Physical Characteristics

  • Half-life: 462.6 days
  • Emissions: Principal emissions are 22.1 keV x-rays (83%), accompanied by electrons with energies ranging up to 87 keV.
  • Half-Value Layer: 0.01 mm lead; 2 cm in tissue (The half-value layer is the amount of material required to reduce the radiation intensity by 50%.)

Dose Rate and Shielding

  • Dose rate to the skin at 10 cm: 0 mrem/hour/mCi (for an unshielded point source)
  • Gamma Dose rate (deep tissue dose) at 30 cm: 0.778 mrem/hour/mCi (for an unshielded point source)
  • Dose rate to epidermal basal cells from skin contamination of 1 µCi/cm2: 2000 mrem/hour
  • Shielding: Lead foil or sheets are used to shield the x-rays (the electrons are too low in energy to require shielding) when Cd-109 is used in hundreds of microcuries or in millicurie quantities. Shielding is not needed when Cd-109 is used in low microcurie amounts.
  • Annual Limit on Intake (ALI): 40 microcuries via inhalation, and 300 microcuries via ingestion. The intake of one ALI will produce a dose of 5 rem. The critical organ for protection are the kidneys.


A sodium iodide crystal scintillation detector is the preferred method for detecting Cd-109. G-M detectors are not likely to detect the presence of Cd-109 in amounts less than about 100,000 dpm (0.05 µCi).


Skin contamination and ingestion are the chief concerns when working with Cd-109, and appropriate precautions must be taken to limit contamination. Contamination of work areas and individuals is a more significant hazard than the external dose, unless working with millicurie quantities.

Radiation Monitoring Requirements: Radiation monitoring badges must be worn by any person who uses open sources of Cd-109 in amounts of 1 mCi or more for extended operations (applies to most operations other than simple aliquoting from a stock vial).

Waste Disposal

  • Solid Wastes/ Liquid Scintillation Wastes:  through the Off-Site Radioactive Waste Disposal Program
  • Liquid Wastes: through the Sewer Disposal Program. The laboratory disposal limit for Cd-109 is 0.3 mCi per month.