Radium-226

A gigabecquerel of Ra-226 is 27.3 mg of material, which follows from a specific activity of 3.66e+10 Bq/g (0.989 Ci/g). Radium-226 decays by alpha decay with a half-life of 1600 years; forty years leaves 98.28% of today's activity and ten thousand years leaves 1.31%.

The air kerma rate constant recorded for Ra-226 is 0.000879 mGy·m²/(GBq·h). 1 GBq at 1 m reads 0.000879 mGy/h, and 1 Ci at the same distance 0.0325 mGy/h — for that emission alone. No ranking against the other emitters here is drawn: for most of them the constant is the whole field, and for Ra-226 it is not.

7 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 186.2 keV at 91.1% of the total — its emission probability is 3.565%, which is also the highest.

For Ra-226's own photons, 0.473 mm of lead halves the air kerma rate, 5.07 mm if the material is steel, and 1.65 mm of lead takes it to a tenth. What thickness the source in front of you needs is a question about its chain, not about this line list.

These photon figures are Ra-226's own. What it decays into is not in this dataset, so nothing below Ra-226 is counted here. A source left sealed long enough for the chain to build up reads higher than this — how much higher is outside what this page can compute.

Alpha emission is led by 4.784 MeV at 94.07%, one of 2 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.