Antimony-124m2
Sb-124m2 · Antimony, Z = 51, A = 124 · isomeric state m2
Antimony-124m2 (Sb-124m2) carries 2.78e+18 Bq/g, or 7.51e+7 Ci/g: a gigabecquerel is 360 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 20.2 minutes, which leaves 3.5e-20% of today's activity after a day and 6.1e-149% after a week.
The air kerma rate constant is small — 9.64e-7 mGy·m²/(GBq·h), 7.7e+4× less than Cs-137 and 3.1e+5× less than Co-60, ranking 95 of 96 by Γ — near the bottom of the photon emitters, but above the cutoff, which 51 nuclides in this dataset are not. 1 GBq at 1 m reads 9.64e-7 mGy/h, and 1 Ci at the same distance 0.0000357 mGy/h. It takes 20.7 TBq at a metre to reach 20 µSv/h from the photons alone.
One line at 26.0 keV carries all of that.
Shielding barely arises: 0.0139 mm of lead halves the air kerma rate and 0.0462 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.0717 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.
Half-life, specific activity and dose rate
| Half-life | 20.2 minutes (1212 s) |
| Decay mode | isomeric transition |
| Specific activity | 2.78e+18 Bq/g (7.51e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 9.64e-7 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 9.64e-7 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.0000357 mGy/h |
| Kerma-weighted mean photon energy | 25.98 keV |
A single photon line at 26.0 keV
2 further lines below the 20 keV cutoff, the highest at 10.86 keV and 12.9% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 25.98 | 0.00337 | 100.00 |
0.0139 mm of lead halves this spectrum
Solved numerically across all 1 line, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.0139 | 0.0462 | 3.32 |
| tungsten | 0.0108 | 0.0360 | 3.32 |
| iron | 0.0717 | 0.238 | 3.32 |
| copper | 0.0474 | 0.158 | 3.32 |
| concrete | 2.15 | 7.13 | 3.32 |
| water | 14.1 | 46.7 | 3.32 |
| aluminum | 1.53 | 5.09 | 3.32 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 3.37 hours, 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 29.1 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 | 1.12 hours |
| Time to fall to 1 % | 2.24 hours |
| Time to fall to 0.1 % | 3.36 hours |
Limits of these dose rates
- 9.64e-7 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0139 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 12.9% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sb-124m2
Other Antimony nuclides: Sb-124, Sb-124m1, Sb-125
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.