[en] The solution of the general form of the integral transport equation is expanded in terms of orders of scattering. The method is applied to the analysis of multiply Bragg-reflected neutrons in mosaic crystals, making evident the vertical broadening of the reflected beam as well as the consequential neutron leakage in crystal spectrometers. A wavelength-dependent effective vertical mosaic spread of the crystal is defined as a function of the mean number of reflections. A detailed mathematical treatment is performed within the frames of an infinite-plane slab model (both in reflection and in transmission geometry) largely used in the literature in the studies of crystal reflectivity. Experimental results are also presented. (Auth.)