Thorium-228

Thorium-228 decays by alpha decay, half-life 1.9116 years. Specific activity is 3.03e+13 Bq/g (820 Ci/g), so a gigabecquerel comes to 33.0 µg — weighable, but on an analytical balance. Over a year the activity falls to 69.59%, and over forty years to 5.0e-5%.

The air kerma rate constant is small — 0.000227 mGy·m²/(GBq·h), 3.4e+2× less than Cs-137 and 1.3e+3× less than Co-60, ranking 84 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 0.000227 mGy/h, and 1 Ci at the same distance 0.00840 mGy/h. It takes 88.1 GBq at a metre to reach 20 µSv/h from the photons alone.

10 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 84.4 keV at 48.4% of the total — its emission probability is 1.188%, which is also the highest.

A half-value layer of 0.363 mm in lead puts this in foil and thin sheet, with 2.81 mm needed if the material is steel; ten-fold attenuation comes at 1.45 mm of lead. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 4.0 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 5.423 MeV at 73.4%, one of 5 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.