The Pixel Pattern

The Pad Chamber readout is based on the induced charge on the cathode planes, which are segmented into pads. To reduce the number of readout channels, the pads consist of 9 copper rectangles called pixels, arranged in an interleaved pattern, as illustrated in fig. 2.3.

Figure 2.3: A pad consists of 9 pixels (left). The pixel layout showing three fired pads combined to form one fired cell (right). The sizes of the center and side pixels are chosen so that they collect an equal amount of charge. The center pixel is closer to the avalanche and therefore somewhat smaller. (Values are for PC1.)

A cell consists of one center pixel and two side pixels from the adjacent pads. A valid hit is formed when all three pads corresponding to these pixels fire, i.e. when the collected charge of each pad is above a set threshold.