Cobalt-60m
Co-60m · Cobalt, Z = 27, A = 60 · isomeric state m
Cobalt-60m (Co-60m) carries 1.11e+19 Bq/g, or 3.00e+8 Ci/g: a gigabecquerel is 90.2 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 10.467 minutes, which leaves 3.9e-40% of today's activity after a day and 1.3e-288% after a week.
The air kerma rate constant is small — 0.000586 mGy·m²/(GBq·h), 1.3e+2× less than Cs-137 and 5.2e+2× less than Co-60, ranking 81 of 96 by Γ — near the bottom of the photon emitters, but above the cutoff, which 51 nuclides in this dataset are not. 1 GBq at 1 m reads 0.000586 mGy/h, and 1 Ci at the same distance 0.0217 mGy/h. It takes 34.2 GBq at a metre to reach 20 µSv/h from the photons alone.
3 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 1332.5 keV at 68.5% of the total — its emission probability is 0.25%, which is lower than the 2.07% of the 58.6 keV line it outranks.
This is a shield that has to be designed: 5.20 mm of lead for a factor of two and 30.2 mm for a factor of ten, or 8.19 mm of steel to halve it, at which point the mass of the shield is part of the problem. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 5.8 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 10.467 minutes (628.0 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.11e+19 Bq/g (3.00e+8 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.000586 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.000586 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.0217 mGy/h |
| Kerma-weighted mean photon energy | 942.4 keV |
1332 keV leads, and 2 lines make 90%
3 further lines below the 20 keV cutoff, the highest at 7.68 keV and 31.6% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 1332.49 | 0.25 | 68.50 |
| 58.60 | 2.07 | 30.05 |
| 826.10 | 0.0078 | 1.45 |
5.20 mm of lead halves this spectrum
Solved numerically across all 3 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 5.20 | 30.2 | 5.81 |
| tungsten | 3.21 | 18.7 | 5.81 |
| iron | 8.19 | 47.5 | 5.80 |
| copper | 7.31 | 42.4 | 5.80 |
| concrete | 32.2 | 150 | 4.66 |
| water | 76.0 | 317 | 4.17 |
| aluminum | 28.1 | 135 | 4.80 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 1.74 hours, so the activity moves measurably while a count is running: every figure has to carry the time it was referred to. The mean life 1/λ is 15.1 minutes.
| 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 | 34.8 minutes |
| Time to fall to 1 % | 1.16 hours |
| Time to fall to 0.1 % | 1.74 hours |
Limits of these dose rates
- 0.000586 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 5.20 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 31.6% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Co-60m
Other Cobalt nuclides: Co-57, Co-60
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.