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Local position determination


The hit position in local coordinates is extracted with simple forms using projections of the pixel information; if desired, more sophisticated techniques should be easy to substitute.

In the direction perpendicular to the padrow and parallel to the padplane (i.e. ``radial''), the reconstructed hit is positioned at the center of the padrow. The two-dimensional pixel pattern gives no information regarding this direction.

The position along the padrow is determined using a 3-point Gaussian fit [3]:

where is the pad pitch, and and are the position and the sum of ADC values on the pad with the largest signal. and are sums on the neighboring pads.

In the time direction, an offset weighted mean is used.


where is the ADC sum in time bucket i, projected onto the time axis.

The STAR shaper amplifier (SAS) has response function [1]


to a TPC signal originating at time . The weighted mean of this function, as well as the convolution of it with a time-symmetric function (like Gaussian longitudinal diffusion), is offset from by

This holds even for non-zero crossing angles .

Michael A. Lisa
Tue Feb 6 15:49:35 EST 1996