THE ANALYTICAL CALCULATION OF ABSORPTION IN MULTIFACETED CRYSTALS

The exact analytic method of evaluating the absorption during scatteri ng in multifaceted convex crystals is developed in a way that permits efficient computation. A fast and accurate algorithm is given for find ing the Howells polyhedra whose determination is fundamental to the an alytic method. The algorithm allows for the evaluation of cases when t he sample is only partly illuminated, can be adapted to more general s ituations and can be used to generate an estimate of the error in the computation. In most cases this is to 1 part in 10(14). Results of sta ndard tests are given to greater accuracy than previously available an d results for multifaceted approximations to a cylinder and a sphere a re given to illustrate the power of the method.