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Inner Tracking MilestoneFrom: U. Straumann Comments
1. Status of Milestone. At the time of the technical proposal we have set ourselfes a milestone called ``fix chamber parameters'', to be met in September 2000. The idea behind was, that after this milestone a detector in full size and with all final details necessary to fit it into the LHCb experiment would be constructed and tested before writing the TDR by end of 2001. We always had foreseen to build prototypes in different technologies at this stage. The decision on the technology would then be taken on the basis of the results of this detailed prototype studies, shortly before writing the TDR. Unfortunately we have presently not yet enough information to fix all these parameters, therefore we suggest to move this milestone to January 2001. Presently we miss the following information: a) Outer dimension of the inner tracking stations, b) possible improvement of background by reducing the radiation length of the beam pipe, c) effect of thickness of the inner tracking in terms of radiation length on the physics performance of LHCb, d) effect of readout pitch on the physics performance of LHCb. All this information should evolve from ongoing simulation studies. In order not to loose time and to produce a TDR of the inner tracking in time, we decided to go ahead with the final prototype studies, using a set of parameters, which seemed reasonable to us.
2. Status of inner tracking development The inner tracking detector setup, which is presently being discussed, is quite different from the description in the technical proposal. On the basis of results from extensive R\&D on various different micropattern gas detectors as well as background simulation studies (V. Talanov, LHCb note 2000-015) with a realistic beam pipe the inner tracking group decided in February 2000 to concentrate now on triple GEM gas and silicon microstrip detectors and drop all other gas micropattern technologies, since those prooved to be unstable in high density hadronic beams. Detailed design studies for an implementation of the remaining two technologies and prototype construction are presently going on. 2.1. Triple GEM We have built a fullsize prototype of a {\bf triple GEM} detector and have measured all its properties. The results are summarized in LHCb note 2000-056 (M. Ziegler et al.). It shows, that the triple GEM is a stable and robust detector with low spark probabilities and enough safety margins in operating parameters to be used as an inner tracking technology of LHCb. However it has two significant drawbacks: First the size of the charged clusters arriving at the readout plane is rather large, a FWHM diameter of 0.5 mm is observed. Secondly tracks entering the chamber with a non vertical crossing angle produce signals on several adjacent channels, which can not easily be distinguished from signals originating from several vertical tracks, entering the detector simultanously. It seems, therefore, unlikely that the inner tracking of LHCb will be a pure triple GEM solution. However triple GEM detectors have the advantage over silicon microstrip devices, that larger sensor planes can be built without increasing the number of readout channels. Now, the outer dimensions of the inner tracking are determined by occupancy considerations of the outer tracking system. The experimental setup has changed quite significantly since the technical proposal, especially the beampipe is now more realistic, being a strong source of charged particle background (converted photons). We expect therefore different optimal boundaries between inner and outer tracking, to be determined in new {\bf simulation studies}, which are presently under way. The result of these considerations might require to increase the size of the inner tracking detectors, for which case triple GEM could be a valuable option at some places. Presently we are preparing for construction of a new full size triple GEM prototype with correct geometry for LHCb, suited for one of the tracking stations 7 to 10. It will have a different readout board, with much lower capacity (``zig-zag'' geometry), allowing to run the detector at lower gain. Readout pitch is 400 $\mu$m. 2.2. Silicon microstrip detector As explained above, the importance of the {\bf silicon solution} has increased, despite its higher costs and bigger radiation length. We have started to plan the arrangement of silicon detectors in ladders, which will be mounted vertically. The conical beam pipe of LHCb makes it necessary to have different arrangements for each of the 11 tracking stations (see note by O. Steinkamp, in preparation). The size of most stations is limited to $\pm 20$ cm vertically and $\pm 40$ cm horizontally. It seems unlikely, that strip length larger than 20 cm are realistic. An increase in the outer dimension of the inner tracking would therefore increase the number of readout channels and thus the costs significantly. We hope presently to be able to use only one sensor type, with an active size of 100 mm length and 90 mm width, which can be produced on a 6 inch wafer. These would be similar detectors, as they are used in the CMS tracking system, however our sensors will have a different pitch of 235 $\mu$m. An extended evaluation of the optimal parameters of the silicon technology has been performed, and detailed specification on material type, implant size and type, strip geometry etc. have been fixed (A. Iglesias, O. Steinkamp et al.) A few pieces of two different prototype sensors for the silicon option have been ordered from two different companies. We plan to have complete detectors available for performance measurements at particle beams at PSI (high rate) and CERN (large momentum to measure position resolution) in 2001.
2.3. Electronics We also have tested a first version of the {\bf readout chip} to be used in LHCb (Beetle from ASIC Laboratory in Heidelberg), by connecting it to a silicon microstrip detector and watch the signals from radioactive sources. Discussions on the design of detector fanout, readout boards and data link to the off detector electronics (ODE) have started (MPI Heidelberg and Uni Zurich). For the ODE a slightly simplified version of the VELO system, developped at Lausanne University will be used.
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