A radon measurement is always based on the counting of decays within a time interval in a measuring chamber. In addition to radon, its daughter nuclides also develop and decay in the chamber. At first only a few, then more and more, and after a certain time, exactly the same number of atoms of each daughter nuclide as radon atoms decay. This process is dependent on the half-lives of the daughter nuclides and takes approximately three hours. Only in this state of radioactive equilibrium is the number of decays counted from the progeny proportional to the radon concentration. The reverse is of course the same. After all radon has left the measuring chamber, it takes three hours until all daughter nuclides in the measuring chamber have decayed.

Devices with ionization chambers or scintillation chambers (Lucas cell) cannot differentiate between the decay of radon and its progeny. Their physical response time is therefore three hours. This can only be shortened by selective measurement of the direct radon decay product Polonium-218 (polonium), which due to its short half-life is already in radioactive equilibrium with radon after about 15 minutes. This measuring principle is called α-spectroscopy.

Higher-quality devices with an ionization or scintillation chamber use mathematical algorithms to determine the actual concentration from the time course of the counting rate. After the first measurement, however, the measurement uncertainty and detection limit increase dramatically, since the decays of all daughter nuclides are carried along as an underground signal.

Some manufacturers advertise that the first measured value is displayed after a few minutes. This refers to the time interval used for the measurement and has nothing to do with the actual radon concentration.

Attention! The term α-spectroscopy is often used in a completely different sense. Therefore, always ask the manufacturer!