In using thick-source alpha counting with a ZnS screen, the preferred procedure is to count the sample unsealed and to evaluate the Rn escape by means of a separate measurement using a gas cell. For the cell described, an equilibrium count-rate of approximately 10 per ks isobtained from one pCi of Rn. By utilizing the 10.6 h half-life of ²¹²Pb, the contributions from ²²⁰Rn and ²²²Rn can be separated. In the case of the latter the extent to which laboratory evaluation is relevant to the burial situation can be assessed by alpha spectrometric measurement of ²¹⁰Po. -- AATA