# Iridium-192

#### Overview

Iridium-192 ($$^{192}Ir$$) is the most common isotope used in high dose rate (HDR) brachytherapy. It is a man-made isotope produced by neutron bombardment of Iridium-191. $$^{192}Ir$$ decays primarily through $$\beta ^{-}$$ decay to an excited state of Platinum-192 which is stable. Approximately 5% of decays occur via electron capture yielding an excited state of Osmium-192 which is also stable. It is regarded as superior to cobalt-60 and cesium-137 due to it’s higher specific activity which allows for smaller source sizes. One disadvantage to $$^{192}Ir$$ is it’s short half life (73.83 days) which results in a loss of activity of approximately 1% per day. This necessitates source changes every 3-4 months.

#### Quick Facts

• Half life: 73.83 days
• Activity loss per day (quick estimate): 1%/day
• Decay modes
• 95.6% $$\beta ^{-}$$ decay to excited state of $$^{192}Pt$$
• 4.4% Electron capture to excited states of $$^{192}Os$$
• Exposure rate constant1
• $$(\Gamma_{\delta})_\chi = 4.69\frac{R \ cm^2}{mCi-h}$$
• Air Kerma Rate Constant
• $$(\Gamma_{\delta})_\kappa = 4.11 \ \frac{cGy \ cm^2 }{mCi \ h}$$
• Photon Energy
• 0.136-1.06 (0.38 average) MeV
• Half-value layer (mm lead): 2.5mm

#### Works Cited

1. 1.
Nath R, Anderson L L, Luxton G, Weaver K A, Williamson J F, Meigooni A S. Dosimetry of Interstitial Brachytherapy Sources: Recommendations of the AAPM Radiation Therapy Committee Task Group No. 43. Medical Physics; 1995:209-234.