"Sealed Source" is a term used to describe radioactive sources that have been designed to prevent spread of radioactive material under normal working condition. Here is the definition from the State of Oregon Rules for Control of Radiation:

"Sealed Source" means radioactive material that is permanently bonded or fixed in a capsule or matrix designed to prevent release and dispersal of the radioactive material under the most severe conditions which are likely to be encountered in normal use and handling.
(Oregon Administrative Rules (OAR) 333-100-0005)

Examples of sealed sources include

  • Nickel-63 Electron Capture Detector (Ni-63 ECD) - the radioactive isotope Ni-63 is electroplated to a metal foil. The foil is installed by the manufacturer in an inaccessible chamber inside the ECD cell.
  • Cesium-137 (Cs-137) or Cobalt-60 (Co-60) gamma sources. These usually contain a small, pea sized source which is sealed in a small welded capsule. The capsule is encased in a shield, usually lead, with a small shuttered opening which controls the gamma beam.