The importance of having attenuation curves available of radiation scattered from objects in the useful beam, such as the walls of a radiation room or a patient, has already been pointed out (1). Each irradiated object becomes a source of secondary radiations which may be hazardous to personnel located in areas away from the direction of the useful beam. Only by knowing the attenuation curves of the secondary radiation in common protective materials, such as lead or concrete, can the most economical secondary protective barriers be designed to eliminate this hazard. The data available on the attenuation of scattered gamma rays are of two types. For one a suitable phantom is used to duplicate the conditions under which a patient scatters radiation out of the useful beam (2, 3). The second type consists of measurements in which the radiation is scattered from a concrete wall or barrier (2). The scattered radiation may traverse a longer path within the phantom than in the wall and thus be more greatly attenu...