Charge Collection Studies with Pixel Sensor Prototypes for ALICE ITS3

Abstract

During the upcoming LHC Long Shutdown 3 the ALICE experiment will upgrade its 3 Inner Tracking System (ITS2) layers to ITS3. The final design of ITS3 will have silicon sensors of up to 26 cm × 10 cm, thinned to 50 μm and bent into a cylindrical shape. The sensor prototypes developed are MOnolithic Stitched Sensors (MOSS)
that are made of 10 Repeated Sensor Units (RSU). A babyMOSS sensor, consisting of a single-RSU, is manufactured to facilitate in-beam sensor characterization. Each babyMOSS has a top and bottom half-unit (HU). The difference between these half units is the pitch of the pixels, with the top HU having a pixel pitch of 22.5 μm and the bottom HU having a pixel pitch of 18 μm. The design of the pixel features a low-doped, deep n-type implant in the pixel to deplete the pixel over its full area. Implants of neighboring pixels are separated by a gap, which enhances charge collection near pixel edges. For a subset of sensors, and only for pixels with a 22.5 μm pitch, the pixel-to-pixel gap width was increased from the baseline of 2.5 μm to 5 μm.
This thesis provides a Time over Threshold analysis from a test beam with a babyMOSS telescope performed at the CERN PS and from a Cosmics Telescope constructed with babyMOSS sensors at Lund University. For the test beam, two babyMOSS sensors representing different pixel variants were tested with negatively charged pions of 7 GeV/c. A cooling plate was provided to keep the babyMOSS devices under test at a constant temperature of 27 ◦C. Time over Threshold (ToT) data were collected in the test beam, along with source and pulsing data separately for a complete ToT analysis. Afterwards, the Lund Cosmics Telescope was designed and constructed with 5 babyMOSS planes, 4 tracking planes and 1 Device Under Test, with a looped trigger sequence. Time over Threshold data was collected with and without cosmics along pulsing data for a simplified ToT analysis.

Popular Abstract

The Large Hadron Collider (LHC) is currently the biggest and most powerful particle accelerator in the world. The LHC is made to collide two high-energy particle beams, usually consisting of protons, that travel close to the speed of light. This accelerator is used by four main experiments to study a wide variety of particle physics, including studies done by A Large Ion Collider Experiment (ALICE). A portion of this experiment uses the LHC to collide lead ions, creating very hot and dense conditions that are comparable to those just after the big bang. To study these collisions, it is necessary to have highly advanced detectors. Studying these collisions can help to understand what the universe was like close to the big bang. Many phenomena that scientists want to observe with collisions at CERN with the LHC have an extremely low probability of occurring in a single collision. Thus, there must be as many collisions or data events as we can possibly achieve to increase our chances of observing a rare event. When the luminosity, in other words the light and energy per second, of the accelerator increases, we sample more collisions. To further increase the luminosity, it has been decided to upgrade the Large Hadron Collider. With an increase in collisions, detectors need to be upgraded as well to collect these larger sets of data. The detectors will need to withstand more radiation over the same amount of time, otherwise they would need to be replaced too often. Replacement of these components is difficult because the beam needs to be shut down and the experiment disassembled to replace them, and would be too costly over time. To withstand the LHC upgrade, as expected to be completed in 2030, and to improve the detector precision and performance, the ALICE experiment will upgrade its Inner Tracking System to ITS3 (Inner Tracking System 3). This will consist of replacing the three innermost layers of the current tracking system (ITS2). The design of ITS3 is made up of ultra thin silicon detectors that can be bent into a cylindrical shape. To prepare for this bent detector, sensor prototypes called Monolithic Stitched Sensors (MOSS) are being tested. This thesis mainly focuses on how much charge from particles is collected in these sensors and the results are compared to theoretical expectations.

• author: Schafer, Macaila Catherine
• supervisor: David Silvermyr; Iaroslav Panasenko
• organization: Department of Physics; Particle and nuclear physics
• course: FYSM64 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 9245278
• date added to LUP: 2026-07-13 09:34:58
• date last changed: 2026-07-13 09:34:58