Mercury-203m
Hg-203m · Mercury, Z = 80, A = 203 · isomeric state m
Mercury-203m (Hg-203m) carries 9.31e+25 Bq/g, or 2.52e+15 Ci/g: a gigabecquerel is 10.7 ag, no weighable quantity at all. It decays by isomeric transition with a half-life of 22.1 microseconds, which leaves 0% of today's activity after a day and 0% after a week.
1 GBq at 1 m reads 0.107 mGy/h, and 1 Ci at the same distance 3.95 mGy/h, from an air kerma rate constant of 0.107 mGy·m²/(GBq·h) — 1.4× Cs-137 and 2.9× less than Co-60, and 20 of 96 among the photon emitters carried here.
7 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 591.4 keV at 73.8% of the total — its emission probability is 98.213%, which is also the highest.
Halving the air kerma rate calls for 3.61 mm of lead, or 10.1 mm of steel where lead is unwelcome, and a factor of ten calls for 13.9 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 9.74 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 22.1 microseconds (0.00002210 s) |
| Decay mode | isomeric transition |
| Specific activity | 9.31e+25 Bq/g (2.52e+15 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.107 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.107 mGy/h |
| Dose rate, 1 Ci at 1 m | 3.95 mGy/h |
| Kerma-weighted mean photon energy | 516.3 keV |
591 keV leads, and 2 lines make 90%
1 further line below the 20 keV cutoff, the highest at 11.78 keV and 14.0% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 591.40 | 98.213 | 73.85 |
| 341.50 | 52.74038 | 22.53 |
| 70.82 | 17.94153 | 1.45 |
| 68.89 | 10.58371 | 0.85 |
| 81.28 | 7.87297 | 0.66 |
| 80.32 | 6.08891 | 0.51 |
| 82.48 | 1.78405 | 0.15 |
3.61 mm of lead halves this spectrum
Solved numerically across all 7 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 3.61 | 13.9 | 3.86 |
| tungsten | 2.48 | 9.41 | 3.80 |
| iron | 10.1 | 35.1 | 3.49 |
| copper | 8.87 | 31.0 | 3.50 |
| concrete | 33.3 | 113 | 3.41 |
| water | 71.0 | 240 | 3.39 |
| aluminum | 29.9 | 102 | 3.42 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 0.000221 seconds, 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 0.0000319 seconds.
| 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 | 0.0000734 seconds |
| Time to fall to 1 % | 0.000147 seconds |
| Time to fall to 0.1 % | 0.000220 seconds |
Limits of these dose rates
- 0.107 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 3.61 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 14.0% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Hg-203m
Other Mercury nuclides: Hg-203
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.