Samarium-151
Sm-151 · Samarium, Z = 62, A = 151
A gigabecquerel of Sm-151 is 1.03 mg of material, which follows from a specific activity of 9.74e+11 Bq/g (26.3 Ci/g). Samarium-151 decays by beta-minus decay with a half-life of 90 years; a year leaves 99.23% of today's activity and forty years leaves 73.49%.
The beta endpoint is 0.0759 MeV, mean 0.0196 MeV over 2 branches. That endpoint stops in 0.0710 mm of acrylic or 0.0335 mm of glass. Of the beta energy, 0.02% turns into X-rays in acrylic and 0.22% in lead.
Decay is the only handle: a year leaves 99.23% of today's activity and forty years leaves 73.49%, with ten half-lives at 900 years and a mean life of 130 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 90 years (2.840e+9 s) |
| Decay mode | beta-minus decay |
| Specific activity | 9.74e+11 Bq/g (26.3 Ci/g) |
| Beta endpoint / mean | 0.0759 MeV / 0.0196 MeV |
0.0710 mm of acrylic stops the 0.0759 MeV endpoint
Katz–Penfold fit to the 0.0759 MeV endpoint — ±10%, and a stopping thickness rather than an attenuation length. Why the material matters more than the thickness.
| Absorber | Range for the endpoint (mm) |
|---|---|
| acrylic | 0.0710 |
| aluminum | 0.0310 |
| water | 0.0837 |
| glass | 0.0335 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.02% |
| lead (Z = 82) | 0.22% |
Activity over decades and centuries
Ten half-lives is 900 years, which puts decay storage out of reach: 73.49% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 130 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 | 299 years |
| Time to fall to 1 % | 598 years |
| Time to fall to 0.1 % | 897 years |
Limits of these range figures
- 0.0710 mm of acrylic for the 0.0759 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Sm-151
Other Samarium nuclides: Sm-147, Sm-153, Sm-153m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.