Sealed containers for radioactive material are presented herein. A sealed container forms an interior envelope for housing a radioactive material and prevents escape of the radioactive material into a surrounding environment. The sealed container provides a diffusion window for gaseous decay products to escape at a particular diffusion rate. In one example, an apparatus, comprises a container forming a sealed interior envelope for a radioactive material. The container has an aperture covered by a window material, and properties of the window material are selected to provide for diffusion of at least one gas produced by radioactive decay of the radioactive material.