Calcium-45
Ca-45 · Calcium, Z = 20, A = 45
At 6.61e+14 Bq/g — 1.79e+4 Ci/g — a gigabecquerel of Ca-45 amounts to 1.51 µg, which is why activity rather than mass is how anyone states the quantity. Calcium-45 decays by beta-minus decay with a half-life of 162.61 days, falling to 88.00% of today's activity in a month and 21.08% in a year.
The beta endpoint is 0.258 MeV, mean 0.0769 MeV over 2 branches. That endpoint stops in 0.528 mm of acrylic or 0.249 mm of glass. Of the beta energy, 0.05% turns into X-rays in acrylic and 0.74% in lead.
Decay is the only handle: a month leaves 88.00% of today's activity and a year leaves 21.08%, with ten half-lives at 4.45 years and a mean life of 235 days per atom.
Half-life, specific activity and beta endpoint
| Half-life | 162.61 days (1.405e+7 s) |
| Decay mode | beta-minus decay |
| Specific activity | 6.61e+14 Bq/g (1.79e+4 Ci/g) |
| Beta endpoint / mean | 0.258 MeV / 0.0769 MeV |
0.528 mm of acrylic stops the 0.258 MeV endpoint
Katz–Penfold fit to the 0.258 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 | 0.528 |
| aluminum | 0.231 |
| water | 0.623 |
| glass | 0.249 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.05% |
| lead (Z = 82) | 0.74% |
Activity over months and years
Ten half-lives is 4.45 years — a storage problem rather than a disposal one, with 21.08% of today's activity still there after a year. The mean life 1/λ is 235 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 | 1.48 years |
| Time to fall to 1 % | 2.96 years |
| Time to fall to 0.1 % | 4.44 years |
Limits of these range figures
- 0.528 mm of acrylic for the 0.258 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for Ca-45
Other Calcium nuclides: Ca-41
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.