Cesium-137
Cs-137 · Cesium, Z = 55, A = 137
Cesium-137 decays by beta-minus decay, half-life 30.08 years. Specific activity is 3.21e+12 Bq/g (86.8 Ci/g), so a gigabecquerel comes to 311 µg — weighable, but on an analytical balance. Over a year the activity falls to 97.72%, and over forty years to 39.78%.
1 GBq at 1 m reads 0.0771 mGy/h, and 1 Ci at the same distance 2.85 mGy/h, from an air kerma rate constant of 0.0771 mGy·m²/(GBq·h) — 4.0× less than Co-60, and 28 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 661.7 keV at 98.1% of the total — its emission probability is 85.1%, which is also the highest.
This is a shield that has to be designed: 5.34 mm of lead for a factor of two and 18.1 mm for a factor of ten, or 11.7 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 10.6 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 30.08 years (9.492e+8 s) |
| Decay mode | beta-minus decay |
| Specific activity | 3.21e+12 Bq/g (86.8 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0771 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0771 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.85 mGy/h |
| Kerma-weighted mean photon energy | 649.8 keV |
662 keV carries 98% of the dose rate
1 further line below the 20 keV cutoff, the highest at 4.97 keV and 0.915% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 661.66 | 85.1 | 98.11 |
| 32.19 | 3.66705 | 0.88 |
| 31.82 | 1.99048 | 0.49 |
| 36.83 | 1.35045 | 0.26 |
| 36.48 | 1.07864 | 0.21 |
| 37.26 | 0.27182 | 0.05 |
5.34 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 | 5.34 | 18.1 | 3.39 |
| tungsten | 3.54 | 12.0 | 3.39 |
| iron | 11.7 | 39.5 | 3.39 |
| copper | 10.4 | 35.1 | 3.39 |
| concrete | 37.2 | 126 | 3.39 |
| water | 79.0 | 266 | 3.37 |
| aluminum | 33.5 | 113 | 3.39 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over decades and centuries
Ten half-lives is 301 years, which puts decay storage out of reach: 39.78% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 43.4 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 | 99.9 years |
| Time to fall to 1 % | 200 years |
| Time to fall to 0.1 % | 300 years |
Limits of these dose rates
- 0.0771 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 5.34 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.915% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Cs-137
Other Cesium nuclides: Cs-134, Cs-134m, Cs-135
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.