Neutron-induced autoradiography is a further step in the development of techniques for the examination of paintings that complements x-ray radiography and often yields unique information that cannot be obtained by other means. After exposure to a field of thermal neutrons (<0.3eV), selected elements in the painting form radioactive nuclides that in turn decay at known half-lives with the emission of gamma rays and charged particles. The charged particles, mainly electrons, expose radiographic film that is held in close contact with the painting by a very slight vacuum. Unlike x-ray radiography, which involves detecting x-rays that have passed through the painting to expose a film, autoradiography uses one activation with neutrons followed by sequential film exposures to capture the images of activated elements that have different half-lives. The autoradiographs show the distribution patterns of the pigments in which the elements occur. These pigments may be on the surface and visible in the painting or they may be beneath the surface. Often autoradiography uncovers the preliminary sketch or underpainting preceding the rough blocking in of the subject and the subsequent paint layers that create the final image. X-rays reveal almost exclusively the presence of pigments containing heavy elements such as lead and barium; autoradiography identifies the presence of activated pigments and distinguishes the fine details of paint application as well as changes in composition.