Open Access

Ievgeniia Momot

• High-energetic heavy-ion collisions offer the unique opportunity to produce and to study dense nuclear matter in the laboratory. The future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, will provide beams of heavy nuclei up to kinetic energies of 11 GeV/nucleon. At these energies, the nuclear matter in the collision zone of two nuclei will be compressed to densities of up to 5 − 10 times the saturation density of atomic nuclei, similar to matter densities existing in the core of massive neutron stars. Under those conditions, nucleons are expected to melt and form a new state of matter, which consists of quarks and gluons, the so called Quark-Gluon Plasma (QGP). The search for such a phase transition from hadronic to partonic matter, and the exploration of the nuclear matter equation-of-state at high densities are the major goals of heavy ion experiments worldwide. The observables, which are proposed to probe the properties of dense nuclear matter and possible phase transitions, include multi-strange hyperons, antibaryons, lepton pairs, collective flow of identified particles, fluctuations and correlations of various particles, particles containing charm quarks, and hypernuclei. These observables have to be measured in multi-dimensions, i.e. as function of collision centrality, rapidity, transverse momentum, energy, emission angle, etc., which requires extremely high statistics. Moreover, some of these particles are produced very rarely. Therefore, the Compressed Baryonic Matter (CBM) experiment at FAIR is designed to run at collision rates of up to 10 MHz, in order to perform measurements with unprecedented precision. Due to the complicated decay topology of many observables, no hardware trigger can be applied, and the data have to be analysed online in order to filter out the interesting events. This strategy requires free-streaming read-out electronics, which provides time stamps to all detector signals, a high performance computer center, and high-speed reconstruction algorithms, which provide an online track and event reconstruction based on time and position information of the detector hits (”4-D“ reconstruction). The core detector of the CBM experiment is the Silicon Tracking System (STS). The main task of the STS is to provide track reconstruction and momentum de- termination of charged particles originating from beam-target interactions. To fulfil the whole tasks the STS is located in the large gap of a superconducting dipole magnet with a bending power of 1 Tm providing momentum measurements for charged particles. The STS comprises 8 detector stations, which are positioned from 30 cm to 100 cm downstream the target. The corresponding active area of the stations grows up from 40×50 cm 2 up to 100×100 cm 2 with a totalarea of 4 m2. The silicon double-sided sensors exhibit 1024 strips on each side with a stereo angle at p-side of 7.5 ◦ and a strip pitch of 58 μm. The strip length ranges from 2 cm for sensors located in a close vicinity to the beam axis, up to 12 cm for other sensors where the flux of the reaction products drops down substantially. In total, the STS consist of 896 sensors mounted on 106 detector ladders. The detector readout electronics dissipates 40 kW and will be equipped with a CO 2 bi-phase cooling system. The detector including electronics will be mounted in a thermal enclosure to allow for sensor operation at below −5 ◦ C which minimizes radiation induced leakage currents. The task of the STS is to measure the trajectories of up to 800 charged particles per collision with an efficiency of more than 95% and a momentum resolution of 1 − 2%. In order to guarantee the required performance over the full lifetime of the CBM experiment, the detector system has to have a low material budget, a high granularity, a high signal-to-noise (SNR) ratio, and a high radiation tolerance. As a result of optimisation studies, the STS consists of double-sided silicon microstrip sensors, about 300 μm thick, which have to provide a SNR ratio of more than 10, even after radiation with the expected equivalent lifetime fluence of 10 14 1 MeV n eq cm −2. This thesis is devoted to the characterization of double-sided silicon microstrip sensors with an emphasis on investigation of their radiation hardness. Different prototypes of double sided silicon sensors produced by two vendors have been irradiated by 23 MeV protons up to the double life time fluence for the CBM experiment (2 × 10 14 1 MeV n eq cm −2 ). The sensor properties have been characterised before and after irradiation. It was found, that after irradiation with a double lifetime fluence the leakage current increased 1000 times, which results in an increased shot noise. Moreover, the relative charge collection efficiency of irradiated with respect to non-irradiated sensors drops down to 85% for the lifetime equivalent fluence, and down to 73% for the double lifetime fluence, both for the p-side and n-side. For non-irradiated sensors the SNR was found to be in the range of 20 − 25, whereas for irradiated sensors it dropped down to 12 − 17. In addition to the sensor characterization, a part of this thesis was devoted to the optimisation of the sensor readout scheme. In order to investigate the possible increase of SNR, and to reduce the number of readout channels in the outer aperture of STS, three versions of routing lines have been realized for the p-side readout of the sensor prototype, and have been tested in the laboratory and under beam conditions. The tests have been performed with different inclination angles between beam direction and sensor surface, corresponding to the polar angle acceptance of the CBM experiment, which is from 2.5 ◦ to 25 ◦. As a result of the studies carried out in this thesis work, the radiation hardness of the double-sided silicon microstrip sensors developed for the CBM STS detector was confirmed. Also the advantage of individual read-out of sensor channels in the lateral regions of the detector was verified. This allowed to start the tendering process for sensor series production in industry, an important step towards the construction of the detector in the coming years.
