A calibrated radioactive source comprises a container and a material labelled by at least one radionuclide. The labelled material is contained in the container and the container is made of a material that is transparent to the radiation emitted by the at least one radionuclide. The source is characterized in that the labelled material is a self-hardening polymer that is chemically inert relative to the material used for the container and in that the container is a flexible sheath. The calibrated source is placed into a hole of a brick of a tissue-equivalent phantom. An assembly is formed by such a calibrated source and a brick of tissue-equivalent phantom.