Argon-39
Ar-39 · Argon, Z = 18, A = 39
Argon-39 decays by beta-minus decay, half-life 268 years. Specific activity is 1.27e+12 Bq/g (34.2 Ci/g), so a gigabecquerel comes to 789 µg — weighable, but on an analytical balance. Over forty years the activity falls to 90.17%, and over ten thousand years to 5.9e-10%.
The beta endpoint is 0.565 MeV, mean 0.219 MeV over 1 branch. That endpoint stops in 1.64 mm of acrylic or 0.776 mm of glass. Of the beta energy, 0.12% turns into X-rays in acrylic and 1.62% in lead.
Decay is the only handle: forty years leaves 90.17% of today's activity and ten thousand years leaves 5.9e-10%, with ten half-lives at 2,680 years and a mean life of 387 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 268 years (8.457e+9 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.27e+12 Bq/g (34.2 Ci/g) |
| Beta endpoint / mean | 0.565 MeV / 0.219 MeV |
1.64 mm of acrylic stops the 0.565 MeV endpoint
Katz–Penfold fit to the 0.565 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 | 1.64 |
| aluminum | 0.719 |
| water | 1.94 |
| glass | 0.776 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.12% |
| lead (Z = 82) | 1.62% |
Activity over millennia
Ten half-lives is 2,680 years, which puts decay storage out of reach: 90.17% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 387 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 | 890 years |
| Time to fall to 1 % | 1,781 years |
| Time to fall to 0.1 % | 2,671 years |
Limits of these range figures
- 1.64 mm of acrylic for the 0.565 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Ar-39
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.