Iridium-192
Ir-192 · Iridium, Z = 77, A = 192
At 3.41e+14 Bq/g — 9.21e+3 Ci/g — a gigabecquerel of Ir-192 amounts to 2.93 µg, which is why activity rather than mass is how anyone states the quantity. Iridium-192 decays by electron capture with a half-life of 73.829 days, falling to 75.45% of today's activity in a month and 3.24% in a year.
1 GBq at 1 m reads 0.109 mGy/h, and 1 Ci at the same distance 4.05 mGy/h, from an air kerma rate constant of 0.109 mGy·m²/(GBq·h) — 1.4× Cs-137 and 2.8× less than Co-60, and 19 of 96 among the photon emitters carried here.
39 lines clear the 20 keV cutoff, but 6 of them carry 90% of the dose rate. The leading one is 316.5 keV at 31.8% of the total — its emission probability is 82.86%, which is also the highest.
Halving the air kerma rate calls for 2.25 mm of lead, or 8.94 mm of steel where lead is unwelcome, and a factor of ten calls for 9.01 mm of lead — sheet thicknesses that a glovebox or a transport container can carry. Reaching 20 µSv/h from 1 GBq at a metre takes 6.20 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 73.829 days (6.379e+6 s) |
| Decay mode | electron capture |
| Specific activity | 3.41e+14 Bq/g (9.21e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.109 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.109 mGy/h |
| Dose rate, 1 Ci at 1 m | 4.05 mGy/h |
| Kerma-weighted mean photon energy | 396.8 keV |
39 lines above the cutoff, and what each contributes
2 further lines below the 20 keV cutoff, the highest at 11.07 keV and 5.43% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 316.51 | 82.86 | 31.78 |
| 468.07 | 47.84 | 27.83 |
| 308.46 | 29.7 | 11.07 |
| 295.96 | 28.71 | 10.22 |
| 604.41 | 8.216 | 6.15 |
| 612.46 | 5.34 | 4.05 |
| 588.58 | 4.522 | 3.30 |
| 484.58 | 3.19 | 1.92 |
| 205.79 | 3.31 | 0.77 |
| 66.83 | 4.5372 | 0.35 |
| 416.47 | 0.67 | 0.35 |
| 374.49 | 0.727 | 0.34 |
| 884.54 | 0.292 | 0.31 |
| 489.06 | 0.438 | 0.27 |
Showing the 14 largest contributors of 39 lines above the cutoff; the remainder together carry 1.30% of the dose rate.
2.25 mm of lead halves this spectrum
Solved numerically across all 39 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 2.25 | 9.01 | 4.01 |
| tungsten | 1.60 | 6.27 | 3.92 |
| iron | 8.94 | 30.6 | 3.42 |
| copper | 7.80 | 26.7 | 3.43 |
| concrete | 30.1 | 101 | 3.37 |
| water | 64.0 | 215 | 3.36 |
| aluminum | 27.0 | 91.1 | 3.38 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 2.02 years — a storage problem rather than a disposal one, with 3.24% of today's activity still there after a year. The mean life 1/λ is 107 days.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.13 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 245 days |
| Time to fall to 1 % | 1.34 years |
| Time to fall to 0.1 % | 2.01 years |
Limits of these dose rates
- 0.109 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 2.25 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 5.43% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ir-192
Other Iridium nuclides: Ir-192m1, Ir-192m2
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.