Antimony-125
Sb-125 · Antimony, Z = 51, A = 125
Antimony-125 decays by beta-minus decay, half-life 2.75856 years. Specific activity is 3.84e+13 Bq/g (1.04e+3 Ci/g), so a gigabecquerel comes to 26.0 µg — weighable, but on an analytical balance. Over a year the activity falls to 77.78%, and over forty years to 4.3e-3%.
1 GBq at 1 m reads 0.0702 mGy/h, and 1 Ci at the same distance 2.60 mGy/h, from an air kerma rate constant of 0.0702 mGy·m²/(GBq·h) — 1.1× less than Cs-137 and 4.4× less than Co-60, and 32 of 96 among the photon emitters carried here.
40 lines clear the 20 keV cutoff, but 8 of them carry 90% of the dose rate. The leading one is 427.9 keV at 24.5% of the total — its emission probability is 29.6%, which is also the highest.
Halving the air kerma rate calls for 2.52 mm of lead, or 7.16 mm of steel where lead is unwelcome, and a factor of ten calls for 12.3 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 5.80 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 | 2.75856 years (8.705e+7 s) |
| Decay mode | beta-minus decay |
| Specific activity | 3.84e+13 Bq/g (1.04e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0702 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0702 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.60 mGy/h |
| Kerma-weighted mean photon energy | 426.4 keV |
40 lines above the cutoff, and what each contributes
2 further lines below the 20 keV cutoff, the highest at 19.80 keV and 4.84% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 427.87 | 29.6 | 24.48 |
| 600.60 | 17.64752 | 20.48 |
| 635.95 | 11.2184 | 13.72 |
| 463.37 | 10.4932 | 9.42 |
| 27.47 | 24.53925 | 8.90 |
| 606.71 | 4.98168 | 5.83 |
| 27.20 | 13.17758 | 4.88 |
| 31.36 | 8.54663 | 2.38 |
| 671.44 | 1.7905 | 2.30 |
| 176.31 | 6.84056 | 2.05 |
| 31.09 | 7.02271 | 1.99 |
| 380.45 | 1.5167 | 1.11 |
| 35.49 | 4.37488 | 0.97 |
| 31.70 | 1.52393 | 0.42 |
Showing the 14 largest contributors of 40 lines above the cutoff; the remainder together carry 1.09% of the dose rate.
2.52 mm of lead halves this spectrum
Solved numerically across all 40 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 2.52 | 12.3 | 4.88 |
| tungsten | 1.73 | 8.34 | 4.83 |
| iron | 7.16 | 31.9 | 4.46 |
| copper | 6.31 | 28.2 | 4.48 |
| concrete | 23.5 | 103 | 4.41 |
| water | 54.0 | 219 | 4.05 |
| aluminum | 21.0 | 92.9 | 4.42 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over decades and centuries
Ten half-lives is 27.6 years, which puts decay storage out of reach: 4.3e-3% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 3.98 years.
| 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 | 9.16 years |
| Time to fall to 1 % | 18.3 years |
| Time to fall to 0.1 % | 27.5 years |
Limits of these dose rates
- 0.0702 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 2.52 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 4.84% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sb-125
Other Antimony nuclides: Sb-124, Sb-124m1, Sb-124m2
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.