Status of Barrel Imaging Calorimeter in Korea for the Electron-Ion Collider

This paper outlines the recent progress and future R&D plans of the Korean group's contributions to the Barrel Imaging Calorimeter (BIC) for the Electron-Ion Collider, covering their work in silicon chip testing, module assembly, prototype development, beam tests, readout system design, and simulations.

The Gist

Imagine the universe is a giant, complex puzzle made of tiny building blocks called protons and neutrons. Scientists want to take these apart to see exactly how they are built, what gives them their weight, and how they spin. To do this, they are building a massive machine called the Electron-Ion Collider (EIC). Think of this machine as a super-powered "microscope" that smashes particles together at incredible speeds to reveal their hidden inner workings.

However, to see the results of these crashes, you need a very special camera. That's where the Barrel Imaging Calorimeter (BIC) comes in.

The "Smart Camera" for Particle Crashes

The BIC is essentially a high-tech camera designed to catch the debris from these particle collisions. Its main job is to spot two specific things: electrons and photons (particles of light). It needs to be incredibly good at telling the difference between these particles and the "background noise" (like pions, which are messy, unwanted particles).

To do this, the BIC uses a clever sandwich design, like a very dense, multi-layered cake:

1. The Heavy Layers: It has layers of lead and special plastic fibers (scintillating fibers). When a particle hits these, it creates a burst of light, kind of like a sparkler. This helps measure the particle's energy.
2. The "Eyes": Sandwiched between the heavy layers are ultra-sensitive silicon chips (called AstroPix). These act like the pixels in a digital camera, but they are so fine they can take a 3D picture of the particle's path as it crashes through the layers.

The goal is to create a 3D movie of how a particle breaks apart, rather than just a flat photo.

What the Korean Team is Doing

A team of scientists from Pusan National University in Korea is playing a crucial role in building this "camera." You can think of them as the engineers and quality control experts making sure every part works perfectly before the big show.

Here is what they are doing, broken down simply:

• Testing the "Pixels": They are checking the tiny silicon chips (AstroPix) to make sure they are sensitive enough to catch even the faintest signals. They are testing them in bulk, like checking thousands of lightbulbs to ensure none are burnt out.
• Building the "Sandwich": They are manufacturing the lead-and-fiber layers. Imagine stacking thin sheets of lead with tiny glass fibers in between, then gluing and polishing them until they are perfect. They have already built 33 of these prototype blocks.
• The "Wiring": They are designing the cables and boxes that connect all these parts so the data can be read quickly. They even tested flexible cables that can snake between the layers, which is like finding a way to plug a camera into a sandwich without squishing it.

Putting It to the Test

You can't just build a camera and hope it works; you have to test it with real light. The Korean team recently took their prototypes to two famous particle labs: CERN in Europe and KEK in Japan.

• The CERN Test (2024): They shot a beam of electrons at their lead-and-fiber blocks. It was like shining a flashlight through a stack of paper to see how the light spreads. They successfully measured the energy of the electrons and started analyzing the data.
• The KEK Test (2025): This was the big upgrade. They combined the lead blocks with the silicon "eyes" (AstroPix) and shot electrons through the whole setup. They successfully recorded data from both the lead blocks and the silicon chips at the exact same time. This proved that their "3D camera" can actually work together to track a particle's journey.

What's Next?

The team has successfully shown that their design works in small tests. Now, they are preparing for even bigger tests in 2025 and 2026. They are building larger prototypes (some as long as 70 cm) to make sure the whole system can handle the massive scale of the final Electron-Ion Collider.

In short, the Korean team is helping build the most advanced "particle camera" in the world, ensuring that when the EIC turns on, we can finally see the fundamental secrets of how our universe is put together.

