Mercury-203
Hg-203 · Mercury, Z = 80, A = 203
At 5.11e+14 Bq/g — 1.38e+4 Ci/g — a gigabecquerel of Hg-203 amounts to 1.96 µg, which is why activity rather than mass is how anyone states the quantity. Mercury-203 decays by beta-minus decay with a half-life of 46.610 days, falling to 64.01% of today's activity in a month and 0.44% in a year.
At 0.0310 mGy·m²/(GBq·h) the air kerma rate constant is 2.5× less than Cs-137 and 9.9× less than Co-60, placing it 48 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0310 mGy/h, and 1 Ci at the same distance 1.15 mGy/h.
6 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 279.2 keV at 95.7% of the total — its emission probability is 81.56%, which is also the highest.
Halving the air kerma rate calls for 1.21 mm of lead, or 7.16 mm of steel where lead is unwelcome, and a factor of ten calls for 4.20 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 0.745 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 46.610 days (4.027e+6 s) |
| Decay mode | beta-minus decay |
| Specific activity | 5.11e+14 Bq/g (1.38e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0310 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0310 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.15 mGy/h |
| Kerma-weighted mean photon energy | 270.4 keV |
279 keV carries 96% of the dose rate
1 further line below the 20 keV cutoff, the highest at 12.15 keV and 5.38% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 279.20 | 81.56 | 95.69 |
| 72.87 | 6.10264 | 1.71 |
| 70.83 | 3.6152 | 1.01 |
| 83.63 | 2.69184 | 0.80 |
| 82.63 | 2.07544 | 0.61 |
| 84.87 | 0.6164 | 0.18 |
1.21 mm of lead halves this spectrum
Solved numerically across all 6 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 1.21 | 4.20 | 3.47 |
| tungsten | 0.888 | 3.09 | 3.48 |
| iron | 7.16 | 24.8 | 3.47 |
| copper | 6.11 | 21.2 | 3.47 |
| concrete | 26.0 | 87.3 | 3.36 |
| water | 55.9 | 187 | 3.34 |
| aluminum | 23.2 | 78.2 | 3.37 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 1.28 years — a storage problem rather than a disposal one, with 0.44% of today's activity still there after a year. The mean life 1/λ is 67.2 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 | 155 days |
| Time to fall to 1 % | 310 days |
| Time to fall to 0.1 % | 1.27 years |
Limits of these dose rates
- 0.0310 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 1.21 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 5.38% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Hg-203
Other Mercury nuclides: Hg-203m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.