Plutonium-241
Pu-241 · Plutonium, Z = 94, A = 241
Plutonium-241 decays by alpha decay, half-life 14.329 years. Specific activity is 3.83e+12 Bq/g (104 Ci/g), so a gigabecquerel comes to 261 µg — weighable, but on an analytical balance. Over a year the activity falls to 95.28%, and over forty years to 14.44%.
All 1 recorded photon lines sit below the 20 keV cutoff, so Γ is zero by convention and no external dose rate is quoted. The strongest line is 16.68 keV. A monitor calibrated on Cs-137 under-responds at these energies, so a reading taken without an energy correction understates Pu-241 rather than missing it outright. The recorded spectrum runs 16.7 to 16.7 keV and carries 0.00125% emission probability in all, which sets what a thin-window or proportional detector has to see.
The beta endpoint is 0.0208 MeV, mean 0.00523 MeV over 1 branch. That endpoint stops in 0.00621 mm of acrylic or 0.00293 mm of glass. Of the beta energy, 4.4e-3% turns into X-rays in acrylic and 0.06% in lead.
Decay is the only handle: a year leaves 95.28% of today's activity and forty years leaves 14.44%, with ten half-lives at 143 years and a mean life of 20.7 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 14.329 years (4.522e+8 s) |
| Decay mode | alpha decay |
| Specific activity | 3.83e+12 Bq/g (104 Ci/g) |
| Beta endpoint / mean | 0.0208 MeV / 0.00523 MeV |
0.00621 mm of acrylic stops the 0.0208 MeV endpoint
Katz–Penfold fit to the 0.0208 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 | 0.00621 |
| aluminum | 0.00272 |
| water | 0.00733 |
| glass | 0.00293 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.00% |
| lead (Z = 82) | 0.06% |
Activity over decades and centuries
Ten half-lives is 143 years, which puts decay storage out of reach: 14.44% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 20.7 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 | 47.6 years |
| Time to fall to 1 % | 95.2 years |
| Time to fall to 0.1 % | 143 years |
Limits of these range figures
- Γ excludes 1 line under 20 keV, carrying 0.00125% of all emissions.
- 0.00621 mm of acrylic for the 0.0208 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Pu-241
Other Plutonium nuclides: Pu-238, Pu-239, Pu-240, Pu-242, Pu-244
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.