We found a strange behaviour of our ADC gates for the x-ray detectors.
The setting as shown in the foto below corresponds to a delay of 5 us and a gate length of 15 us.
This is the only setting that produces nice spectra, even though the signals and gate are not aligned.
The setting is extremely sensitive to the delay 4 us still works but 3 or 7 us give already reduced energy values and beyond these values the spectra disapears.

For now it was decided to operate with this not-understood settings because the result is pretty good...
However, further investigation to understand whats happening would be desirable.

Additional note:
This effect appeared after using the new ortec SpecAmps. Before this we used old Tennelec Amps which delivered horrible resolution, but the gates and signals could be matched as expected at this time.