Gold-198m
Au-198m · Gold, Z = 79, A = 198 · isomeric state m
Gold-198m (Au-198m) carries 1.07e+16 Bq/g, or 2.90e+5 Ci/g: a gigabecquerel is 93.1 ng, no weighable quantity at all. It decays by isomeric transition with a half-life of 2.272 days, which leaves 11.82% of today's activity after a week and 8.8e-11% after three months.
1 GBq at 1 m reads 0.0647 mGy/h, and 1 Ci at the same distance 2.39 mGy/h, from an air kerma rate constant of 0.0647 mGy·m²/(GBq·h) — 1.2× less than Cs-137 and 4.7× less than Co-60, and 35 of 96 among the photon emitters carried here.
11 lines clear the 20 keV cutoff, but 6 of them carry 90% of the dose rate. The leading one is 214.9 keV at 31.8% of the total — its emission probability is 77%, which is also the highest.
A half-value layer of 0.481 mm in lead puts this in foil and thin sheet, with 4.46 mm needed if the material is steel; ten-fold attenuation comes at 2.36 mm of lead. Reaching 20 µSv/h from 1 GBq at a metre takes 0.960 mm of lead. The tenth-value layer runs 4.9 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 2.272 days (1.963e+5 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.07e+16 Bq/g (2.90e+5 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0647 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0647 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.39 mGy/h |
| Kerma-weighted mean photon energy | 189.5 keV |
11 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 11.42 keV and 57.3% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 214.89 | 77 | 31.76 |
| 180.31 | 49.28 | 16.43 |
| 204.10 | 38.5 | 14.94 |
| 333.82 | 17.71 | 12.16 |
| 97.21 | 68.53 | 11.03 |
| 68.81 | 41.55732 | 5.52 |
| 66.99 | 24.41077 | 3.22 |
| 78.98 | 18.07526 | 2.46 |
| 78.05 | 14.0227 | 1.91 |
| 80.13 | 4.05256 | 0.55 |
| 115.20 | 0.04004 | 0.01 |
0.481 mm of lead halves this spectrum
Solved numerically across all 11 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.481 | 2.36 | 4.90 |
| tungsten | 0.344 | 1.75 | 5.08 |
| iron | 4.46 | 18.8 | 4.21 |
| copper | 3.56 | 15.6 | 4.38 |
| concrete | 21.5 | 74.3 | 3.46 |
| water | 48.2 | 163 | 3.39 |
| aluminum | 18.9 | 66.1 | 3.49 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 22.7 days — a storage problem rather than a disposal one, with 4.0e-47% of today's activity still there after a year. The mean life 1/λ is 3.28 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 | 7.55 days |
| Time to fall to 1 % | 15.1 days |
| Time to fall to 0.1 % | 22.6 days |
Limits of these dose rates
- 0.0647 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.481 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 57.3% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Au-198m
Other Gold nuclides: Au-198
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.