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Some studies with @philip_adam_hart on the sagging region (high #column ), we take the photon counts ratio between single-pixel-cluster and single & double-pixel-cluster (run29, -50V), it seems like we have less two-photon cluster in sagging area (top: Module1, bottom: Module2)
We also checked the got smoother date at -110V (run 148, primary beam energy 11kev )
I guess the sensor really doesn't work well at -50V.