Detector Development

No matter if protons or lead ions are collided, to explore the physics behind such collisions and test theories like Quantum Chromo Dynamics (QCD) or the evolution and properties of the so-called Quark-Gluon Plasma (see Data Analysis and Phenomenology sections), a particle detector is required. Its purpose is to track and identify (charged) particles, which are produced and emerge from the collision point. Often many detectors are composed to make up larger detector systems like the ALICE detector to get the best combined performance.

Naturally, an ideal detector provides unlimited precision of the variables of interest. In reality the measured particle needs to interact with the detector material to produce a signal that then can be read out. Therefore, the detector itself has an impact on the measurement and thus poses limits to the ideal case. However, to get as close to the ideal as possible Research and Development (R&D) of new detectors and technologies never stops. If either existing detectors are being upgraded or new ones being built, there is always work to do.

ALICE TPC Upgrade and GEM Studies

In the past, our group played a major role in the upgrade of the ALICE Time Projection Chamber (TPC). There the readout chambers were changed from Multi-Wire Proportional Chambers (MWPC) to Gas Electron Multiplier (GEM) based chambers. The group was mainly involved in the effort of testing the GEMs for quality assurance and actually building and testing the ready chambers. Furthermore, some fundamental research about the nature and origin of discharges happening in GEM detectors has been carried out.

Curved Silicon Sensors

From late 2019 on, the group joined the effort of research on curved silicon detectors, which are based on CMOS technology. As Monolithic Active Pixel Sensors (MAPS) signal-processing electronic circuits are integrated in the chip and more precisely in every pixel. This allows tasks like signal amplification and discrimination to be directly performed inside the pixel.

For this studies our group works with the so-called ALPIDE chip, which has been developed for the upgrade of the ALICE Inner Tracking System (ITS). It features 1024x512 pixels and has a position resolution of better than 5 um. After having learned to operate the chip and to perform necessary electrical tests of the chip, members of the group in close contact to other ALICE groups, especially at CERN, now joined the research on bent chips for future applications in e.g. another upgrade of the ITS or a follow-up experiment for ALICE.

Laboratory Measurements

In order to tune the chip and find out the optimal working point, laboratory tests need to be done. An important example is the "threshold scan". As ALPIDE is a fully digital chip that only outputs if a pixel was hit or not, but nothing about the amplitude of the detected signal, a threshold needs to be set for each pixel. If the amplitude of the signal produced by the charged particle to be detected exceeds this threshold the pixel counts as "hit". There are parameters to set this threshold. To get back the corresponding threshold for a certain set of parameters a threshold scan is performed. Additional electrical tests are also available.

Beam Tests

In order to investigate the performance of the sensor, i.e. detection efficiency, position resolution, etc., a beam test is performed. In such a campaign the Device Under Test (DUT) is subject to a particle beam at an accelerator facility like GSI, CERN or DESY. In comparison to reference detectors (beam telescope), which are "normal", flat ALPIDE chips in our case, the quantities of interest can be extracted from the data taken via a thorough analysis.

Our group is involved in performing test beams and the according data analysis process. For that reason tasks like setting up and testing a trigger system and learning how to set up and operate a full test beam setup were successfully completed. Nevertheless, there are always new tasks and issues to be looked into.

Construction of a Telescope

Our group has designed and built a cheap, modular and highly-flexible beam telescope that can accommodate a large number of sensors. The whole concept is based on a rail system, allowing quick and easy adjustments on all 3 major axes of translation for each individual sensor plane, for a group of planes (upstream/downstream reference arms), or for the full telescope. The default precision is ~1mm in each direction, which can be considerably improved by usage of micrometer screws.
The design also allows rotations around the vertical position for each plane, to study the impact of inclined beam tracks with respect to the sensor. The design of the rotational plate is basic, but robust, all while being extremely advantageous from a price perspective. Moreover, the new design allows easy inclusion of various other detectors (eg: ITS2 Outer Barrel modules) or targets at any point along the beam direction.
With the new system, the transportation of the equipment becomes easy, the build-up times of the whole setup are drastically reduced as everything is modular and the sensor alignment can now be done in a matter of minutes.

Tools for Data Acquisition and Analysis

There is a lot of software involved in the research on detectors, which is predominately based on C++ and Python. For data taking there is the EUDAQ framework, which also allows for an online monitor to have a first look on the recorded data. For the analysis of test beam data the Corryvreckan framework is used. Both frameworks involve CERN's data analysis framework ROOT.

Operating, understanding and eventually extending the functionality of software is a central part of the work with detectors.

Contact Persons in the Group

Bogdan Blidaru (PhD student)
Pascal Becht (PhD student)
Bent Buttwil (Master student)
Maurice Donner (Master student)
Simon Groß-Bölting (Master student)
Johannes Hensler (HiWi)
Fabian Königstein (HiWi)