AbstractA general expression for the intensity of unscattered gamma rays at points on the axis of a cylindrical absorber, surrounded by a ring-shaped self-absorbing source of rectangular cross section, is derived in terms of two integrals that have been evaluated on the IBM 704 computer. The results are presented in the form of curves from which the intensity can be determined for a wide variety of cases. For points not on the axis of symmetry a solution is given for the special case of source with large peripheral dimensions.