ECAL stability checks

During the NA64 weekly meeting of 8/12/2023, it was shown that background-like events with E_{ECAL}\lesssim 70 GeV and E_{HCAL}\simeq 0 are associated with specific runs and spills (e.g. 8941, spill 118 and 119), and this is probably a PMT-related effect due to intensity variations of the beam.

The plot below shows for run 8941 and spill 118 the energy of central ECAL cell vs the event number in the spill (proxy for the time):

The same for run 8941, spill 20:

It is clear that the first interval of the spill behaves differently for the two spills. In the first, a saturation effect is visible. Maybe we can try to use this data to implement some event filtering / correction?