Cesium-134
Cs-134 · Cesium, Z = 55, A = 134
Cesium-134 decays by beta-minus decay, half-life 2.0652 years. Specific activity is 4.78e+13 Bq/g (1.29e+3 Ci/g), so a gigabecquerel comes to 20.9 µg — weighable, but on an analytical balance. Over a year the activity falls to 71.49%, and over forty years to 1.5e-4%.
Among the strong external emitters here: Γ of 0.207 mGy·m²/(GBq·h) ranks 9 of 96, 2.7× Cs-137 and 1.5× less than Co-60. 1 GBq at 1 m reads 0.207 mGy/h, and 1 Ci at the same distance 7.67 mGy/h — a metre-scale hazard at gigabecquerel activities, and 96.4 MBq is already 20 µSv/h at that distance.
17 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 795.9 keV at 43.4% of the total — its emission probability is 85.46%, which is lower than the 97.62% of the 604.7 keV line it outranks.
This is a shield that has to be designed: 5.85 mm of lead for a factor of two and 20.2 mm for a factor of ten, or 12.3 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 20.5 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 2.0652 years (6.517e+7 s) |
| Decay mode | beta-minus decay |
| Specific activity | 4.78e+13 Bq/g (1.29e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.207 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.207 mGy/h |
| Dose rate, 1 Ci at 1 m | 7.67 mGy/h |
| Kerma-weighted mean photon energy | 719.9 keV |
17 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 4.97 keV and 0.105% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 795.86 | 85.46 | 43.40 |
| 604.72 | 97.62 | 38.55 |
| 569.33 | 15.373 | 5.73 |
| 801.95 | 8.688 | 4.44 |
| 563.25 | 8.338 | 3.07 |
| 1365.18 | 3.017 | 2.38 |
| 1167.97 | 1.79 | 1.25 |
| 1038.61 | 0.99 | 0.63 |
| 475.37 | 1.477 | 0.46 |
| 32.19 | 0.439 | 0.04 |
| 31.82 | 0.238 | 0.02 |
| 36.83 | 0.1618 | 0.01 |
| 36.48 | 0.1292 | 0.01 |
| 242.74 | 0.027 | 0.00 |
Showing the 14 largest contributors of 17 lines above the cutoff; the remainder together carry 0.01% of the dose rate.
5.85 mm of lead halves this spectrum
Solved numerically across all 17 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 5.85 | 20.2 | 3.44 |
| tungsten | 3.86 | 13.2 | 3.42 |
| iron | 12.3 | 41.3 | 3.35 |
| copper | 11.0 | 36.8 | 3.35 |
| concrete | 39.4 | 132 | 3.35 |
| water | 83.2 | 278 | 3.34 |
| aluminum | 35.4 | 118 | 3.35 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over decades and centuries
Ten half-lives is 20.7 years, which puts decay storage out of reach: 1.5e-4% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 2.98 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 | 6.86 years |
| Time to fall to 1 % | 13.7 years |
| Time to fall to 0.1 % | 20.6 years |
Limits of these dose rates
- 0.207 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 5.85 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.105% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Cs-134
Other Cesium nuclides: Cs-134m, 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.