Cadmium-109
Cd-109 · Cadmium, Z = 48, A = 109
Cadmium-109 decays by electron capture, half-life 461.9 days. Specific activity is 9.60e+13 Bq/g (2.60e+3 Ci/g), so a gigabecquerel comes to 10.4 µg — weighable, but on an analytical balance. Over a year the activity falls to 57.80%, and over forty years to 3.0e-8%.
At 0.0452 mGy·m²/(GBq·h) the air kerma rate constant is 1.7× less than Cs-137 and 6.8× less than Co-60, placing it 44 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0452 mGy/h, and 1 Ci at the same distance 1.67 mGy/h.
6 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 22.2 keV at 48.7% of the total — its emission probability is 55.12889%, which is also the highest.
Shielding barely arises: 0.00999 mm of lead halves the air kerma rate and 0.0342 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.0500 mm. Reaching 20 µSv/h from 1 GBq at a metre takes 0.0118 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 461.9 days (3.991e+7 s) |
| Decay mode | electron capture |
| Specific activity | 9.60e+13 Bq/g (2.60e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0452 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0452 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.67 mGy/h |
| Kerma-weighted mean photon energy | 23.35 keV |
6 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 3.22 keV and 10.4% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 22.16 | 55.12889 | 48.68 |
| 21.99 | 29.24588 | 26.25 |
| 25.21 | 17.92964 | 12.09 |
| 25.03 | 15.27226 | 10.45 |
| 25.45 | 2.65737 | 1.76 |
| 88.03 | 3.644 | 0.77 |
0.00999 mm of lead halves this spectrum
Solved numerically across all 6 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.00999 | 0.0342 | 3.42 |
| tungsten | 0.00776 | 0.0266 | 3.42 |
| iron | 0.0500 | 0.173 | 3.45 |
| copper | 0.0333 | 0.115 | 3.45 |
| concrete | 1.53 | 5.25 | 3.43 |
| water | 11.0 | 37.3 | 3.38 |
| aluminum | 1.08 | 3.71 | 3.44 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over decades and centuries
Ten half-lives is 12.6 years, which puts decay storage out of reach: 3.0e-8% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 1.82 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 | 4.20 years |
| Time to fall to 1 % | 8.40 years |
| Time to fall to 0.1 % | 12.6 years |
Limits of these dose rates
- 0.0452 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.00999 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 10.4% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Cd-109
Other Cadmium nuclides: Cd-109m1, Cd-109m2
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.