A project I recently joined is the Insertable B-Layer (IBL) for the ATLAS Pixel Detector. This is an upgrade of the ATLAS Pixel Detector. Why do we already need an upgrade for the LHC detectors when they are actually just completed?
In case of the pixel detector, made of silicon and sitting only a few centimeters from the beam pipe, one reason is the radiation damage. The particles passing through the material not only create a signal giving us the possibility to measure their tracks, they also can displace silicon atoms from their lattice position. Depending on the type of radiation, this leads to point defects or damage clusters. The more damaged the silicon gets, the more the electrical properties are changed up to the point when the detector stops operating.
This properties and the subsequent detector operation were studied in great detail during the last decades by all LHC experiments and the systems designed are the best what was on the “market” when the detector productions started. Nevertheless it was clear from the beginning that the innermost layer of the ATLAS Pixel Detector called B-Layer* cannot survive 10 years of LHC operation. Therefore a replacement of this layer surviving longer has to be prepared. The idea is, to put in an additional layer inside of the current ATLAS pixel detector to run in parallel to the other layers until the radiation damage takes its toll.
Even so the insertion of this new layer will not be before 2014, it is now time to work on the details. Sensors have to be developed which are more radiation hard. The technology advanced quite a bit in the recent years. Many properties are understood much better and this knowledge helps to design the silicon devices such a way that they survive longer. But many tests have to be done before we are sure that this is good enough. Devices will be irradiated at different facilities and then put in the test beam to see how the detector operation is affected (here our telescope comes in).
Another challenge is the mechanical integration. As already mentioned, there is a detector in place and will not be removed before the installation of this new layer. Therefore the new layer has to be squeezed in a very tight gap between the beam-pipe and the current detector, while not damaging anything. One gains more space by reducing the beam pipe diameter, meaning that the current beam pipe has to be removed without touching anything. To be honest, I am rather happy that I am not one of the mechanical engineers.
While people working on the analysis of the LHC data are waiting for LHC to be turned on, detector developers are already thinking of the future…
*Why is this layer called B-Layer if there is no A or C Layer. The naming was chosen because this innermost layer of the Pixel Detector is used for b-tagging. Quarks cannot be directly measured in a detector, they hadronize (find other quarks to make a particle) directly after they are created at the interaction point. The newly created particles are creating so-called jets. The measured jet leaving the interaction point are called primary vertex.
Some particles (b-or c-hadrons, and τ-leptons) can travel a considerable distance before decaying. The jets are only starting a little outside the interaction point. The jets created by these particles are called secondary vertices. Measuring the starting points of these vertices is called vertexing. A detector with good vertexing ability can separate primary and secondary vertices, and thus perform identification of b/c/τ-particles. Measuring the distance between the interaction point and the start of the secondary vertex is called b tagging.
