We investigated the thermal behavior of the photorefractive fanning effect in dehydrated lithium niobate doped with iron. Thermal dark decay of backward scattering did not follow the law determined for thermal fixing of two-wave mixing gratings. It can be explained by a combination of reflection and small-angle fanning. The concentration of hydrogen in the sample was determined by comparing the decay of 90° scattering with that of the small-angle one. A calibration factor of A/NH = (2.5 ± 0.5) × 10-18 cm, and a cross section of (2.1 ± 0.4)×10-19 cm2 were also determined for the 3480 cm-1 IR absorption band of OH– ions.