[en] Liquid scintillation samples were filled with "2"2"2Rn and the activity was measured with good precision after reaching the secular equilibrium with the progeny "2"1"8Po, "2"1"4Pb, "2"1"4Bi and "2"1"4Po. After decay of most of "2"2"2Rn activity, the samples contain "2"1"0Pb and progeny. The activities of "2"1"0Pb and progeny can be calculated as a function of time using the initial "2"2"2Rn activity. The samples were measured in a TDCR counter and the experimentally determined counting efficiencies are in accordance with previously published results. - Highlights: • LS samples were filled with "2"2"2Rn and the activity was determined by LS counting. • Activity of "2"1"0Pb can be computed from the initial "2"2"2Rn activity. • The samples were re-measured in a TDCR system after decay of the major part of "2"2"2Rn. • This method allows the preparation of "2"1"0Pb standards for the calibration of LSC counters.