Xenon-133m
Xe-133m · Xenon, Z = 54, A = 133 · isomeric state m
Xenon-133m (Xe-133m) carries 1.65e+16 Bq/g, or 4.47e+5 Ci/g: a gigabecquerel is 60.5 ng, no weighable quantity at all. It decays by isomeric transition with a half-life of 2.198 days, which leaves 11.00% of today's activity after a week and 3.4e-11% after three months.
At 0.0124 mGy·m²/(GBq·h) the air kerma rate constant is 6.2× less than Cs-137 and 25× less than Co-60, placing it 65 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0124 mGy/h, and 1 Ci at the same distance 0.461 mGy/h.
6 lines clear the 20 keV cutoff, but 5 of them carry 90% of the dose rate. The leading one is 29.8 keV at 36.0% of the total — its emission probability is 20.83169%, which is also the highest.
Shielding barely arises: 0.0324 mm of lead halves the air kerma rate and 1.08 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.175 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 33.4 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 2.198 days (1.899e+5 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.65e+16 Bq/g (4.47e+5 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0124 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0124 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.461 mGy/h |
| Kerma-weighted mean photon energy | 79.17 keV |
6 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 4.54 keV and 5.94% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 29.78 | 20.83169 | 35.97 |
| 233.22 | 10.12 | 23.90 |
| 29.46 | 11.24494 | 19.86 |
| 34.03 | 7.46423 | 10.09 |
| 33.73 | 6.03902 | 8.30 |
| 34.41 | 1.42521 | 1.89 |
0.0324 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 | 0.0324 | 1.08 | 33.43 |
| tungsten | 0.0254 | 0.805 | 31.65 |
| iron | 0.175 | 8.44 | 48.10 |
| copper | 0.116 | 7.07 | 61.06 |
| concrete | 4.60 | 31.8 | 6.92 |
| water | 23.3 | 91.5 | 3.93 |
| aluminum | 3.41 | 28.2 | 8.27 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 22.0 days — a storage problem rather than a disposal one, with 9.5e-49% of today's activity still there after a year. The mean life 1/λ is 3.17 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 | 7.30 days |
| Time to fall to 1 % | 14.6 days |
| Time to fall to 0.1 % | 21.9 days |
Limits of these dose rates
- 0.0124 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0324 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 5.94% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Xe-133m
Other Xenon nuclides: Xe-133
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.