The pixel detector, though about the size of a shoebox, contains 65 million pixels, allowing it to track the paths of particles emerging from the collision with extreme accuracy. It is also the closest detector to the beam pipe, with cylindrical layers at 4cm, 7cm and 11cm and disks at either end, and so will be vital in reconstructing the tracks of very short-lived particles.
However, being so close to the collision means that the number of particles passing through is huge: the rate of particles received 8cm from the beam line will be around 10 million particles per square centimetre per second. The pixel detector is able to disentangle and reconstruct all the tracks they leave behind, and withstand such a pummeling over the ten-year duration of the experiment.
Each layer is spilt into segments like tiny kitchen tiles, each a little silicon sensor, 100µm by 150µm, about two hairs widths. When a charged particle passes through it gives enough energy for electrons to be ejected from the silicon atoms, creating electron-hole pairs. Each pixel uses an electric current to collect these charges on the surface as a small electric signal. A electronic silicon chip, one for each tile is attached, using an almost microscopic spot of solder using the so-called bump bonding technique, which amplifies the signal.
Knowing which pixels have been touched allows us to deduce the particle's trajectory. And because the detector is made of 2D tiles, rather than strips, and has a number of layers, we can create a three-dimensional picture.
Because there are 65 million channels, the power for each pixel must be kept to a minimum. Even with each only generating around 50 microwatts, the total power output is around the same as the energy produced by a hot plate. So as not to overheat the detector, the pixels are mounted on cooling tubes.