Statistical dynamical theory in the case of a large correlation length

Any available form of the statistical dynamical theory contains an approxim ation based on the assumption that the correlation length tau of the random imperfection of the crystal is much smaller than the Pendellosung length L ambda. A new formulation is developed for the coherent part of the rocking curve, without this short correlation approximation. Numerical calculations are carried out in order to discuss the validity of this approximation whe n the ratio tau/Lambda is varied.