When germanium is illuminated by photons whose energy exceeds 11.2 keV, the crystal fluoresces, emitting a photon whose energy is 9.9 keV. Since the energy resolution of the BCS detectors is around 20%, the tail of the distribution of these fluorescent photons falls within the energy range that the BCS is sensitive to and indeed part of the tail lies under the distribution of the photons detected by channels 1 and 2. In order to try to reject the fluorescence, the upper level discriminator is set midway between the two energy distributions. germanium fluorescence is less of a problem in channels 3 and 4 since there is a greater difference in the energy between the channel photons and the fluorescent photons.