Technetium-99
Tc-99 · Technetium, Z = 43, A = 99
Technetium-99 (Tc-99) is slow enough to be handled as a material rather than as a trace: 6.34e+8 Bq/g, or 0.0171 Ci/g, puts a gigabecquerel at 1.58 g. Decay is by beta-minus decay, half-life 2.111 × 10⁵ years, and shedding even one per cent of the activity takes 3,061 years.
The beta endpoint is 0.297 MeV, mean 0.0846 MeV over 2 branches. That endpoint stops in 0.655 mm of acrylic or 0.309 mm of glass. Of the beta energy, 0.06% turns into X-rays in acrylic and 0.85% in lead.
Decay is the only handle: forty years leaves 99.99% of today's activity and ten thousand years leaves 96.77%, with ten half-lives at 2.11 million years and a mean life of 304,546 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 2.111 × 10⁵ years (6.662e+12 s) |
| Decay mode | beta-minus decay |
| Specific activity | 6.34e+8 Bq/g (0.0171 Ci/g) |
| Beta endpoint / mean | 0.297 MeV / 0.0846 MeV |
0.655 mm of acrylic stops the 0.297 MeV endpoint
Katz–Penfold fit to the 0.297 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 | 0.655 |
| aluminum | 0.286 |
| water | 0.773 |
| glass | 0.309 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.06% |
| lead (Z = 82) | 0.85% |
Activity over geological time
Ten half-lives is 2.11 million years. On any timescale a facility can be planned over the activity is constant — 99.99% is left after forty years — and the mean life 1/λ is 304,546 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 | 701,244 years |
| Time to fall to 1 % | 1.40 million years |
| Time to fall to 0.1 % | 2.10 million years |
Limits of these range figures
- 0.655 mm of acrylic for the 0.297 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Tc-99
Other Technetium nuclides: Tc-99m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.