The Silicon Ministrip Detector of the DELPHI Very Forward Tracker

The subject of this work is the design, test and construction of a new silicon tracking detector for the extreme forward region of the DELPHI experiment at LEP. I joined the Very Forward Tracker (VFT) Ministrip group in 1993, at a time when the upgrade of the DELPHI tracking system was proposed. My first task was to participate in the design of the ministrip detector for the VFT. This included the optimisation of the detector layout in simulations and the study of prototype detectors in the testbeam. In 1994 I became responsible for the tests and assembly' of the VFT ministrip detector at CERN. The main focus of my work was the study of the performance of a large variety of detectors in beam tests. This included the preparation of the test setup, the tests of different detectors and the analysis of the measurements. With these measurements it is possible to compare the advantages and disadvantages of various new layouts for large pitch silicon strip detectors. In particular the signal response and spatial resolution of the VFT ministrip detector was precisely measured and modelled. The results of this study form the central part of my thesis. During 1995, prior to the assembly of the VFT detector, my main task was the quality monitoring of the final VFT ministrip detectors in acceptance tests at CERN. The experience gained during these tests was subsequently used to optimise the control of the detector to assure reliable operation in DELPHI. In the following I will give a brief overview of the contents of this thesis: In chapter 1 an overview of the DELPHI detector and its components, in particular the silicon tracking detector, is presented. Chapter 2 is dedicated to the design of the DELPHI Very For- ward Tracker. The requirements for the VFT are given together with the considerations infiuencing the layout and capability of the detector. The chapter shows the complex environment in the extreme forward region of collider experiments. A new unconven- tional design with inclined detectors was necessary to optimise efficiency and acceptance area. The only way to cope with the tight space constraints in the forward region is to mount the readout electronics on top of the active detector surface. The advantages and technical problems of this solution are described. Chapter 3 presents the results of testbeam studies carried out on different large pitch strip detectors. The signal response and spatial resolution of well known and newly developed detectors was precisely measured. This study allows the comparison of many different layouts concerning their track reconstruction capability and intrinsic problems like insufficient charge measurement. The study provides useful information for the VFT ministrip layout and demonstrates the influence of layout parameters. It also provides necessary information for the design of similar detectors to be used in the future LHC (Large Hadron Collider) experiments. The tracking capability of the VFT ministrip detector is pre$ented in chapter 4. As the tracks in DELPHI will be inclined with respect to the detector surface, dedicated measurements at different track angles were carried out with the VFT ministrip detector. The measured signal response and spatial resolution could be modelled in a simulation, which proves excellent agreement with measurement data. Chapter 4 is concluded by an evaluation of the effects infiuencing the spatial resolution. In chapter 5 the production of the VFT ministrip detector is summarised. The chapter prescnts test results from the acceptance test of the full VFT ministrip detector prior to the installation in DELPHI. Extensive tests with the final configuration helped us to op- timise the operation parameters and insure reliable detector operation. Throughout the last three years I have been given the possibility to report on my work for this thesis. The considerations and results of the detector design are sumrnerisecl in Nucl.Phys.B(Proc.Supp.)44(1995)292-295, which I had the pleasure to present at the 4th Int. Conference on Aclvanced Technology and Particle Physics 1994. The predictions of the detector simulation for the final layout has been accomplished with testbeam meas- urements on VFT prototype detectors (NIM A349(1994)424-430, DELPHI internal note DELPHI 94-44 Track 78). I had also the pleasure to report the results of the testbeam analysis with different large pitch detectors to the CMS collaboration ( CMS collaboration meeting, Feb. 1996). Write-ups of the results presented in chapter 3 and 4 are currently in preparation and will be submitted for publication. I owe special thanks to Dr. M. Krammer, head of the serniconductor group of the lnstitute for High Energy Physics, for the motivating work in his group and the many hours of fruitful discussions. His attention, encouragement and knowledge was essential for this thesis. I would like to thank my thesis superviser, Prof. M. Regier, for his constant support and interest over many years. His advise and guidance was important for the analysis presentecl in this thesis. I want to express my gratefullness to Prof. W. Majerotto, director of the Institute for High Energy Physics, for financial support during this work. Furthermore I want to thank all my colleagues, in particular W. Adam, D. Rakoczy, N. Ncufelcl, V. Cindro, V. Rykalin and R. Turchetta. I also want to a.cknowlcclge the help of Prof. P. Weilhammer and Dr. W. Dulinski for their support during the test.bca.m rneasurements and the supply of many test detectors. Finally I want to thank my beloved girl-friend Bruna for all her patience and encour- agement throughout the years. I dedicate this thesis to Bruna.