Design and performance studies of the Micro-Vertex-Detector for the CBM experiment at FAIR

The CBM experiment is a fixed target experiment to be installed at the future accelerator facility at GSI/FAIR. It will investigate the properties of nuclear matter at extreme conditions and its underlying strong interaction. The research of the CBM experiment, which focuses on the regime of highest net-baryon densities and moderate temperatures, is complementary to this of the experiments at RHIC/BNL (STAR) and LHC/CERN (ALICE), which mainly focuses on the regime of high energy and zero net-baryon densities. The corresponding conditions in the CBM experiment can be produced in heavy-ion collisions at beam energies between 10 and 40 AGeV . Heavy particles, as e.g. charm carrying particles, could be sensitive to the properties of the medium in the early phase of the collision. However due to the short lifetime of open charm particles, they can only be reconstructed via their decay products and the corresponding track topology. Consequently in order to reconstruct the decay vertex with a high accuracy an ultrathin detector system with excellent spatial resolution is required. Due to the high particle fluxes expected, additional requirements to the rate capabilities as well as to the radiation hardness are set. For the precise vertexing a microvertex detector is envisaged, which has to be located directly behind the target and has to operate in the vacuum. Monolithic Active Pixel Sensors (MAPS) are the most promising candidates for the underlying sensor technology for the MVD of the CBM experiment. As currently no suited detector is available, substantial research and development is needed in order to meet these requirements. In the context of this thesis first attempts haven been initiated in order to integrate mechanically MAPS sensors into an ultra-thin detector dedicated to the CBM experiment. The mechanical integration necessarily needs to contain the MAPS sensors, electrical services and a support structure to cool and mount the sensors. As, apart from the intrinsic properties of the sensor, the support structures contribute notably to the specific functions and properties of the detector, particular care has to be taken during its development. Its implementation is not meant to push already the limits, rather it is meant to provide an efficient and secure integration solution for MAPS sensors to a detector. In order to not run a risk of failures only state-of the art technologies were used. Thus the ultimate goal, regarding the demands of the CBM experiment, could not be achieved. Based on MAPS MIMOSA-20 sensors the concept was developed and implemented. MIMOSA- 20 was a prototype sensor available at that time. Although this particular sensors do not meet the I Abstract requirements of the CBM experiment, they could be used for this study. Due to their lightweight construction, flexprint cables have been adapted to the use as electrical interconnection. Flexprint cables comprises thin conducting traces applied on thin kapton foils. For the demonstrator a standard flexprint cable with copper traces was chosen, focusing on electrical properties rather than on the material budget. In this respect only moderate demands on the material budget were made, in order to avoid any risk, related to malfunctions. Since for small areas cooling pipes contribute stronger to the mean material budget, no cooling pipes inside the acceptance were included. Instead highly conducting materials for the support structure were provided, in order to the evacuate the heat to an actively cooled heat sink. In this context synthetic high performance carbon materials turned out to be most suitable. Apart from their high heat conductivity, they essentially provide good rigidity at low material budget. The performance and the dimensioning of the support structure have been evaluated and optimized in consideration of the material budget by means of Finite Element Method (FEM) simulations and analytic calculations. As determining factors both the requirements of the CBM experiment and the requirements of the sensors have been accounted for. As the requirements on a temperature profile of MIMOSA-20 were not known initially, different determining analysis have been conducted. The MVD demonstrator was evaluated and validated by test runs performed in the laboratory and in a beam time at CERN SPS. Thereby the demonstrator setup was characterized extensively and the interplay of the components was studied. By means of the measurements at the beam time the positional stability could be evaluated very accurately. In the course of the systematic evaluation of the measured demonstrator data an extensive ROOT based analysis software was developed. Further on the strength of the systematic evaluation, evaluation approaches and optimization tools in terms of the operation under experimental conditions were concluded. Throughout the development comparison to similar projects were drawn, which could provide suggestions to be considered for the MVD demonstrator