Samarium-153m
Sm-153m · Samarium, Z = 62, A = 153 · isomeric state m
Samarium-153m (Sm-153m) carries 2.58e+23 Bq/g, or 6.96e+12 Ci/g: a gigabecquerel is 3.88 fg, no weighable quantity at all. It decays by isomeric transition with a half-life of 10.6 milliseconds, which leaves 0% of today's activity after a day and 0% after a week.
At 0.0104 mGy·m²/(GBq·h) the air kerma rate constant is 7.4× less than Cs-137 and 29× less than Co-60, placing it 69 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0104 mGy/h, and 1 Ci at the same distance 0.385 mGy/h.
9 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 32.9 keV at 86.1% of the total — its emission probability is 50.1%, which is also the highest.
Shielding barely arises: 0.0280 mm of lead halves the air kerma rate and 0.0982 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.153 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 | 10.6 milliseconds (0.01060 s) |
| Decay mode | isomeric transition |
| Specific activity | 2.58e+23 Bq/g (6.96e+12 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0104 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0104 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.385 mGy/h |
| Kerma-weighted mean photon energy | 34.46 keV |
32.9 keV carries 86% of the dose rate
3 further lines below the 20 keV cutoff, the highest at 11.90 keV and 28.2% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 32.90 | 50.1 | 86.08 |
| 40.12 | 3.75099 | 4.50 |
| 39.52 | 2.07242 | 2.55 |
| 46.10 | 2.2044 | 2.21 |
| 46.00 | 1.47507 | 1.48 |
| 45.52 | 1.17255 | 1.20 |
| 58.00 | 1.1523 | 0.95 |
| 53.60 | 0.8517 | 0.74 |
| 46.58 | 0.30252 | 0.30 |
0.0280 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 | 0.0280 | 0.0982 | 3.51 |
| tungsten | 0.0220 | 0.0774 | 3.52 |
| iron | 0.153 | 0.543 | 3.54 |
| copper | 0.101 | 0.357 | 3.54 |
| concrete | 4.15 | 14.3 | 3.44 |
| water | 21.4 | 71.6 | 3.35 |
| aluminum | 3.07 | 10.6 | 3.46 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 0.106 seconds, 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 0.0153 seconds.
| 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 | 0.0352 seconds |
| Time to fall to 1 % | 0.0704 seconds |
| Time to fall to 0.1 % | 0.106 seconds |
Limits of these dose rates
- 0.0104 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0280 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 28.2% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sm-153m
Other Samarium nuclides: Sm-147, Sm-151, Sm-153
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.