Antimony-124
Sb-124 · Antimony, Z = 51, A = 124
At 6.48e+14 Bq/g — 1.75e+4 Ci/g — a gigabecquerel of Sb-124 amounts to 1.54 µg, which is why activity rather than mass is how anyone states the quantity. Antimony-124 decays by beta-minus decay with a half-life of 60.20 days, falling to 70.79% of today's activity in a month and 1.49% in a year.
Among the strong external emitters here: Γ of 0.226 mGy·m²/(GBq·h) ranks 8 of 96, 2.9× Cs-137 and 1.4× less than Co-60. 1 GBq at 1 m reads 0.226 mGy/h, and 1 Ci at the same distance 8.35 mGy/h — a metre-scale hazard at gigabecquerel activities, and 88.6 MBq is already 20 µSv/h at that distance.
59 lines clear the 20 keV cutoff, but 6 of them carry 90% of the dose rate. The leading one is 1691.0 keV at 40.2% of the total — its emission probability is 47.56506%, which is lower than the 97.79% of the 602.7 keV line it outranks.
This is a shield that has to be designed: 7.97 mm of lead for a factor of two and 31.1 mm for a factor of ten, or 15.0 mm of steel to halve it, at which point the mass of the shield is part of the problem. Reaching 20 µSv/h from 1 GBq at a metre takes 33.1 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 60.20 days (5.201e+6 s) |
| Decay mode | beta-minus decay |
| Specific activity | 6.48e+14 Bq/g (1.75e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.226 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.226 mGy/h |
| Dose rate, 1 Ci at 1 m | 8.35 mGy/h |
| Kerma-weighted mean photon energy | 1198 keV |
59 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 4.13 keV and 0.0447% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 1690.97 | 47.56506 | 40.25 |
| 602.73 | 97.79 | 35.38 |
| 2090.93 | 5.48602 | 5.39 |
| 722.78 | 10.7569 | 4.60 |
| 645.85 | 7.42031 | 2.86 |
| 1368.16 | 2.62371 | 1.90 |
| 1325.50 | 1.58029 | 1.12 |
| 1045.13 | 1.83258 | 1.08 |
| 968.20 | 1.88246 | 1.04 |
| 713.78 | 2.27557 | 0.96 |
| 1436.55 | 1.21651 | 0.92 |
| 1355.20 | 1.03755 | 0.75 |
| 709.34 | 1.35341 | 0.57 |
| 1488.89 | 0.67182 | 0.52 |
Showing the 14 largest contributors of 59 lines above the cutoff; the remainder together carry 2.67% of the dose rate.
7.97 mm of lead halves this spectrum
Solved numerically across all 59 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 7.97 | 31.1 | 3.90 |
| tungsten | 5.11 | 19.3 | 3.78 |
| iron | 15.0 | 52.5 | 3.49 |
| copper | 13.4 | 46.8 | 3.50 |
| concrete | 47.9 | 167 | 3.49 |
| water | 101 | 354 | 3.49 |
| aluminum | 43.1 | 150 | 3.49 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 1.65 years — a storage problem rather than a disposal one, with 1.49% of today's activity still there after a year. The mean life 1/λ is 86.9 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 | 200 days |
| Time to fall to 1 % | 1.10 years |
| Time to fall to 0.1 % | 1.64 years |
Limits of these dose rates
- 0.226 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 7.97 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.0447% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sb-124
Other Antimony nuclides: Sb-124m1, Sb-124m2, Sb-125
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.