Open Access

Adrian Rodríguez Rodríguez

• The last decades have brought tremendous progress in understanding the phase structure of the strongly interacting matter. This has been driven by studying heavy-ion collisions on the experimental side and Lattice QCD, functional approaches to QCD, perturbation theory and effective theories on the theoretical side. Of particular interest is the transition from hadrons to partonic degrees of freedom which is expected to occur at high temperatures or high baryon densities. These phases play an important role in the early universe and the core of neutron stars. Nowadays, the existence of a deconfined phase, i.e. Quark Gluon Plasma (QGP) and its phase transition at vanishing and small net-baryon densities, are well established. However, the situation at larger densities is less clear. Complementary to the studies of matter at high temperatures and low net-baryon densities performed at RHIC and LHC, the proposed Compressed Baryonic Matter (CBM) experiment at the future FAIR facility, aims to explore the QCD phase diagram at very high baryon-net densities and moderate temperatures. The CBM research program includes the search for the deconfinement phase transition, the study of chiral symmetry restoration in super dense baryonic matter, the search for the critical endpoint, and the study of the nuclear equation of state at high densities. While other experiments (STAR-BES at BNL, BM@N at NICA) are suited to measure bulk observables, CBM is explicitly designed to access rare observables, such as multi-strange hadrons, dileptons, hypernuclei and charmonium. Therefore, a key feature of CBM is the very high interaction rate, exceeding those of contemporary and proposed nuclear collision experiments by several orders of magnitude. However, some of the rare probes have a complex signature, hidden in a background of several hundreds of charged tracks. This forbids a conventional, hardware-triggered readout; instead, the experiment combines self-triggered front-end electronics, fast and free-streaming data transport, online event reconstruction and online event selection. The central detector for tracking and momentum determination of charged particles in the CBM experiment is the Silicon Tracking System (STS). It is designed to measure up to 700 charged particles in nucleus-nucleus collisions between 0.1 and 10 MHz interaction rate, to achieve a momentum resolution in 1 Tm dipole magnetic field better than 2%, and to be capable of identifying complex particle decays topologies, e.g., such with strangeness content. The STS comprises 8 tracking stations equipped with double-sided silicon microstrip sensors. Two million channels are read out with self-triggering electronics, matching the data streaming and on-line event analysis concept applied throughout the experiment. The detector’s functional building block consists of a silicon sensor, aluminum-kapton microcables and two front-end electronics boards integrated in a module. The custom-designed ASIC (STS-XYTER) implements the analog front-end, the digitizer and the generation of individual hit data for each signal. Design of the front-end chip requires finding an optimal solution for time and input charge measurements with tight constraints: small area (58 μm channel pitch), low noise levels (below 1500 ENC(e− )), low power consumption (610 mW/channel), radiation hard architecture and speed requirements. Being a part of the first processing stage in the full readout and data acquisition chain, the characterization of the chip and its integration with the detector components is a crucial task. In this work, various methods and tools are established for testing and qualifying the ASIC analog front-end. A procedure for amplitude and timing calibration is developed using different functionalities of the chip. The procedure is optimized for our prototype system in order to achieve the best accuracy in the shortest amount of time. Results were verified using a gamma source and an external pulse generator, showing discrepancies below 5%. Among the multiple operation requirements of the ASIC, the noise performance is of essential importance. The characterization of the chip noise is carried out as a function of a large number of parameters such as: low-voltage power regulators, input capacitance, shaping time, temperature and bond’s protective glue (glob-top). These studies allowed to optimize the ASIC configuration settings, to identify possible malfunctions in the low voltage powering scheme and to select possible glob-top materials to be used in the module assembly. Moreover, important differences are found among odd and even channels, which main cause was related to the bias scheme of the amplifiers of the two groups of channels. This effect has been corrected in the new version (v2.1) of the ASIC. Despite the STS front-end electronics being located outside of the physics acceptance, they will be exposed to high fluxes of charged particles. Considering the SIS100 possible running scenario, the lifetime dose at the location of the electronics is expected not to exceed 800 krad. Consequently, the STS-XYTERv2 ASIC implements a radiation hard design based on dual-interlocked cells (DICE), and triple modular redundancy (TMR). Multiple dedicated beam campaigns were carried out to evaluate the ASIC’s design in terms of immunity to single event upsets (SEU) errors and overall performance after a lifetime doses. The DICE cell SEU cross section was measured in a high-intensity proton beam. Result show a significant improvement of the SEU immunity in the STS-XYTERv2 compared to its predecessor, and allows to estimate the upset rate in the CBM running scenario, resulting in less than one SEU/ASIC/day. The studies on the total ionizing dose (TID) show that the overall noise levels for the ASIC, at the end of the experiment lifetime, are expected to increase by approximately 40 – 60%. Moreover, they demonstrated that short periods of annealing at room temperature can favorably influence the noise performance of the chip. The assembly and test of the STS modules, a complex process with multiple stages and a long learning curve, is illustrated in different parts of this work. The first prototype modules were built with the front-end board type B (FEBs-B), capable of reading out 128 channels for p and n side respectively. The studies were conducted with a relativistic proton beam of 1.7 GeV/c momentum at the COSY accelerator facility, Research Center Juelich, in March 2018. The campaign brought valuable insights to the development of an effective grounding and powering scheme for reading out the detectors. The signal-to-noise was measured for one of the prototype modules, resulting in values larger than 15 for both polarities. A deeper analysis into the collected data allowed the identification of a logic error in the ASIC that affected the readout rate and the quality of the data. This issue was corrected in the new version of the chip. A precursor of the STS detector, named mini-STS (mSTS), has been built within the mCBM project carried out in FAIR Phase0. mSTS was built from 4 fully assembled detector modules. To ensure the proper operation of the ASICs that were used in the module assembly, it was required to develop a rigorous quality assurance procedure. A dedicated setup was built based on a custom designed pogo-pin station and a total of 339 chips were tested. More than 90% of good-quality and operational ASICs were obtained. In the mCBM beam campaign of March 2019, four detector modules were successfully operated in a close-to-final readout chain and valuable data were collected. The mSTS detector was exposed to the products of Ag+Au collisions at energies above 1.58 AGeV and overall interaction rates up to 106 , which resembles the real conditions of the CBM experiment. Along this work, significant progress for the development of the STS detector modules was achieved. Techniques for characterization of the front-end electronics and the complete detector system were developed and worked out. They will be applied for QA of the components during the series production.