Samarium-153
Sm-153 · Samarium, Z = 62, A = 153
Samarium-153 (Sm-153) carries 1.64e+16 Bq/g, or 4.43e+5 Ci/g: a gigabecquerel is 61.0 ng, no weighable quantity at all. It decays by beta-minus decay with a half-life of 46.284 hours, which leaves 8.08% of today's activity after a week and 6.2e-13% after three months.
At 0.0119 mGy·m²/(GBq·h) the air kerma rate constant is 6.5× less than Cs-137 and 26× less than Co-60, placing it 67 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0119 mGy/h, and 1 Ci at the same distance 0.441 mGy/h.
38 lines clear the 20 keV cutoff, but 5 of them carry 90% of the dose rate. The leading one is 41.5 keV at 30.1% of the total — its emission probability is 30.03632%, which is also the highest.
Shielding barely arises: 0.0673 mm of lead halves the air kerma rate and 0.274 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.485 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 4.1 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 46.284 hours (1.666e+5 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.64e+16 Bq/g (4.43e+5 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0119 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0119 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.441 mGy/h |
| Kerma-weighted mean photon energy | 66.54 keV |
38 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 6.60 keV and 10.7% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 41.54 | 30.03632 | 30.11 |
| 103.18 | 29.14 | 27.07 |
| 40.90 | 16.64613 | 17.03 |
| 47.65 | 11.90402 | 10.00 |
| 47.15 | 9.46266 | 8.06 |
| 69.67 | 4.66823 | 3.37 |
| 48.25 | 2.44137 | 2.02 |
| 97.43 | 0.76347 | 0.67 |
| 531.40 | 0.05467 | 0.33 |
| 533.20 | 0.02955 | 0.18 |
| 83.37 | 0.19087 | 0.15 |
| 75.42 | 0.19232 | 0.14 |
| 89.49 | 0.15736 | 0.13 |
| 539.10 | 0.02066 | 0.13 |
Showing the 14 largest contributors of 38 lines above the cutoff; the remainder together carry 0.62% of the dose rate.
0.0673 mm of lead halves this spectrum
Solved numerically across all 38 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.0673 | 0.274 | 4.07 |
| tungsten | 0.0499 | 0.193 | 3.88 |
| iron | 0.485 | 4.13 | 8.52 |
| copper | 0.322 | 2.96 | 9.20 |
| concrete | 8.91 | 35.4 | 3.97 |
| water | 31.0 | 107 | 3.44 |
| aluminum | 7.05 | 29.5 | 4.18 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 19.3 days — a storage problem rather than a disposal one, with 9.7e-56% of today's activity still there after a year. The mean life 1/λ is 2.78 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 | 6.41 days |
| Time to fall to 1 % | 12.8 days |
| Time to fall to 0.1 % | 19.2 days |
Limits of these dose rates
- 0.0119 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0673 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 10.7% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sm-153
Other Samarium nuclides: Sm-147, Sm-151, Sm-153m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.