Plutonium-239

A gigabecquerel of Pu-239 is 436 mg of material, which follows from a specific activity of 2.30e+9 Bq/g (0.0620 Ci/g). Plutonium-239 decays by alpha decay with a half-life of 24110 years; forty years leaves 99.89% of today's activity and ten thousand years leaves 75.01%.

The air kerma rate constant is small — 0.00000823 mGy·m²/(GBq·h), 9.6e+3× less than Cs-137 and 3.8e+4× less than Co-60, ranking 90 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 0.00000823 mGy/h, and 1 Ci at the same distance 0.000305 mGy/h. It takes 2.43 TBq at a metre to reach 20 µSv/h from the photons alone.

11 lines clear the 20 keV cutoff, but 8 of them carry 90% of the dose rate. The leading one is 51.6 keV at 30.5% of the total — its emission probability is 0.02722%, which is also the highest.

Shielding barely arises: 0.120 mm of lead halves the air kerma rate and 2.45 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 1.07 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 20.4 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 5.157 MeV at 70.77%, one of 9 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.