Holmium-166
Ho-166 · Holmium, Z = 67, A = 166
Holmium-166 (Ho-166) carries 2.61e+16 Bq/g, or 7.04e+5 Ci/g: a gigabecquerel is 38.4 ng, no weighable quantity at all. It decays by beta-minus decay with a half-life of 26.824 hours, which leaves 1.30% of today's activity after a week and 3.1e-23% after three months.
The beta endpoint is 1.85 MeV, mean 0.665 MeV over 7 branches. That endpoint stops in 7.35 mm of acrylic or 3.47 mm of glass. Of the beta energy, 0.39% turns into X-rays in acrylic and 5.32% in lead.
Decay is the only handle: a week leaves 1.30% of today's activity and three months leaves 3.1e-23%, with ten half-lives at 11.2 days and a mean life of 1.61 days per atom.
Half-life, specific activity and beta endpoint
| Half-life | 26.824 hours (9.657e+4 s) |
| Decay mode | beta-minus decay |
| Specific activity | 2.61e+16 Bq/g (7.04e+5 Ci/g) |
| Beta endpoint / mean | 1.85 MeV / 0.665 MeV |
7.35 mm of acrylic stops the 1.85 MeV endpoint
Katz–Penfold fit to the 1.85 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 | 7.35 |
| aluminum | 3.22 |
| water | 8.68 |
| glass | 3.47 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.39% |
| lead (Z = 82) | 5.32% |
Activity over days and weeks
Ten half-lives is 11.2 days — a storage problem rather than a disposal one, with 4.2e-97% of today's activity still there after a year. The mean life 1/λ is 1.61 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 | 3.71 days |
| Time to fall to 1 % | 7.43 days |
| Time to fall to 0.1 % | 11.1 days |
Limits of these range figures
- 7.35 mm of acrylic for the 1.85 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Ho-166
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.