Thallium-204
Tl-204 · Thallium, Z = 81, A = 204
Thallium-204 decays by beta-minus decay, half-life 3.783 years. Specific activity is 1.71e+13 Bq/g (463 Ci/g), so a gigabecquerel comes to 58.3 µg — weighable, but on an analytical balance. Over a year the activity falls to 83.26%, and over forty years to 0.07%.
The beta endpoint is 0.763 MeV, mean 0.244 MeV over 1 branch. That endpoint stops in 2.46 mm of acrylic or 1.16 mm of glass. Of the beta energy, 0.16% turns into X-rays in acrylic and 2.19% in lead.
Decay is the only handle: a year leaves 83.26% of today's activity and forty years leaves 0.07%, with ten half-lives at 37.8 years and a mean life of 5.46 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 3.783 years (1.194e+8 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.71e+13 Bq/g (463 Ci/g) |
| Beta endpoint / mean | 0.763 MeV / 0.244 MeV |
2.46 mm of acrylic stops the 0.763 MeV endpoint
Katz–Penfold fit to the 0.763 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 | 2.46 |
| aluminum | 1.08 |
| water | 2.91 |
| glass | 1.16 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.16% |
| lead (Z = 82) | 2.19% |
Activity over decades and centuries
Ten half-lives is 37.8 years, which puts decay storage out of reach: 0.07% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 5.46 years.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.12 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 12.6 years |
| Time to fall to 1 % | 25.1 years |
| Time to fall to 0.1 % | 37.7 years |
Limits of these range figures
- 2.46 mm of acrylic for the 0.763 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Tl-204
Other Thallium nuclides: Tl-201, Tl-201m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.