Cobalt-57
Co-57 · Cobalt, Z = 27, A = 57
At 3.12e+14 Bq/g — 8.44e+3 Ci/g — a gigabecquerel of Co-57 amounts to 3.20 µg, which is why activity rather than mass is how anyone states the quantity. Cobalt-57 decays by electron capture with a half-life of 271.74 days, falling to 92.63% of today's activity in a month and 39.39% in a year.
At 0.0133 mGy·m²/(GBq·h) the air kerma rate constant is 5.8× less than Cs-137 and 23× less than Co-60, placing it 64 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0133 mGy/h, and 1 Ci at the same distance 0.494 mGy/h.
8 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 122.1 keV at 86.5% of the total — its emission probability is 85.6%, which is also the highest.
Shielding barely arises: 0.190 mm of lead halves the air kerma rate and 0.655 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 3.35 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.
Half-life, specific activity and dose rate
| Half-life | 271.74 days (2.348e+7 s) |
| Decay mode | electron capture |
| Specific activity | 3.12e+14 Bq/g (8.44e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0133 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0133 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.494 mGy/h |
| Kerma-weighted mean photon energy | 130.4 keV |
122 keV carries 87% of the dose rate
4 further lines below the 20 keV cutoff, the highest at 14.41 keV and 65.7% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 122.06 | 85.6 | 86.50 |
| 136.47 | 10.68 | 12.31 |
| 692.41 | 0.149 | 1.04 |
| 570.09 | 0.0158 | 0.09 |
| 706.54 | 0.005 | 0.04 |
| 339.69 | 0.0037 | 0.01 |
| 352.33 | 0.003 | 0.01 |
| 366.80 | 0.0012 | 0.00 |
0.190 mm of lead halves this spectrum
Solved numerically across all 8 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.190 | 0.655 | 3.44 |
| tungsten | 0.141 | 0.486 | 3.44 |
| iron | 3.35 | 11.3 | 3.37 |
| copper | 2.50 | 8.45 | 3.38 |
| concrete | 19.3 | 64.4 | 3.34 |
| water | 43.7 | 146 | 3.33 |
| aluminum | 17.0 | 56.7 | 3.34 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 7.44 years — a storage problem rather than a disposal one, with 39.39% of today's activity still there after a year. The mean life 1/λ is 1.07 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 | 2.47 years |
| Time to fall to 1 % | 4.94 years |
| Time to fall to 0.1 % | 7.41 years |
Limits of these dose rates
- 0.0133 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.190 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 4 lines under 20 keV, carrying 65.7% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Co-57
Other Cobalt nuclides: Co-60, Co-60m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.