Uranium-238m
U-238m · Uranium, Z = 92, A = 238 · isomeric state m
Uranium-238m (U-238m) carries 6.26e+27 Bq/g, or 1.69e+17 Ci/g: a gigabecquerel is 0.160 ag, no weighable quantity at all. It decays by isomeric transition with a half-life of 280 ns, which leaves 0% of today's activity after a day and 0% after a week.
Among the strong external emitters here: Γ of 0.229 mGy·m²/(GBq·h) ranks 7 of 96, 3.0× Cs-137 and 1.3× less than Co-60. 1 GBq at 1 m reads 0.229 mGy/h, and 1 Ci at the same distance 8.48 mGy/h — a metre-scale hazard at gigabecquerel activities, and 87.3 MBq is already 20 µSv/h at that distance.
7 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 2512.7 keV at 71.2% of the total — its emission probability is 65%, which is also the highest.
This is a shield that has to be designed: 13.6 mm of lead for a factor of two and 45.2 mm for a factor of ten, or 21.8 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 47.9 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 280 ns (2.800e-7 s) |
| Decay mode | isomeric transition |
| Specific activity | 6.26e+27 Bq/g (1.69e+17 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.229 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.229 mGy/h |
| Dose rate, 1 Ci at 1 m | 8.48 mGy/h |
| Kerma-weighted mean photon energy | 2330 keV |
2513 keV leads, and 2 lines make 90%
1 further line below the 20 keV cutoff, the highest at 16.68 keV and 0.0682% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 2512.70 | 65 | 71.24 |
| 1879.00 | 32 | 28.75 |
| 98.44 | 0.06468 | 0.00 |
| 94.66 | 0.0404 | 0.00 |
| 112.64 | 0.03131 | 0.00 |
| 111.24 | 0.02333 | 0.00 |
| 114.45 | 0.00798 | 0.00 |
13.6 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 | 13.6 | 45.2 | 3.33 |
| tungsten | 8.31 | 27.6 | 3.33 |
| iron | 21.8 | 72.7 | 3.33 |
| copper | 19.4 | 64.6 | 3.33 |
| concrete | 71.2 | 237 | 3.33 |
| water | 152 | 506 | 3.34 |
| aluminum | 63.7 | 212 | 3.33 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 0.00000280 seconds, 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 4.04e-7 seconds.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.12 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 9.30e-7 seconds |
| Time to fall to 1 % | 0.00000186 seconds |
| Time to fall to 0.1 % | 0.00000279 seconds |
Limits of these dose rates
- 0.229 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 13.6 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.0682% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for U-238m
Other Uranium nuclides: U-232, U-233, U-234, U-235, U-236, U-238
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.