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John Mallinckrodt said, "My guess is that the meter is recording the
rate of beta decay from an initially stable population of Po-218 created
by the alpha decay of Rn-222 in the air. When the replenishing Radon
source is removed, the Po-218 quickly (3 minute half life) alpha decays
to Pb-214."
This is close, but not exactly correct.
If the 222-Rn level would stay constant in a room, then an equilibrium
would establish and eventually all the daughters would be present at the
same level. It is not that one decays building up the next, and then
that decays and builds up the next, etc.
For example, suppose the 222-Rn level in the room would be 20 picocuries
per liter of air (20 pCi/L), and this level is maintained, despite the
cut