Uranium-238

Uranium-238 (U-238) is slow enough to be handled as a material rather than as a trace: 1.24e+4 Bq/g, or 3.36e-7 Ci/g, puts a gigabecquerel at 80.4 kg. Decay is by alpha decay, half-life 4.468 × 10⁹ years, and shedding even one per cent of the activity takes 64.8 million years.

The air kerma rate constant recorded for U-238 is 0.00000747 mGy·m²/(GBq·h). 1 GBq at 1 m reads 0.00000747 mGy/h, and 1 Ci at the same distance 0.000276 mGy/h — for that emission alone. No ranking against the other emitters here is drawn: for most of them the constant is the whole field, and for U-238 it is not.

3 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 49.5 keV at 81.6% of the total — its emission probability is 0.064%, which is also the highest.

For U-238's own photons, 0.0828 mm of lead halves the air kerma rate, 0.555 mm if the material is steel, and 0.291 mm of lead takes it to a tenth. What thickness the source in front of you needs is a question about its chain, not about this line list. The tenth-value layer runs 3.5 times the half-value layer, not the 3.32 a single energy would give.

These photon figures are U-238's own. Its decay product Th-234 is not in this dataset, and neither is what follows it, so nothing below U-238 is counted here. A source left sealed long enough for the chain to build up reads higher than this — how much higher is outside what this page can compute.

Alpha emission is led by 4.198 MeV at 79%, one of 3 recorded lines. None of it reaches through skin, so the alpha is an intake hazard rather than an external one — the photon figures above are the separate question.