Actinium-227
Ac-227 · Actinium, Z = 89, A = 227
Actinium-227 decays by beta-minus decay, half-life 21.772 years. Specific activity is 2.68e+12 Bq/g (72.3 Ci/g), so a gigabecquerel comes to 374 µg — weighable, but on an analytical balance. Over a year the activity falls to 96.87%, and over forty years to 27.99%.
The beta endpoint is 0.0448 MeV, mean 0.00987 MeV over 4 branches. That endpoint stops in 0.0274 mm of acrylic or 0.0129 mm of glass. Of the beta energy, 9.4e-3% turns into X-rays in acrylic and 0.13% in lead.
Alpha emission is led by 4.953 MeV at 0.6583%, one of 6 recorded lines. None of it reaches through skin, so the limit here is intake, not external dose.
Decay is the only handle: a year leaves 96.87% of today's activity and forty years leaves 27.99%, with ten half-lives at 218 years and a mean life of 31.4 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 21.772 years (6.871e+8 s) |
| Decay mode | beta-minus decay |
| Specific activity | 2.68e+12 Bq/g (72.3 Ci/g) |
| Beta endpoint / mean | 0.0448 MeV / 0.00987 MeV |
0.0274 mm of acrylic stops the 0.0448 MeV endpoint
Katz–Penfold fit to the 0.0448 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 | 0.0274 |
| aluminum | 0.0120 |
| water | 0.0323 |
| glass | 0.0129 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.01% |
| lead (Z = 82) | 0.13% |
6 alpha lines, strongest 4.953 MeV
6 recorded lines from 4.768 to 4.953 MeV, the strongest 4.953 MeV at 0.6583% of 1.413% total alpha emission.
| Energy (MeV) | Emission probability (%) |
|---|---|
| 4.9533 | 0.6583 |
| 4.9407 | 0.5465 |
| 4.8727 | 0.0869 |
| 4.8550 | 0.0828 |
| 4.7680 | 0.0248 |
| 4.7960 | 0.0138 |
Activity over decades and centuries
Ten half-lives is 218 years, which puts decay storage out of reach: 27.99% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 31.4 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 | 72.3 years |
| Time to fall to 1 % | 145 years |
| Time to fall to 0.1 % | 217 years |
Limits — intake, not external dose
- 0.0274 mm of acrylic for the 0.0448 MeV endpoint: a ±10% fit, and not a skin dose.
- Alpha reaches nothing through skin — Ac-227 is an intake problem, not an external one.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Ac-227
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.