Open Access

Borislav Milanovic

• The upcoming CBM Experiment at FAIR aims at exploring the region of highest net baryonic densities reproducible in energetic heavy ion collisions. Due to the very high beam intensities expected at FAIR, unprecedented data regarding rare observables such as charm quarks and hyperons will be accessible. Open charm mesons are particularly interesting, since they support the reconstruction of the total charm cross-section in order to search for exotic phenomena, e.g. a phase transition towards the quark-gluon plasma which is predicted by several theoretical models. Open charm studies will be performed via secondary vertex reconstruction with a suitable Micro-Vertex Detector (MVD). The CBM-MVD is currently in the development and prototyping phase with primary design goals concentrating on spatial resolution, radiation hardness, material budget, and readout performance. CMOS Monolithic Active Pixel Sensors (MAPS) provide an excellent spatial resolution for the MVD in the order of few um in combination with a low material budget (50 um thickness) and high radiation hardness. The active volume of the devices is formed from the epitaxial layer of standard CMOS wafers. This allows for integration of pixels together with analogue and digital data processing circuits on one single chip. This option was explored with the MIMOSA-26 prototype, which integrates functionalities like pedestal correction, correlated double sampling, discrimination and data sparsification based on zero suppression combined with a small and dense pixel matrix. The pixel array composed of 576 lines of 1152 pixels is read out in a column-parallel rolling shutter mode. One discriminator per column and the digital data processing circuits are located on the same chip in a 3 mm wide area beneath the pixel matrix allowing for binary hit encoding. This area also contains the circuits for pedestal correction and the configuration memory, which is programmed via JTAG. The preprocessed digital data is read out via two 80 Mbit/s LVDS links per sensor, which stream their data continuously based on a low-level protocol. Within the scope of this thesis, a readout concept of the CBM-MVD is proposed and studied based on the current MIMOSA sensor generation. The backbone of the system is formed by the Readout Controller boards (ROCs) featuring FPGA microchips and optical links. Several ROC prototypes are considered using the synergy with the HADES Experiment. Finally, the TRB3 board is selected as a possible candidate for the initial FAIR experiments. Furthermore, a highly scalable, hardware independent FPGA firmware is implemented in order to steer and read out multiple MIMOSA-26 sensors. The reconfigurable firmware is also designed with the support for future MIMOSA sensor generations. The free-streaming sensor data is deserialized and error-checked, prior to its transmission over a suitable network interface. In order to demonstrate the validity of the concept, a readout network similar to the HADES Data Acquisition (DAQ) system is developed. The ROC is tested on the HADES TRB2 boards and data is acquired using suitable MAPS add-on boards and the TrbNet protocol. In the context of the CBM-MVD prototype project, a readout network with 12 MIMOSA-26 sensors has been prepared for an in-beam test at the CERN SPS facility. A comprehensive control system is designed comprising customized software tools. The subsequent in-beam test is used to validate the design choices. As a result, the system could be operated synchronously and dead-time free for several days. The readout network behavior in a realistic operating environment has been carefully studied with the outcome the the TrbNet based approach handles the MVD prototype setup without any difficulties. A procedure to keep the sensors synchronous even in case of a data overflow has been pioneered as well. After the beam test, improvements and conceptual changes to the readout systems are being addressed which allow an integration into the global CBM DAQ system.