Lead-210

Lead-210 decays by beta-minus decay, half-life 22.20 years. Specific activity is 2.84e+12 Bq/g (76.7 Ci/g), so a gigabecquerel comes to 352 µg — weighable, but on an analytical balance. Over a year the activity falls to 96.93%, and over forty years to 28.68%.

The air kerma rate constant is small — 0.000438 mGy·m²/(GBq·h), 1.8e+2× less than Cs-137 and 7.0e+2× less than Co-60, ranking 82 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 0.000438 mGy/h, and 1 Ci at the same distance 0.0162 mGy/h. It takes 45.6 GBq at a metre to reach 20 µSv/h from the photons alone.

One line at 46.5 keV carries all of that.

Shielding barely arises: 0.0630 mm of lead halves the air kerma rate and 0.209 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.369 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.

The beta endpoint is 0.0635 MeV, mean 0.00608 MeV over 2 branches. That endpoint stops in 0.0517 mm of acrylic or 0.0244 mm of glass. Of Pb-210's own beta energy, 0.01% turns into X-rays in acrylic and 0.18% in lead.

Pb-210 does not stand alone. Bi-210 follows it with a half-life of 5.012 days, so ingrowth is complete within 35.1 days of separation. Its endpoint is 1.16 MeV and stops in 4.21 mm of acrylic, 81× the thickness above. The shield is sized on Bi-210, and so is the bremsstrahlung: 0.24% of that energy turns into X-rays in acrylic and 3.33% in lead.