The beta backscattering method can measure coatings, whose atomic numbers Z differ sufficiently from the atomic numbers of the substrate such as Au on Ni, Lacquer on Fe, Sn on Cu.
This depends on parameters such as coating thickness, surface composition, measurement time, the difference in atomic numbers of coating and substrate.
Yes, this is the case.
At first the measurement instrument is calibrated with a sample and its coating's density has 2 g/cm³. Then a new sample shall be measured with a coating's density of 1 g/cm³. Without any new calibration a systematic measuring error will then occur. This means the measuring values would always be too small. This is the case, because the instrument rates the measuring signals of the new sample in such a manner, as if the coating's density also was 2 g/cm³.