Antimony-124m1
Sb-124m1 · Antimony, Z = 51, A = 124 · isomeric state m1
Antimony-124m1 (Sb-124m1) carries 3.62e+19 Bq/g, or 9.79e+8 Ci/g: a gigabecquerel is 27.6 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 93 seconds, which leaves 2.2e-278% of today's activity after a day and 0% after a week.
1 GBq at 1 m reads 0.0599 mGy/h, and 1 Ci at the same distance 2.22 mGy/h, from an air kerma rate constant of 0.0599 mGy·m²/(GBq·h) — 1.3× less than Cs-137 and 5.1× less than Co-60, and 37 of 96 among the photon emitters carried here.
9 lines clear the 20 keV cutoff, but 3 of them carry 90% of the dose rate. The leading one is 645.8 keV at 36.3% of the total — its emission probability is 25%, which is also the highest.
Halving the air kerma rate calls for 4.70 mm of lead, or 11.3 mm of steel where lead is unwelcome, and a factor of ten calls for 16.0 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 7.49 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 93 seconds (93.00 s) |
| Decay mode | isomeric transition |
| Specific activity | 3.62e+19 Bq/g (9.79e+8 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0599 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0599 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.22 mGy/h |
| Kerma-weighted mean photon energy | 593.7 keV |
646 keV leads, and 3 lines make 90%
3 further lines below the 20 keV cutoff, the highest at 10.86 keV and 4.95% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 645.82 | 25 | 36.30 |
| 602.72 | 25 | 34.08 |
| 498.40 | 25 | 28.31 |
| 1101.00 | 0.5 | 1.15 |
| 27.47 | 0.17103 | 0.07 |
| 27.20 | 0.09184 | 0.04 |
| 31.36 | 0.05957 | 0.02 |
| 31.09 | 0.04895 | 0.02 |
| 31.70 | 0.01062 | 0.00 |
4.70 mm of lead halves this spectrum
Solved numerically across all 9 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 4.70 | 16.0 | 3.40 |
| tungsten | 3.17 | 10.7 | 3.38 |
| iron | 11.3 | 37.7 | 3.34 |
| copper | 10.0 | 33.5 | 3.34 |
| concrete | 36.3 | 121 | 3.34 |
| water | 76.7 | 256 | 3.33 |
| aluminum | 32.6 | 109 | 3.34 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 15.5 minutes, 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 2.24 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 | 5.15 minutes |
| Time to fall to 1 % | 10.3 minutes |
| Time to fall to 0.1 % | 15.4 minutes |
Limits of these dose rates
- 0.0599 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 4.70 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 4.95% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sb-124m1
Other Antimony nuclides: Sb-124, 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.