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14-02-11

Studies for the Silicon Tracking System of the International Large Detector at the International Linear Collider

Autoren: Stephan Hänsel
Gruppe:
Kategorie: PhD-Thesis

The International Large Detector (ILD) is one of the proposed detector concepts for the future International Linear Collider (ILC), which will extend and complement the physics program of the Large Hadron Collider (LHC). One of the perceived priorities of an ILC detector is the reconstruction of charged particle momenta with a precision of sigma(delta pt/pt^2) = 2*10^(-5) (GeV/c)^(-1), required for the use of the particle flow concept. Only with the particle flow approach the needs on jet-energy resolution can be satisfied. The ILD meets this requirement with a central tracking system consisting of a TPC combined with silicon strip detectors. In the barrel region, the silicon tracking system is composed of three double layers of silicon strip detectors. Two of these layers, located between vertex detector and TPC, form the Silicon Internal Tracker (SIT) and the third layer surrounds the TPC, the Silicon External Tracker (SET). In the forward region the silicon tracking system is completed with the End-cap Tracking Detector (ETD), two times three layers of silicon strip detectors just outside the TPC End Plates, and the Forward Tracking Detector (FTD), two times seven discs of silicon detectors, covering the very forward region.

 

After a short description of the International Linear Collider and its possible physics program, the International Large Detector, its tracking system and especially silicon strip sensors are examined in more detail. Based on simulations, presented in this thesis, it could be verified that the high demands on the resolution of charged particle momenta can only be satisfied with the inclusion of precise measured space points just outside the TPC volume. These simulations led to the inclusion of the Silicon External Tracker into the ILD baseline design. It was understood, that the resolution of the SET along the TPC must be in the order of 50µm and that its resolution in r-phi must be below 10µm.

 

At the HEPHY two different silicon strip sensors, a multi-geometry and a big area sensor, were designed with the purpose to provide a deeper insight into the definition of the ILD tracking system. These two sensors could be implemented on one silicon wafer which was produced by Hamamatsu Photonics, Japan.

 

Each of the multi-geometry sensors contains 256 readout strips arranged in 16 zones with different strip geometries and a readout pitch of 50µm, which is the lower limit for reliable mass production of silicon strip detectors, as needed for large scale detector systems. These sensors were designed to determine the optimal geometry in terms of spatial resolution, taking the different charge collection efficiencies and signal to noise ratios into account. After the quality of these sensors was verified at the institute, they were connected to front end electronics, designed by the electronic group of the HEPHY Vienna, and integrated into detector modules. After the functionality of the modules was tested in Vienna, they were used in a test beam at the Super Proton Synchrotron (SPS) at CERN. About 500k events were recorded with a 120GeV/c pion beam, providing enough statistics to reliably determine the spatial resolution of all different geometries. The achieved results are presented and were included into the simulations of the ILD tracking system.

 

The big area micro-strip sensors have a size of 91.5mm² and a readout pitch of 50µm. After their quality was verified at the HEPHY, those sensors were used to build detector modules which could be integrated into a test beam experiment at DESY, Hamburg, where the Large TPC Prototype (LP), a first large scale prototype for the ILD TPC, is installed in a superconducting magnet. This test beam campaign, dedicated to deliver data with different TPC readout systems in a time period of four years, is an important step towards the optimization of the ILD tracking system. At first, the complete silicon system, including the data acquisition system used for the test beam, was verified during a stand-alone test with cosmic muons in IEKP, Karlsruhe. Afterwards, in November 2009, the system was installed into the setup at DESY. In a first combined test beam of the LP and the silicon detectors without magnetic field more than 80k events were recorded with a 5.6GeV/c electron beam. The results of this first experiment and a simulation study for the operation with magnetic field show, that all systems work as expected and that useful insights can be gained with the LP setup.

 

Based on the knowledge gained in the test beams and the simulations an optimal design for the Silicon External Tracker is developed. For cost reasons single sided silicon strip sensors, optimized for the needed resolutions, will be arranged in a double layer with orthogonal readout strips to provide the optimal spatial resolution for both measured coordinates. Next to the spatial resolution the minimization of the material budget of the silicon tracker is most important, because of multiple scattering and the production of unwanted secondary particles. Different possibilities to achieve this goal are discussed, like the thinning of the sensors and the readout chips and the inclusion of on-sensor pitch adapters. It should also be possible to redundantise the need for cooling pipes, a major contributor to the material budget in former silicon tracking systems, with the development of low power front end electronics making forced air-cooling sufficient.