Yttrium-91
Y-91 · Yttrium, Z = 39, A = 91
At 9.08e+14 Bq/g — 2.45e+4 Ci/g — a gigabecquerel of Y-91 amounts to 1.10 µg, which is why activity rather than mass is how anyone states the quantity. Yttrium-91 decays by beta-minus decay with a half-life of 58.51 days, falling to 70.09% of today's activity in a month and 1.32% in a year.
The beta endpoint is 1.54 MeV, mean 0.603 MeV over 2 branches. That endpoint stops in 5.95 mm of acrylic or 2.81 mm of glass. Of the beta energy, 0.32% turns into X-rays in acrylic and 4.43% in lead.
Decay is the only handle: a month leaves 70.09% of today's activity and a year leaves 1.32%, with ten half-lives at 1.60 years and a mean life of 84.4 days per atom.
Half-life, specific activity and beta endpoint
| Half-life | 58.51 days (5.055e+6 s) |
| Decay mode | beta-minus decay |
| Specific activity | 9.08e+14 Bq/g (2.45e+4 Ci/g) |
| Beta endpoint / mean | 1.54 MeV / 0.603 MeV |
5.95 mm of acrylic stops the 1.54 MeV endpoint
Katz–Penfold fit to the 1.54 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 | 5.95 |
| aluminum | 2.60 |
| water | 7.02 |
| glass | 2.81 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.32% |
| lead (Z = 82) | 4.43% |
Activity over months and years
Ten half-lives is 1.60 years — a storage problem rather than a disposal one, with 1.32% of today's activity still there after a year. The mean life 1/λ is 84.4 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 | 194 days |
| Time to fall to 1 % | 1.06 years |
| Time to fall to 0.1 % | 1.60 years |
Limits of these range figures
- 5.95 mm of acrylic for the 1.54 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Y-91
Other Yttrium nuclides: Y-88, Y-88m1, Y-88m2, Y-90, Y-90m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.