Technical Summary

Technical Summary: Status of Barrel Imaging Calorimeter in Korea for the Electron-Ion Collider

Problem and Context
The Electron-Ion Collider (EIC) is designed to probe the fundamental structure of matter, specifically the origins of nucleon mass, spin, and the dynamic behavior of quarks and gluons. To achieve the required precision in 3D imaging of nucleons and nuclei, the ePIC experiment necessitates a high-performance electromagnetic calorimeter in the barrel region ($-1.7 < \eta < 1.4$). The Barrel Imaging Calorimeter (BIC) is tasked with meeting stringent performance metrics: an energy resolution of $10\%/\sqrt{E} \oplus (2-3)\%$, electron–pion separation of $10^4$ at low momenta, photon reconstruction down to $\sim100$ MeV, and $\pi^0$ discrimination up to $\sim10$ GeV. These requirements demand an imaging-type calorimeter capable of precise energy measurement and efficient particle separation from background pions.

Methodology and Design
The BIC design integrates two primary subsystems to achieve 3D shower imaging:

1. Pb/SciFi Sampling Layers: Based on the BCAL from the GlueX experiment, these layers utilize Lead (Pb) plates and Scintillating Fibers (SciFi) for energy measurement.
2. AstroPix Silicon Pixel Layers: Developed for the NASA Amego-X mission, these layers provide fine position resolution and 3D shower profiling.

The baseline configuration consists of four AstroPix imaging layers interleaved with five Pb/SciFi sampling layers, followed by a bulk Pb/SciFi section. This arrangement yields a total radiation length of approximately $17 X_0$ with a sampling fraction of roughly 10%. The full barrel system is projected to cover $\sim100$ m$^2$ using approximately $2.5 \times 10^5$ AstroPix chips.

Key Contributions by the Korean Group
The Korean research group, led by Pusan National University, is actively engaged in several core R&D tasks:

• Component Testing: Conducting mass chip testing for AstroPix sensors, including wafer-level evaluation and the development of multi-line single-chip test systems.
• Prototype Fabrication: Producing Pb/SciFi prototype modules. The process involves shaping 0.5 mm Pb plates, stacking them with 1 mm diameter scintillating fibers, curing, and polishing. A total of 33 modules were produced, including one with a light-guide prototype.
• System Integration: Developing readout boxes and a new Data Acquisition (DAQ) system produced in Korea.
• Simulation: Performing GEANT4 simulations and system-level performance studies.

Experimental Results
The paper details results from two recent beam tests:

1. CERN PS T10 (August 2024):

   • Setup: A $3 \times 5$ array of Pb/SciFi unit modules (totaling $\sim11 X_0$ depth) instrumented with PMTs or SiPMs.
   • Outcome: The team successfully collected data using 0.5–3 GeV/c electron beams. Electron calibration was achieved, and data analysis for energy resolution is currently underway.

2. KEK PF-AR (March 2025):

   • Setup: A larger $4 \times 7$ array of Pb/SciFi modules ($\sim15 X_0$ depth) with AstroPix layers inserted between the Pb/SciFi layers. Flexible cables were utilized to connect AstroPix modules to the readout system.
   • Outcome: Using a new Korean-produced DAQ system, the team recorded simultaneous signals from AstroPix (via GECCO + FPGA boards) and the Pb/SciFi layers. Preliminary analysis confirmed correlations between beam positions on AstroPix and wire-chamber drift times. This demonstrated the feasibility of synchronized data taking between silicon and sampling layers, a critical milestone for 3D shower imaging.

Significance and Outlook
The paper asserts that the Korean group's contributions are vital to the BIC project, particularly in silicon testing, module production, and system integration. The successful execution of beam tests at CERN and KEK, resulting in the collection of electron-beam data and the demonstration of synchronized DAQ for hybrid detector layers, represents a significant step toward validating the BIC's 3D imaging capabilities.

Future work includes further beam tests in 2025 and 2026 to validate system performance, the preparation of larger prototypes (including a 70 cm-long bulk sector with HGCROC and SiPM readout), and the eventual mass production of the detector components. The ongoing analysis of collected data will guide the refinement of the calorimeter's performance for the ePIC experiment.