Iridium-192m1
Ir-192m1 · Iridium, Z = 77, A = 192 · isomeric state m1
Iridium-192m1 (Ir-192m1) carries 2.50e+19 Bq/g, or 6.76e+8 Ci/g: a gigabecquerel is 40.0 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 1.45 minutes, which leaves 1.1e-297% of today's activity after a day and 0% after a week.
At 0.0497 mGy·m²/(GBq·h) the air kerma rate constant is 1.6× less than Cs-137 and 6.1× less than Co-60, placing it 42 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0497 mGy/h, and 1 Ci at the same distance 1.84 mGy/h.
9 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 316.5 keV at 77.8% of the total — its emission probability is 92.24%, which is also the highest.
Halving the air kerma rate calls for 1.70 mm of lead, or 8.21 mm of steel where lead is unwelcome, and a factor of ten calls for 6.02 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 2.26 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 1.45 minutes (87.00 s) |
| Decay mode | isomeric transition |
| Specific activity | 2.50e+19 Bq/g (6.76e+8 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0497 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0497 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.84 mGy/h |
| Kerma-weighted mean photon energy | 328.5 keV |
317 keV leads, and 2 lines make 90%
2 further lines below the 20 keV cutoff, the highest at 11.07 keV and 25.8% 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 | 92.24 | 77.80 |
| 295.96 | 18.92 | 14.81 |
| 612.46 | 3.6736 | 6.12 |
| 66.83 | 2.94986 | 0.51 |
| 65.12 | 1.72567 | 0.30 |
| 76.72 | 1.27642 | 0.23 |
| 75.82 | 0.99255 | 0.17 |
| 77.83 | 0.28387 | 0.05 |
| 56.71 | 0.0351 | 0.01 |
1.70 mm of lead halves this spectrum
Solved numerically across all 9 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 1.70 | 6.02 | 3.54 |
| tungsten | 1.24 | 4.35 | 3.52 |
| iron | 8.21 | 27.8 | 3.38 |
| copper | 7.11 | 24.1 | 3.39 |
| concrete | 28.1 | 94.2 | 3.35 |
| water | 60.0 | 200 | 3.34 |
| aluminum | 25.2 | 84.6 | 3.35 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 14.5 minutes, so the activity moves measurably while a count is running: every figure has to carry the time it was referred to. The mean life 1/λ is 2.09 minutes.
| 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 | 4.82 minutes |
| Time to fall to 1 % | 9.63 minutes |
| Time to fall to 0.1 % | 14.5 minutes |
Limits of these dose rates
- 0.0497 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 1.70 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 25.8% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ir-192m1
Other Iridium nuclides: Ir-192, Ir-192m2
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.