Isotopes: Iodine 125

Physical Characteristics:
  • Half-life: 60.1 days
  • Emmissions: Principal emissions are a 35 keV gamma ray and 27 - 32 keV x-rays.
  • Half-Value Layer: 0.02 mm lead; 2 cm in tissue.
  • The half-value layer is the amount of material required to reduce the radiation intensity by 50%.
Dose and Shielding:
  • Dose rate at 10 cm: 15 mrem/hour/mCi
  • (for an unshielded point source)
  • Shielding:
  • Annual Limit on Intake (ALI): 60 microcuries via inhalation, and 40 microcuries via ingestion. The intake of one ALI will produce a dose of 5 rem. The critical organ for protection is the thyroid gland.

A sodium iodide crystal scintillation detector is the preferred method for detecting I-125. G-M detectors are not likely to detect the presence of I-125 in amounts less than about 100,000 dpm (0.05 μCi).


Volatile iodine can be released from Na125I or from iodinated compounds containing hundreds of microcuries or more of I-125. Containers of I-125, including sample vials of iodinated compounds, should always be opened in a fume hood. Personnel using I-125 in hundreds of microcuries or more must wear double gloves and should change gloves as soon as the gloves become contaminated. Iodinations must be performed under EHS surveillance and thyroid count bioassays must be performed following an iodination.

Waste Disposal:
  • Solid Wastes: through the Off-Site Radioactive Waste Disposal Program