An approach for preparation of short-lived alpha sources of energy 6.0 MeV and 7.69 MeV is proposed. The sources are prepared by taking a sample of 222Rn progeny on an alpha spectrometric filter. The activities (or related parameters) of 218Po, 214Pb and 214Bi at the end of sampling are precisely determined by a reference measurement with an alpha spectrometer. Further they are used as input values to calculate with a sufficient precision the number of emitted alpha particles of any energy and at any time interval of interest. Theoretical modelling and experimental results demonstrated that such sources could be prepared with a sufficient purity. There is a potential for the number of alpha particles emitted in a given time interval to be certified with an accuracy of 1-2%.