Thorium-232

Thorium-232 (Th-232) is slow enough to be handled as a material rather than as a trace: 4.07e+3 Bq/g, or 1.10e-7 Ci/g, puts a gigabecquerel at 246 kg. Decay is by alpha decay, half-life 1.40 × 10¹⁰ years, and shedding even one per cent of the activity takes 203 million years.

The air kerma rate constant is small — 0.0000262 mGy·m²/(GBq·h), 3.0e+3× less than Cs-137 and 1.2e+4× less than Co-60, ranking 86 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 0.0000262 mGy/h, and 1 Ci at the same distance 0.000970 mGy/h. It takes 763 GBq at a metre to reach 20 µSv/h from the photons alone.

5 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 63.8 keV at 85.0% of the total — its emission probability is 0.263%, which is also the highest.

Shielding barely arises: 0.152 mm of lead halves the air kerma rate and 0.523 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.997 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 3.4 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 4.012 MeV at 78.2%, 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.