Ruthenium-106
Ru-106 · Ruthenium, Z = 44, A = 106
At 1.23e+14 Bq/g — 3.32e+3 Ci/g — a gigabecquerel of Ru-106 amounts to 8.15 µg, which is why activity rather than mass is how anyone states the quantity. Ruthenium-106 decays by beta-minus decay with a half-life of 371.8 days, falling to 50.61% of today's activity in a year and 1.5e-10% in forty years.
The beta endpoint is 0.0394 MeV, mean 0.0100 MeV over 1 branch. That endpoint stops in 0.0215 mm of acrylic or 0.0102 mm of glass. Of Ru-106's own beta energy, 8.3e-3% turns into X-rays in acrylic and 0.11% in lead.
Ru-106 does not stand alone. Rh-106 follows it with a half-life of 30.07 seconds, so ingrowth is complete within 3.51 minutes of separation. Its endpoint is 3.54 MeV and stops in 15.0 mm of acrylic, 697× the thickness above. The shield is sized on Rh-106, and so is the bremsstrahlung: 0.74% of that energy turns into X-rays in acrylic and 10.16% in lead. Rh-106 also carries the photons here — 0.0275 mGy·m²/(GBq·h), against nothing recorded for Ru-106.
Decay is the only handle: a year leaves 50.61% of today's activity and forty years leaves 1.5e-10%, with ten half-lives at 10.2 years and a mean life of 1.47 years per atom.
Half-life, specific activity and beta endpoint
| Half-life | 371.8 days (3.212e+7 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.23e+14 Bq/g (3.32e+3 Ci/g) |
| Beta endpoint / mean | 0.0394 MeV / 0.0100 MeV |
0.0215 mm of acrylic stops the 0.0394 MeV endpoint
Katz–Penfold fit to the 0.0394 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 | 0.0215 |
| aluminum | 0.00941 |
| water | 0.0254 |
| glass | 0.0102 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.01% |
| lead (Z = 82) | 0.11% |
Activity over decades and centuries
Ten half-lives is 10.2 years, which puts decay storage out of reach: 1.5e-10% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 1.47 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 | 3.38 years |
| Time to fall to 1 % | 6.76 years |
| Time to fall to 0.1 % | 10.1 years |
Limits of these range figures
- 0.0215 mm of acrylic for the 0.0394 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Ru-106
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.