Radium-223

At 1.90e+15 Bq/g — 5.12e+4 Ci/g — a gigabecquerel of Ra-223 amounts to 528 ng, which is why activity rather than mass is how anyone states the quantity. Radium-223 decays by alpha decay with a half-life of 11.43 days, falling to 65.41% of today's activity in a week and 0.40% in three months.

The air kerma rate constant recorded for Ra-223 is 0.0171 mGy·m²/(GBq·h). 1 GBq at 1 m reads 0.0171 mGy/h, and 1 Ci at the same distance 0.631 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-223 it is not.

57 lines clear the 20 keV cutoff, but 10 of them carry 90% of the dose rate. The leading one is 269.5 keV at 27.2% of the total — its emission probability is 13.3042%, which is lower than the 25.4% of the 83.8 keV line it outranks.

For Ra-223's own photons, 0.541 mm of lead halves the air kerma rate, 4.15 mm if the material is steel, and 3.84 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. The tenth-value layer runs 7.1 times the half-value layer, not the 3.32 a single energy would give.

These photon figures are Ra-223's own. What it decays into is not in this dataset, so nothing below Ra-223 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 5.716 MeV at 51.2%, one of 12 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.