Cerium-139m
Ce-139m · Cerium, Z = 58, A = 139 · isomeric state m
Cerium-139m (Ce-139m) carries 5.22e+19 Bq/g, or 1.41e+9 Ci/g: a gigabecquerel is 19.2 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 57.58 seconds, which leaves 0% of today's activity after a day and 0% after a week.
1 GBq at 1 m reads 0.0938 mGy/h, and 1 Ci at the same distance 3.47 mGy/h, from an air kerma rate constant of 0.0938 mGy·m²/(GBq·h) — 1.2× Cs-137 and 3.3× less than Co-60, and 23 of 96 among the photon emitters carried here.
6 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 754.2 keV at 98.9% of the total — its emission probability is 92.6%, which is also the highest.
This is a shield that has to be designed: 6.32 mm of lead for a factor of two and 21.2 mm for a factor of ten, or 12.6 mm of steel to halve it, at which point the mass of the shield is part of the problem. Reaching 20 µSv/h from 1 GBq at a metre takes 14.2 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 57.58 seconds (57.58 s) |
| Decay mode | isomeric transition |
| Specific activity | 5.22e+19 Bq/g (1.41e+9 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0938 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0938 mGy/h |
| Dose rate, 1 Ci at 1 m | 3.47 mGy/h |
| Kerma-weighted mean photon energy | 746.6 keV |
754 keV carries 99% of the dose rate
1 further line below the 20 keV cutoff, the highest at 5.41 keV and 0.796% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 754.24 | 92.6 | 98.94 |
| 34.72 | 2.85722 | 0.49 |
| 34.28 | 1.56004 | 0.28 |
| 39.75 | 1.07914 | 0.15 |
| 39.37 | 0.86056 | 0.12 |
| 40.23 | 0.21858 | 0.03 |
6.32 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 | 6.32 | 21.2 | 3.36 |
| tungsten | 4.12 | 13.8 | 3.36 |
| iron | 12.6 | 42.2 | 3.36 |
| copper | 11.2 | 37.6 | 3.36 |
| concrete | 40.0 | 134 | 3.36 |
| water | 84.7 | 284 | 3.35 |
| aluminum | 36.0 | 121 | 3.36 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 9.60 minutes, 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 1.38 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 | 3.19 minutes |
| Time to fall to 1 % | 6.38 minutes |
| Time to fall to 0.1 % | 9.56 minutes |
Limits of these dose rates
- 0.0938 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 6.32 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.796% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ce-139m
Other Cerium nuclides: Ce-139, 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.