Cerium-144

At 1.18e+14 Bq/g — 3.18e+3 Ci/g — a gigabecquerel of Ce-144 amounts to 8.49 µg, which is why activity rather than mass is how anyone states the quantity. Cerium-144 decays by beta-minus decay with a half-life of 284.91 days, falling to 92.96% of today's activity in a month and 41.12% in a year.

The air kerma rate constant is small — 0.00334 mGy·m²/(GBq·h), 23× less than Cs-137 and 91× less than Co-60, ranking 79 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 0.00334 mGy/h, and 1 Ci at the same distance 0.124 mGy/h. It takes 5.98 GBq at a metre to reach 20 µSv/h from the photons alone.

11 lines clear the 20 keV cutoff, but 5 of them carry 90% of the dose rate. The leading one is 133.5 keV at 49.7% of the total — its emission probability is 11.09%, which is also the highest.

Shielding barely arises: 0.0859 mm of lead halves the air kerma rate and 0.552 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.731 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 6.4 times the half-value layer, not the 3.32 a single energy would give.

The beta endpoint is 0.318 MeV, mean 0.0821 MeV over 3 branches. That endpoint stops in 0.724 mm of acrylic or 0.342 mm of glass. Of Ce-144's own beta energy, 0.07% turns into X-rays in acrylic and 0.91% in lead.

Ce-144 does not stand alone. Pr-144 follows it with a half-life of 17.28 minutes, so ingrowth is complete within 2.02 hours of separation. Its endpoint is 3.00 MeV and stops in 12.6 mm of acrylic, 17× the thickness above. The shield is sized on Pr-144, and so is the bremsstrahlung: 0.63% of that energy turns into X-rays in acrylic and 8.60% in lead.