Americium-241

A gigabecquerel of Am-241 is 7.88 mg of material, which follows from a specific activity of 1.27e+11 Bq/g (3.43 Ci/g). Americium-241 decays by alpha decay with a half-life of 432.6 years; forty years leaves 93.79% of today's activity and ten thousand years leaves 1.1e-5%.

The air kerma rate constant is small — 0.00369 mGy·m²/(GBq·h), 21× less than Cs-137 and 83× less than Co-60, ranking 78 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 0.00369 mGy/h, and 1 Ci at the same distance 0.137 mGy/h. It takes 5.41 GBq at a metre to reach 20 µSv/h from the photons alone.

14 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 59.5 keV at 81.8% of the total — its emission probability is 35.9%, which is also the highest.

Shielding barely arises: 0.0865 mm of lead halves the air kerma rate and 0.362 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.517 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 4.2 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 5.486 MeV at 84.8%, one of 7 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.