontaneous parametric down-conversion (SPDC) in strongly frequency nondegenerateregime as a function of temperature (297 K < T < 4 K) of the nonlinear crystal. Frequency-angular SPDCspectra of signal photons allowed determining the dispersion of the refractive indices and absorption coefficientsof the crystal at idler THz frequencies. We have measured these characteristics of Mg : LiNbO3 crystal in a wide rangeof temperatures 297 K < T < 4 K. Obtained data can be used in calculation of the spatial-frequency spectra of opticalterahertzbiphoton fields.