Cesium-134m
Cs-134m · Cesium, Z = 55, A = 134 · isomeric state m
Cesium-134m (Cs-134m) carries 2.97e+17 Bq/g, or 8.04e+6 Ci/g: a gigabecquerel is 3.36 ng, no weighable quantity at all. It decays by isomeric transition with a half-life of 2.912 hours, which leaves 0.33% of today's activity after a day and 4.3e-16% after a week.
At 0.00880 mGy·m²/(GBq·h) the air kerma rate constant is 8.8× less than Cs-137 and 35× less than Co-60, placing it 73 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.00880 mGy/h, and 1 Ci at the same distance 0.326 mGy/h.
7 lines clear the 20 keV cutoff, but 5 of them carry 90% of the dose rate. The leading one is 31.0 keV at 37.5% of the total — its emission probability is 16.5162%, which is also the highest.
Shielding barely arises: 0.0315 mm of lead halves the air kerma rate and 0.214 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.179 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 6.8 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 2.912 hours (1.048e+4 s) |
| Decay mode | isomeric transition |
| Specific activity | 2.97e+17 Bq/g (8.04e+6 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.00880 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.00880 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.326 mGy/h |
| Kerma-weighted mean photon energy | 51.47 keV |
7 lines above the cutoff, and what each contributes
2 further lines below the 20 keV cutoff, the highest at 11.24 keV and 16.7% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 30.97 | 16.5162 | 37.49 |
| 30.63 | 8.94022 | 20.71 |
| 127.50 | 12.6 | 20.33 |
| 35.41 | 6.00262 | 10.68 |
| 35.09 | 4.82526 | 8.73 |
| 35.82 | 1.17736 | 2.05 |
| 138.73 | 0.00391 | 0.01 |
0.0315 mm of lead halves this spectrum
Solved numerically across all 7 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.0315 | 0.214 | 6.78 |
| tungsten | 0.0247 | 0.161 | 6.50 |
| iron | 0.179 | 3.56 | 19.82 |
| copper | 0.118 | 2.67 | 22.58 |
| concrete | 4.64 | 23.2 | 4.99 |
| water | 22.9 | 82.8 | 3.61 |
| aluminum | 3.45 | 19.2 | 5.57 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 1.21 days, 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 4.20 hours.
| 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 | 9.67 hours |
| Time to fall to 1 % | 19.3 hours |
| Time to fall to 0.1 % | 1.21 days |
Limits of these dose rates
- 0.00880 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0315 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 16.7% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Cs-134m
Other Cesium nuclides: Cs-134, Cs-135, Cs-137
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.