• Hochenergetische Schwerionenkollisionen stellen die einzige Möglichkeit dar, dichte Kernmaterie im Labor herzustellen und zu untersuchen. Die zukünftige Beschleunigereinrichtung “Facility for Antiproton and Ion Research (FAIR)” in Darmstadt, Deutschland, wird Strahlen schwerer Kerne bis zu kinetischen Energien von 11 GeV/Nukleon liefern. Bei diesen Energien wird die Kernmaterie in der Kollisionszone zweier Kerne auf Dichten bis zu 5-10 mal der Sättigungsdichte von Atomkernen komprimiert, ähnlich den im Kern massereicher Neutronensterne vorhandenen Materiedichten. Unter diesen Bedingungen wird erwartet, dass Nukleonen schmelzen und einen neuen Materiezustand bilden, der aus Quarks und Gluonen besteht, das sogenannte Quark-Gluon-Plasma (QGP). Die Suche nach einem solchen Phasenübergang von hadronischer zu partonischer Materie und die Untersuchung der Zustandsgleichung von Kernmaterie bei hohen Dichten sind die Hauptziele von Schwerionenexperimenten weltweit. Zu den Observablen, die zur Untersuchung der Eigenschaften dichter Kernmaterie und möglicher Phasenübergänge dienen können, gehören Hyperonen mit mehreren Strange-Quarks, Antibaryonen, Leptonpaare, kollektiver Fluss identifizierter Teilchen, Fluktuationen und Korrelationen verschiedener Teilchenproduktionsgrössen und -raten, ferner Teilchen, die Charm-Quarks enthalten, und Hyperkerne. Diese Observablen müssen in mehreren Dimensionen gemessen werden, d. h. als Funktion der Kollisionszentralität, der Rapidität, des Transversalimpulses, der Energie, des Emissionswinkels usw., was extrem hohe Statistiken erfordert. Darüber hinaus werden einige dieser Teilchen sehr selten produziert. Daher ist das Compressed Baryonic Matter (CBM) Experiment bei FAIR, das Gegenstand der folgenden Diskussion ist, für Kollisionsraten von bis zu 10MHz ausgelegt, um Messungen mit bisher unerreichter Genauigkeit durchzuführen. Aufgrund der komplizierten Zerfallstopologien vieler Observablen kann kein Hardware-Trigger angewendet werden, und die Daten müssen online analysiert werden, um die interessanten Ereignisse herauszufiltern. Diese Strategie erfordert eine Ausleseelektronik mit freiem Streaming, die allen Detektorsignalen Zeitstempel zuweist und an ein Hochleistungsrechenzentrum leitet. Dort wird mit schnellen Rekonstruktionsalgorithmen eine Online-Spur- und Ereignisrekonstruktion basierend auf Zeit- und Positionsinformationen der Detektordaten durchgeführt (“4-D” Rekonstruktion). Der zentrale Detektor des CBM-Experiments ist das Silicon Tracking System (STS). Die Hauptaufgabe des STS besteht darin, Spurrekonstruktion und Impulsbestimmung von geladenen Teilchen vorzunehmen, die aus Strahl-Target Wechselwirkungen stammen. Um diese Aufgaben zu erfüllen, befindet sich der STS zwischen den Polschuhen eines supraleitenden Dipolmagneten mit einem Ablenkvermögen von 1Tm. Der STS umfasst 8 Detektorstationen, die zwischen 30 cm und 100 cm hinter dem Target positioniert sind. Die entsprechende aktive Fläche der Stationen wächst von 40X50 cm2 auf 100X100 cm2 mit einer Gesamtfläche von etwa 4m2. Die doppelseitigen Siliziumsensoren weisen auf jeder Seite 1024 Streifen mit einem Stereowinkel..