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Iodine-125 is a radioisotope of iodine which has uses in biological assays and in radiation therapy to treat prostate cancer and brain tumors. Its half-life is around 60 days and it emits gamma-rays with maximum energies of 35 keV, some of which are internally converted to x-rays. Iodine-125 is created by the electron capture decay of Xenon-125, which is a synthetic isotope of Xenon, itself created by neutron capture of the slightly radioactive Xenon-124, which occurs naturally with an abundance of around 0.1%. Because of the synthetic production route of Iodine-125 and its short half-life, the natural abundance is effectively 0%.

Physical Data

  • Element: Iodine
  • Z: 54
  • A: 125
  • Atomic Mass:
  • Density:
  • Physical state: Solid at room temperature
  • Isotopic abundance: 0%

  • Radioactive: Yes
  • T(1/2): 59.4 days
  • Decay: Electron capture to Tellurium-125
  • Emissions: Gamma-rays at 35.5 keV. 7% emitted, 93% internally converted to:
    • 27.0 keV (113% abundance relative to 7% gamm emission)
    • 31.0 keV (26%)
    • 27-32 keV (14%)
  • Half-value layer: 0.025 mm Pb


  • Harper, P.V. ; Siemens, W.D. ; Lathrop, K.A. ; Brizel, H.E. ; Harrison, R.W. Iodine-125. Proc. Japan Conf. Radioisotopes; Vol: 4th Jan 01, 1961