Strontium-90
Sr-90 · Strontium, Z = 38, A = 90
Strontium-90 decays by beta-minus decay, half-life 28.91 years. Specific activity is 5.09e+12 Bq/g (138 Ci/g), so a gigabecquerel comes to 197 µg — weighable, but on an analytical balance. Over a year the activity falls to 97.63%, and over forty years to 38.33%.
The beta endpoint is 0.546 MeV, mean 0.196 MeV over 1 branch. That endpoint stops in 1.57 mm of acrylic or 0.740 mm of glass. Of Sr-90's own beta energy, 0.11% turns into X-rays in acrylic and 1.57% in lead.
Sr-90 does not stand alone. Y-90 follows it with a half-life of 64.05 hours, so ingrowth is complete within 18.7 days of separation. Its endpoint is 2.28 MeV and stops in 9.28 mm of acrylic, 5.9× the thickness above. The shield is sized on Y-90, and so is the bremsstrahlung: 0.48% of that energy turns into X-rays in acrylic and 6.54% in lead.
Decay is the only handle: a year leaves 97.63% of today's activity and forty years leaves 38.33%, with ten half-lives at 289 years and a mean life of 41.7 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 28.91 years (9.123e+8 s) |
| Decay mode | beta-minus decay |
| Specific activity | 5.09e+12 Bq/g (138 Ci/g) |
| Beta endpoint / mean | 0.546 MeV / 0.196 MeV |
1.57 mm of acrylic stops the 0.546 MeV endpoint
Katz–Penfold fit to the 0.546 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 | 1.57 |
| aluminum | 0.685 |
| water | 1.85 |
| glass | 0.740 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.11% |
| lead (Z = 82) | 1.57% |
Activity over decades and centuries
Ten half-lives is 289 years, which puts decay storage out of reach: 38.33% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 41.7 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 | 96.0 years |
| Time to fall to 1 % | 192 years |
| Time to fall to 0.1 % | 288 years |
Limits of these range figures
- 1.57 mm of acrylic for the 0.546 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Sr-90
Other Strontium nuclides: Sr-85, Sr-85m, Sr-89
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.