Uranium-235

Uranium-235 (U-235) is slow enough to be handled as a material rather than as a trace: 7.99e+4 Bq/g, or 0.00000216 Ci/g, puts a gigabecquerel at 12.5 kg. Decay is by alpha decay, half-life 7.04 × 10⁸ years, and shedding even one per cent of the activity takes 10.2 million years.

The air kerma rate constant recorded for U-235 is 0.0194 mGy·m²/(GBq·h). 1 GBq at 1 m reads 0.0194 mGy/h, and 1 Ci at the same distance 0.719 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-235 it is not.

58 lines clear the 20 keV cutoff, but 6 of them carry 90% of the dose rate. The leading one is 185.7 keV at 65.8% of the total — its emission probability is 57.2%, which is also the highest.

For U-235's own photons, 0.434 mm of lead halves the air kerma rate, 5.07 mm if the material is steel, and 1.64 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.

These photon figures are U-235's own. Its decay product Th-231 is not in this dataset, and neither is what follows it, so nothing below U-235 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 4.396 MeV at 57.79%, one of 12 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.