At the heart of the GAPS experiment is the tracker, which serves as a target for slow antiparticles. Once an antiparticle stops in the material of the tracker, it has a high probability of forming an exotic atom which will emit a series of X-rays with characteristic energies, as well as pions and protons from annihilation. The tracker is segmented in such a way that these processes deposit energies at different locations that allow the event to be reconstructed.
The fundamental unit of the tracker is a 4”-diameter Si(Li) detector segmented into eight strips. Four of these detectors are grouped into a module. The module provides mechanical support, furnishes power, measures the energy deposits and converts them into digital information, and connects the detectors to a cooling system that keeps them below -40°C. Such cold temperatures are necessary to resolve the characteristic X-ray energies.
In total there are 1,440 detectors arranged in 10 layers, where each layer consists of six rows of six detector modules. Seven planes are read out, while the other three planes provide additional target mass and thermal balance. Individual modules rest on antistatic polyethylene foam panels, protecting the modules without introducing too much material into the flight path of particles. To reduce the amount of material even more, the foam panels include cutouts above the detectors. Cooling pipes penetrate the stack vertically to carry heat away from the detectors. They are routed out of the tracker at the corners to the radiator. On the edge of the tracker are interface boards that supply a row of detectors with power and transfer digital signals from that row to the digital back end.