Strontium-89

At 1.07e+15 Bq/g — 2.90e+4 Ci/g — a gigabecquerel of Sr-89 amounts to 930 ng, which is why activity rather than mass is how anyone states the quantity. Strontium-89 decays by beta-minus decay with a half-life of 50.563 days, falling to 66.28% of today's activity in a month and 0.67% in a year.

The air kerma rate constant is small — 0.0000113 mGy·m²/(GBq·h), 7.0e+3× less than Cs-137 and 2.8e+4× less than Co-60, ranking 89 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 0.0000113 mGy/h, and 1 Ci at the same distance 0.000417 mGy/h. It takes 1.77 TBq at a metre to reach 20 µSv/h from the photons alone.

One line at 909.0 keV carries all of that.

This is a shield that has to be designed: 7.82 mm of lead for a factor of two and 26.0 mm for a factor of ten, or 14.0 mm of steel to halve it, at which point the mass of the shield is part of the problem. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.

The beta endpoint is 1.50 MeV, mean 0.587 MeV over 2 branches. That endpoint stops in 5.75 mm of acrylic or 2.71 mm of glass. Of the beta energy, 0.32% turns into X-rays in acrylic and 4.31% in lead.