Iodine-125 (125I) is used primarily as a low dose rate (LDR) brachytherapy source. It’s half-life (59.4 days) and low photon energy (28keV) allows for medium term storage with minimal shielding. 125I decays solely via electron capture to an excited state of tellurium-125 (125Te). This excited state of 125Te is not a metastable state and it decays to a ground state via internal conversion or gamma emission. Because of the low energy and the presence of end welds the sources encapsulation, 125I seeds are often highly ansitropic. Significant differences exist in the published exposure rate constant and mean photon energy of Iodine-125 which are largely the result of differences in handling the low energy photon spectrum.

Quick Facts

  • Half-life
    • 59.4 days
  • Decay Modes
    • Electron Capture to first excited state of 125Te (100%)
    • 125Te decays to ground state of 125Te via
      • Internal conversion (93%)
        • Characteristic x-ray energies of 27 to 35keV
      •  γ-emission (7%)
        • 35.5keV
  • Exposure Rate Constant
    • \((\Gamma_{\delta})_{\chi} = 1.51 \frac{R \ cm^2}{mCi \ h}\)
  • Air Kerma Rate Constant
    • \((\Gamma_{\delta})_{\kappa} = 1.32\frac{cGy \ cm^2}{mCi \ h}\)
  • Average Photon Energy
    • 28 keV
  • Average Photons per Decay
    • 1.4
  • Half-value layer (mm lead)
    • 0.008