Uranium-234

Uranium-234 (U-234) is slow enough to be handled as a material rather than as a trace: 2.30e+8 Bq/g, or 0.00622 Ci/g, puts a gigabecquerel at 4.34 g. Decay is by alpha decay, half-life 2.455 × 10⁵ years, and shedding even one per cent of the activity takes 3,560 years.

The air kerma rate constant is small — 0.0000167 mGy·m²/(GBq·h), 4.5e+3× less than Cs-137 and 1.8e+4× less than Co-60, ranking 88 of 96 by Γ — near the bottom of the photon emitters, but above the cutoff, which 51 nuclides in this dataset are not. 1 GBq at 1 m reads 0.0000167 mGy/h, and 1 Ci at the same distance 0.000619 mGy/h. It takes 1.20 TBq at a metre to reach 20 µSv/h from the photons alone.

6 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 53.2 keV at 66.5% of the total — its emission probability is 0.123%, which is also the highest.

Shielding barely arises: 0.107 mm of lead halves the air kerma rate and 0.382 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.831 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 3.6 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 4.775 MeV at 71.38%, 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.