Curium-244

Curium-244 decays by alpha decay, half-life 18.11 years. Specific activity is 2.99e+12 Bq/g (80.9 Ci/g), so a gigabecquerel comes to 334 µg — weighable, but on an analytical balance. Over a year the activity falls to 96.24%, and over forty years to 21.63%.

The air kerma rate constant is small — 0.00000336 mGy·m²/(GBq·h), 2.6e+4× less than Cs-137 and 1.0e+5× less than Co-60, ranking 94 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 0.00000336 mGy/h, and 1 Ci at the same distance 0.000124 mGy/h. It takes 5.95 TBq at a metre to reach 20 µSv/h from the photons alone.

3 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 42.8 keV at 90.1% of the total — its emission probability is 0.02635%, which is also the highest.

Shielding barely arises: 0.0558 mm of lead halves the air kerma rate and 0.207 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.335 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 3.7 times the half-value layer, not the 3.32 a single energy would give.

Alpha emission is led by 5.805 MeV at 76.9%, 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.