ALICE ITS3 WP4 Research

The ALICE experiment investigates the properties of the 'Quark-Gluon Plasma (QGP),' the state of the universe right after the Big Bang. The key to this research is observing heavy particles like Charm and Beauty quarks that punch through the interior of the QGP.

Just like throwing a heavy bowling ball into a foggy room to determine the fog's density from its trail, these heavy quarks act as probes that allow us to figure out the properties of the QGP.

However, these particles have extremely short lifetimes and decay in a fraction of a moment. Therefore, to capture their minute positions right before they decay, a highly precise detector must be placed as close as possible to the collision point.

In the ALICE experiment at the LHC, the Inner Tracking System (ITS) takes on this role. The next-generation model, ITS3, introduces a revolutionary idea: installing paper-thin silicon sensors by rolling them into a cylindrical shape.

A flat, single piece of silicon is very thin and weak, making it difficult to maintain its shape on its own under the influence of gravity without additional structures, but when rolled into an arch shape, it becomes sturdy. Utilizing this geometric stiffness allows it to maintain its shape without the need for separate support structures or cooling pipes.

Thanks to this, the material budget, which is the obstacle blocking the particles, can be drastically lowered (to about 0.05% X₀ per layer).

As a result, the multiple scattering effect that occurs in low-momentum particles is minimized, and interference experienced by the particles is reduced, allowing for more accurate trajectory measurements.

Once thin silicon sensors that bend to gain geometric stiffness were devised, the next challenge was how to supply power and retrieve data from them. To solve this, the Flexible Printed Circuit (FPC) serves as the backbone for establishing physical and electrical connections.

As a member of Pusan National University's Heavy Ion Physics Experiment Lab (HIPEx), I participated in the ALICE Collaboration's ITS3 WP4 (Mechanics & Engineering) research. The main research task was to verify whether the 50 and 100μm thick silicon sensors break or suffer electrical pixel response degradation when rolled roundly (radii 18mm, 24mm, 30mm) as in the actual design.

To achieve this, the first problem to solve was, "How can we precisely bend and secure a paper-thin silicon sensor to the desired radius without breaking it?" I built the test environment by designing 'PNU Guide & Frame', a custom mechanical jig assembly in 3D CAD, and 3D printing it. This allowed us to bend the sensor-FPC assembly safely.

(Placeholder: 3D design rendering image of the PNU Guide & Frame will be added here)

During the research, while conducting a sensor beamtime test at the Gyeongju Proton Accelerator, a problem arose where continuous, inexplicable anomalies (errors) were detected in the sensor's measurement values.

While exploring various possibilities to find the cause, I focused on the intrinsic physical properties of the silicon. Since the bandgap of silicon is about 1.12 eV, it can absorb near-infrared light (Near IR, wavelength ~1100 nm) and generate electron-hole pairs. Therefore, I hypothesized that the infrared rays emitted by the IR CCTV camera for night vision installed inside the accelerator irradiation room were hitting the thin sensor, causing fake hits and Threshold Scan errors.

To prove this, I returned to the laboratory and conducted a reproduction experiment by intentionally irradiating infrared rays onto the ALPIDE sensor inside a fully controlled dark room. As a result, I confirmed the exact same Threshold Scan anomalies that we had experienced at the accelerator. This process went beyond simply following a given measurement manual; it was an experience that provided deep insight into identifying and independently proving the cause of seemingly unexplainable errors in the field based on physical knowledge.

Ultimately, no significant performance degradation due to bending was observed in the sensor itself, regardless of whether it was flat or curved, but we gathered the practical conclusion that controlling ambient light conditions (especially IR) is highly critical. The bending research itself was eventually halted due to mandatory military service, so a final conclusion agreed upon by the joint research group was not reached.