Plutonium-238

A gigabecquerel of Pu-238 is 1.58 mg of material, which follows from a specific activity of 6.34e+11 Bq/g (17.1 Ci/g). Plutonium-238 decays by alpha decay with a half-life of 87.7 years; a year leaves 99.21% of today's activity and forty years leaves 72.89%.

The air kerma rate constant is small — 0.00000519 mGy·m²/(GBq·h), 1.5e+4× less than Cs-137 and 6.1e+4× less than Co-60, ranking 93 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 0.00000519 mGy/h, and 1 Ci at the same distance 0.000192 mGy/h. It takes 3.85 TBq at a metre to reach 20 µSv/h from the photons alone.

2 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 43.5 keV at 85.0% of the total — its emission probability is 0.0392%, which is also the highest.

Shielding barely arises: 0.0582 mm of lead halves the air kerma rate and 0.204 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.372 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 3.5 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 5.499 MeV at 70.91%, 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.