Cerium-139
Ce-139 · Cerium, Z = 58, A = 139
At 2.53e+14 Bq/g — 6.83e+3 Ci/g — a gigabecquerel of Ce-139 amounts to 3.96 µg, which is why activity rather than mass is how anyone states the quantity. Cerium-139 decays by electron capture with a half-life of 137.63 days, falling to 85.98% of today's activity in a month and 15.89% in a year.
At 0.0309 mGy·m²/(GBq·h) the air kerma rate constant is 2.5× less than Cs-137 and 9.9× less than Co-60, placing it 49 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0309 mGy/h, and 1 Ci at the same distance 1.14 mGy/h.
6 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 165.9 keV at 50.3% of the total — its emission probability is 79.9%, which is also the highest.
Shielding barely arises: 0.0846 mm of lead halves the air kerma rate and 0.903 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.608 mm. Reaching 20 µSv/h from 1 GBq at a metre takes 0.0425 mm of lead. The tenth-value layer runs 10.7 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 137.63 days (1.189e+7 s) |
| Decay mode | electron capture |
| Specific activity | 2.53e+14 Bq/g (6.83e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0309 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0309 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.14 mGy/h |
| Kerma-weighted mean photon energy | 100.6 keV |
6 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 5.18 keV and 11.9% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 165.86 | 79.9 | 50.26 |
| 33.44 | 41.31865 | 23.20 |
| 33.03 | 22.49387 | 12.92 |
| 38.27 | 15.4171 | 6.77 |
| 37.91 | 12.30415 | 5.50 |
| 38.73 | 3.11295 | 1.34 |
0.0846 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 | 0.0846 | 0.903 | 10.67 |
| tungsten | 0.0655 | 0.676 | 10.32 |
| iron | 0.608 | 11.6 | 19.03 |
| copper | 0.422 | 9.19 | 21.77 |
| concrete | 8.84 | 50.9 | 5.76 |
| water | 31.5 | 119 | 3.78 |
| aluminum | 7.03 | 45.3 | 6.44 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 3.77 years — a storage problem rather than a disposal one, with 15.89% of today's activity still there after a year. The mean life 1/λ is 199 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 | 1.25 years |
| Time to fall to 1 % | 2.50 years |
| Time to fall to 0.1 % | 3.76 years |
Limits of these dose rates
- 0.0309 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0846 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 11.9% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ce-139
Other Cerium nuclides: Ce-139m, Ce-141, Ce-144
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.