Starcheck uniformity experiment

Back in January, we mounted Starcheck on a Velmex linear stage, again, to measure the response uniformity of all the detectors in a rigorous way.  By moving the profiler 9mm per step and taking a 300MU profile each step, we tie 22 detectors directly to the central chamber.  By measuring profilers at 3mm and 6mm offset, we managed to tie remaining two groups of 22 detectors to the central chamber, in an indirect way.  Note that the starcheck central chamber has a 6mm gap between it and its neighbours, rather than the 3mm spacing for other chambers.  This was a cause of grief in analysis, as we can't simply multiply the detector number by 3 to get the chamber position.  

Looking at the data, what we see is that all three groups follow the same shape, suggesting a systematic bias.  For now, I speculate that the bias is due to beam quality modification by the wedge.  Since we already know that the Starcheck central chamber show strong energy dependence, it is not surprising that we observe beam quality effect due to beam softening by the wedge.  Note that the detector at -78mm is an obvious outlier, showing a 2% deviation from the central chamber.  Given the 1%, 2SD uncertainty quoted by the manufacturer, this is possible.  Further characterization may be needed before any conclusion about this detector can be made.  The three colors represent the three groups of detectors, offset from each group by 3mm.  The red dots represent detectors tied directly to the central chamber, while the other two colors represent detectors tied through the 3mm and 6mm measurements.