An apparatus to monitor thickness of a crystalline sheet grown from a melt. The apparatus may include a process chamber configured to house the melt and crystalline sheet; an x-ray source disposed on a first side of the crystalline sheet and configured to deliver a first beam of x-rays that penetrate the crystalline sheet from a first surface to a second surface opposite the first surface, at a first angle of incidence with respect to the first surface; and an x-ray detector disposed on the first side of the crystalline sheet and configured to intercept a second beam of x-rays that are generated by reflection of the first beam of x-rays from the crystalline sheet at an angle of reflection with respect to the first surface, wherein a sum of the angle of incidence and the angle of reflection satisfies the equation λ=2d sin θ.