Yttrium-90
Y-90 · Yttrium, Z = 39, A = 90
Yttrium-90 (Y-90) carries 2.01e+16 Bq/g, or 5.44e+5 Ci/g: a gigabecquerel is 49.7 ng, no weighable quantity at all. It decays by beta-minus decay with a half-life of 64.05 hours, which leaves 16.23% of today's activity after a week and 5.4e-9% after three months.
All 4 recorded photon lines sit below the 20 keV cutoff, so Γ is zero by convention and no external dose rate is quoted. The strongest line is 17.82 keV. A monitor calibrated on Cs-137 under-responds at these energies, so a reading taken without an energy correction understates Y-90 rather than missing it outright. The recorded spectrum runs 15.7 to 17.8 keV and carries 0.00856% emission probability in all, which sets what a thin-window or proportional detector has to see.
The beta endpoint is 2.28 MeV, mean 0.932 MeV over 2 branches. That endpoint stops in 9.28 mm of acrylic or 4.38 mm of glass. Of the beta energy, 0.48% turns into X-rays in acrylic and 6.54% in lead.
Decay is the only handle: a week leaves 16.23% of today's activity and three months leaves 5.4e-9%, with ten half-lives at 26.7 days and a mean life of 3.85 days per atom.
Half-life, specific activity and beta endpoint
| Half-life | 64.05 hours (2.306e+5 s) |
| Decay mode | beta-minus decay |
| Specific activity | 2.01e+16 Bq/g (5.44e+5 Ci/g) |
| Beta endpoint / mean | 2.28 MeV / 0.932 MeV |
9.28 mm of acrylic stops the 2.28 MeV endpoint
Katz–Penfold fit to the 2.28 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 | 9.28 |
| aluminum | 4.06 |
| water | 10.9 |
| glass | 4.38 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.48% |
| lead (Z = 82) | 6.54% |
Activity over days and weeks
Ten half-lives is 26.7 days — a storage problem rather than a disposal one, with 6.3e-40% of today's activity still there after a year. The mean life 1/λ is 3.85 days.
| 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 | 8.87 days |
| Time to fall to 1 % | 17.7 days |
| Time to fall to 0.1 % | 26.6 days |
Limits of these range figures
- Γ excludes 4 lines under 20 keV, carrying 0.00856% of all emissions.
- 9.28 mm of acrylic for the 2.28 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Y-90
Other Yttrium nuclides: Y-88, Y-88m1, Y-88m2, Y-90m, Y-91
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.