Iodine-125 (125I) 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. 125I is created by the electron capture decay of 125Xe, which is a synthetic isotope of Xenon, itself created by neutron capture of the slightly radioactive 124Xe, which occurs naturally with an abundance of around 0.1%. Because of the synthetic production route of 125I and its short half-life, the natural abundance is effectively 0%.


125I is reactor-produced radionuclide and is available in large quantities. Its production follows reaction: 124Xe (n,γ)→ 125mXe(57s)→125I (59,4 d)
124Xe (n,γ)→ 125gXe(19,9h)→125I (59,4 d)

The irradiation target is natural xenon gas containing 0,0965% 124Xe. It s filled in zircaloy-2 capsule to pressure about 100 bars. On irradiation in a nuclear reactor, several radionuclides of xenon are produced. Fortunately only the decay of 125Xe leads to radioiodine 125I. The other radioxenons decay either to stable xenon or some cesium isotopes. It needs however, to be pointed out that long irradiations are disadvantageous. Iodine-125 itself has a neutron capture cross section of 900 barns, and consequently during a long irradiation, part of the 125I formed will be converted to 126I. In practice, the irradiation time amounts to a few days. Thereafter the irradiated gas is allowed to decay for several days. For isolating radioiodine, the irradiated capsule is cooled and Xe gas is allowed to escape. The inner walls of the capsule are then rinsed with dilute NaOH solution. In order to eliminate long-lived 135Cs and 137Cs, which may be present in small amounts, the solution is passed through a cation-exchange column. The radioiodine remains in solution.

Availability and purity

Iodine-125 is commercially available in dilute NaOH solution as [125I]iodide. The radioactive concentration lies at 4 to 11 GBq/ml and the specific radioactivity amounts to >75GBq/µmol. The chemical and radiochemical purity is high. The radionuclidic purity is also high; only some 126I (t1/2=13.1d) is unavoidable. Its tolerable content lies at about 0.2%.

Physical Data

  • Element: Iodine
  • Z: 53
  • 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 125Te
  • Emissions: Gamma-rays at 35.5 keV. 7% emitted, 93% internally converted to:
    • 27.0 keV (113% abundance relative to 7% gamma 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

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