Iridium-192m2

A gigabecquerel of Ir-192m2 is 3.50 mg of material, which follows from a specific activity of 2.86e+11 Bq/g (7.73 Ci/g). Iridium-192m2 decays by isomeric transition with a half-life of 241 years; forty years leaves 89.13% of today's activity and ten thousand years leaves 3.2e-11%.

The air kerma rate constant is small — 0.0000819 mGy·m²/(GBq·h), 9.4e+2× less than Cs-137 and 3.7e+3× less than Co-60, ranking 85 of 96 by Γ — near the bottom of the photon emitters, but above the cutoff, which 51 nuclides in this dataset are not. 1 GBq at 1 m reads 0.0000819 mGy/h, and 1 Ci at the same distance 0.00303 mGy/h. It takes 244 GBq at a metre to reach 20 µSv/h from the photons alone.

6 lines clear the 20 keV cutoff, but 5 of them carry 90% of the dose rate. The leading one is 64.9 keV at 31.7% of the total — its emission probability is 0.3055%, which is also the highest.

Shielding barely arises: 0.189 mm of lead halves the air kerma rate and 0.684 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 1.27 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 3.6 times the half-value layer, not the 3.32 a single energy would give.

Ir-192 grows in beneath Ir-192m2 with a half-life of 73.829 days. How much of it is present depends on how long ago the isomer was produced, which this page cannot know — at full ingrowth it brings 0.109 mGy·m²/(GBq·h) of air kerma rate constant, which the figures above do not include.