Characterization of the CMS Pixel Detectors

In 2005 the Large Hadron Collider (LHC) will start the pp collisions at a high luminosity and at a center of mass energy of 14 TeV. The primary goal of the experimental programme is the search of the Higgs boson(s) and the supersymmetric particles. The programme is also proposed to detect a range of diverse signatures in order to provide guidance for future physics. The pixel detector system makes up the innermost part of the CMS experiment, which is one of the two general purpose detectors at the LHC. The main tasks of the system are vertex detection and flavor tagging. The high luminosity and the high particle multiplicity as well as the small bunch spacing at the LHC impose great challenges on the pixel detectors: radiation hardness of sensors and electronics, fast signal processing and a high granularity are the essential requirements. This thesis concentrates on the study of the suitability of two test stands, which are implemented to characterize the CMS pixel detectors: one is con-cerned with test pulses, while the other with laser pulses. The benefits of a laser system are the provision of a flexible test stand, an easy trigger system, high statistic and completely controllable source for the laboratory use. A brief description of the LHC physics and an overview of the CMS experiment are given in Chapter two. The following chapter is primarily concerned with the layout of the CMS pixel system and the design of the pixel detectors, together with a brief introduction of the bump bonding process. Chapter four describes the pixel readout chip - the AC30 chip fabricated in the RICMOS IV-Technology. In this chapter the readout architecture, the operation procedures and the method of data processing of the chip are discussed. In order to understand the charge injection with laser pulses light absorption in semi-conductor detectors is treated in Chapter five. Since Si is chosen as the baseline material of the CMS pixel detectors, whereas GaAs was the candidate material, absorption mechanisms of both materials are outlined and the results of the absorption measurements in Si and GaAs wafers are presented. The results of the measurements of a GaAs Schottky diode with a 1084 nm pulsed laser diode and a GaAs strip detector with a 1064 nm pulsed Nd:YAG laser are also given in this chapter. Chapter six defines and discusses the performance parameters, which are applied to characterize the CMS pixel detectors: I-V characteristics, full depletion voltage, use of the trim bits, threshold distribution, noise, pulse form, peaking time, time walk, linearity, gain factors and spatial resolution. In this chapter two test stands are studied in detail: one employs testpulses, while the other is a laser test stand equipped with a 670 nm laser diode and a 1064 nm Nd:YAG laser.The results of the measurements of the performance parameters are presented and discussed. The suitability of the test stands is then evaluated at the end of the thesis.