Plutonium-240

A gigabecquerel of Pu-240 is 119 mg of material, which follows from a specific activity of 8.40e+9 Bq/g (0.227 Ci/g). Plutonium-240 decays by alpha decay with a half-life of 6561 years; forty years leaves 99.58% of today's activity and ten thousand years leaves 34.77%.

The air kerma rate constant is small — 0.00000542 mGy·m²/(GBq·h), 1.5e+4× less than Cs-137 and 6.1e+4× less than Co-60, ranking 92 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 0.00000542 mGy/h, and 1 Ci at the same distance 0.000201 mGy/h. It takes 3.69 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 45.2 keV at 85.2% of the total — its emission probability is 0.0432%, which is also the highest.

Shielding barely arises: 0.0644 mm of lead halves the air kerma rate and 0.226 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.413 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 5.168 MeV at 72.76%, 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.