Tritium
H-3 · Hydrogen, Z = 1, A = 3
At 3.56e+14 Bq/g — 9.62e+3 Ci/g — a gigabecquerel of H-3 amounts to 2.81 µg, which is why activity rather than mass is how anyone states the quantity. Tritium decays by beta-minus decay with a half-life of 12.32 years, falling to 94.53% of today's activity in a year and 10.53% in forty years.
The beta endpoint is 0.0186 MeV, mean 0.00568 MeV over 1 branch. That endpoint stops in 0.00496 mm of acrylic or 0.00234 mm of glass. Of the beta energy, 3.9e-3% turns into X-rays in acrylic and 0.05% in lead.
Decay is the only handle: a year leaves 94.53% of today's activity and forty years leaves 10.53%, with ten half-lives at 123 years and a mean life of 17.8 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 12.32 years (3.888e+8 s) |
| Decay mode | beta-minus decay |
| Specific activity | 3.56e+14 Bq/g (9.62e+3 Ci/g) |
| Beta endpoint / mean | 0.0186 MeV / 0.00568 MeV |
0.00496 mm of acrylic stops the 0.0186 MeV endpoint
Katz–Penfold fit to the 0.0186 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 | 0.00496 |
| aluminum | 0.00217 |
| water | 0.00585 |
| glass | 0.00234 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.00% |
| lead (Z = 82) | 0.05% |
Activity over decades and centuries
Ten half-lives is 123 years, which puts decay storage out of reach: 10.53% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 17.8 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 | 40.9 years |
| Time to fall to 1 % | 81.9 years |
| Time to fall to 0.1 % | 123 years |
Limits of these range figures
- 0.00496 mm of acrylic for the 0.0186 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for H-3
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.