The calculation of the indoor radon concentration is complex since there are multiple error sources that must be avoided. When radon decays several radioactive daughter products are produced, most notably Polonium-218 and Polonium-214, which themselves emit alpha particles - like radon do.
These alpha particles could be mistaken for alpha particles from radon and give a false radon gas reading, but algorithms are used to eliminate these possible error sources. The main input to these algorithms is the alpha particle energy in combination with the knowledge of the energy spectrum of radon and polonium isotopes.
All our Radon detectors have a very good resolution in the energy measurement, which is required for using such algorithms.