Preliminary Results (1999 testbeam data)

First Results on Position Resolution

Rob Kutschske has obtained results on pixel resolution from data taken on 8/16/99. In this data, the pixel plane was positioned normal to the incident beam. The data was reconstructed by Charles Newsom's "monitor" program. The PAW n-tuple was then manipulated by Rob. Rob used only tracks with hits in both x-measuring SSD planes located ~30 cm upstream of the single pixel plane (an ATLAS ST-1/FPIX0) and both the x-measuring and y-measuring SSD planes located ~30 cm downstream of the pixel plane. He further required that all three x-measuring SSD planes have two adjacent (20 micron pitch) strips on. This requirement yielded a prediction of the x position at the pixel plane with an error of ~5 microns.
Click here for a postscript file showing the predicted x-position (modulo the width of a pixel=50 microns) when a) Exactly one pixel is on, and b)when two adjacent pixels are on.
Click here for a 6-page postscript file of residual plots. Residuals are shown separately for 1-pixel and 2-pixel clusters. For 2-pixel clusters, the position is computed either simply as the position of the line between the two pixels (binary weights), or using the ADC center of gravity. As expected, binary weighting yields a much better result for these data (normal incidence). Marina's simulations indicate that for 50 micron wide pixels, binary weighting is near optimal for normal incidence. The ADC center of gravity is a good position estimator only for large angles.
See also Rob Kutschke's writeup (may be BTeV password protected) for a more complete description & updated plots.

First Results on Charge Measurement

Gabriele Chiodini has produced plots using calibration constants obtained using an x-ray source which show the pulse height recorded in the pixel detector expressed in electrons input to the FPIX0 amplifiers.
The sum of ADC's in two adjacent pixels (for the case in which two pixels are hit) fits well to a Landau distribution, and has approximately the expected peak value (we haven't yet compared the width to simulations).
The ADC histogram for 1-pixel hits is unexpectedly narrow. We haven't yet determined whether this is an artifact. It MAY be the case that the requirement that only one 50 micron wide pixel be hit selects events without delta rays, and this eliminates the Landau tail. Or maybe not!

Last Updated on 8/30/99
By David Christian