Thorium-229
Th-229 · Thorium, Z = 90, A = 229
A gigabecquerel of Th-229 is 136 mg of material, which follows from a specific activity of 7.33e+9 Bq/g (0.198 Ci/g). Thorium-229 decays by alpha decay with a half-life of 7880 years; forty years leaves 99.65% of today's activity and ten thousand years leaves 41.49%.
Alpha emission is led by 4.845 MeV at 56.2%, one of 12 recorded lines. None of it reaches through skin, so the limit here is intake, not external dose.
Decay is the only handle: forty years leaves 99.65% of today's activity and ten thousand years leaves 41.49%, with ten half-lives at 78,798 years and a mean life of 11,368 years per atom.
Half-life 7880 years, 7.33e+9 Bq/g, and no line recorded
| Half-life | 7880 years (2.487e+11 s) |
| Decay mode | alpha decay |
| Specific activity | 7.33e+9 Bq/g (0.198 Ci/g) |
12 alpha lines, strongest 4.845 MeV
12 recorded lines from 4.761 to 5.053 MeV, the strongest 4.845 MeV at 56.2% of 99.75% total alpha emission.
| Energy (MeV) | Emission probability (%) |
|---|---|
| 4.8453 | 56.2 |
| 4.9010 | 10.2 |
| 4.8146 | 9.3 |
| 5.0530 | 6.6 |
| 4.9675 | 5.97 |
| 4.8380 | 5 |
| 4.9785 | 3.17 |
| 4.7978 | 1.5 |
| 4.7610 | 1 |
| 4.8330 | 0.29 |
Activity over millennia
Ten half-lives is 78,798 years. On any timescale a facility can be planned over the activity is constant — 99.65% is left after forty years — and the mean life 1/λ is 11,368 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 | 26,176 years |
| Time to fall to 1 % | 52,352 years |
| Time to fall to 0.1 % | 78,529 years |
Limits — intake, not external dose
- Alpha reaches nothing through skin — Th-229 is an intake problem, not an external one.
- What every sheet leaves out, internal dose included.
Decay and mass calculators for Th-229
Other Thorium nuclides: Th-228, Th-230, Th-232
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.