The ALPHA neutral trap is surrounded by a complex particle detector, called the Silicon Vertex Detector (SVD). The SVD could be described as a four megapixel 3D camera, it 'sees' inside the ALPHA -apparatus and is sensitive enough to tell us where and when a single annihilation event occurs. In ALPHA, the antihydrogen atoms annihilate mainly at the gold coated trap walls, but occasionally the annihilation can take place in the vacuum with the tiny amount of residual gas always present in the vacuum systems. The annihilating particles in the ALPHA trap are positron and antiproton. Positron, being a lepton, annihilates with its counterpart, electron, and produces two gamma rays. The annihilation of antiproton is a more complex event, but during the annihilation process several energetic charged particles called pions are emitted. The pions penetrate through the ALPHA -apparatus as well as the SVD, during which a tiny amount of energy is deposited into the three thin silicon sensor layers forming the SVD. The SVD records the locations of these interactions and, using this information, constructs the pion track (helix). As there are several of these tracks, the intersection of the tracks then gives the annihilation spatial location (vertex). In addition to the annihilation events, there is also cosmic muon background the SVD records. The fingerprint of these events, however, is very different from the annihilations and they can be effectively rejected.
An annihilation event as seen by the SVD. In this case the detector has identified four helices, the white star in the middle denotes the annihilation vertex.