Performance of the AstroPix Prototype Module for the Barrel Imaging Calorimeter at the ePIC Detector and in Space-Based Payloads

This paper presents the performance testing of various AstroPix HV-CMOS prototype configurations designed for high-precision gamma-ray imaging in both space-based payloads and the Barrel Imaging Calorimeter of the future ePIC detector.

The Gist

The "Super-Sensitive Digital Eye": Making Sense of AstroPix

Imagine you are trying to take a photo of a single firefly blinking in a massive, dark forest from miles away. To do this, you don't just need a regular camera; you need a specialized "digital eye" that is incredibly sensitive, can see tiny movements, and doesn't get overwhelmed by the sheer amount of information it has to process.

This paper describes the testing of a new technology called AstroPix. Think of AstroPix as a high-tech, ultra-sensitive digital sensor designed to act as the "retina" for two very different, very ambitious scientific projects.

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1. The Two Big Missions

The researchers are building this "eye" for two main purposes:

• The Space Explorer (AMEGO-X/A-STEP): This is like a high-powered telescope sent into space to watch the universe's biggest explosions (gamma-ray bursts). It needs to catch the faintest "glimmers" of light from deep space.
• The Particle Microscope (ePIC at the EIC): This is a massive machine on Earth that smashes atoms together to see what’s inside them. It’s like using a super-microscope to look at the "gears and springs" of matter itself.

2. What makes AstroPix special? (The "Smart Pixel")

In a normal digital camera, a pixel just records light. But AstroPix pixels are "smart."

The Analogy: Imagine a stadium full of people (the pixels). In a normal camera, a security guard has to walk around and check every single person to see if anyone is cheering. That takes too long.
With AstroPix, every single person in the stadium has their own tiny sensor. The moment someone cheers, they instantly shout, "I'm cheering! Here is how loud I am!" This allows the system to handle massive amounts of information very quickly without getting "confused" or overwhelmed.

3. The "Lego" Test (The Prototype Modules)

The researchers didn't just test one tiny chip; they wanted to see if they could snap these chips together like Lego bricks to build bigger and bigger "eyes." They tested four stages:

1. A Single Chip: One tiny square.
2. The Quad-Chip: Four squares snapped together.
3. The Three-Layer Stack: Like a sandwich of three quad-chips (to see if they can work in sync).
4. The Nine-Chip Module: A long strip of nine chips, which is the "basic building block" for the massive particle microscope.

4. What did the tests prove?

The scientists put these "Lego" modules through several "stress tests":

• The "Noise" Test: They checked if the pixels were "chatty" (reporting hits when nothing was there). They found that almost every pixel worked perfectly—like a classroom where almost every student is paying attention and not just making random noise.
• The "Cosmic Ray" Test: They watched how the sensors reacted to high-speed particles flying through them. It worked! The layers "talked" to each other perfectly, proving they could work together as a team.
• The "Source" Test: They used a small radioactive source (like a tiny, controlled flashlight) to see if the sensors could track a moving light. The sensors followed the "light" accurately, proving they have great "vision."
• The "Speed" Test: They checked how many "shouts" (hits) the sensor could handle per second before it got overwhelmed. It turns out, the sensor is plenty fast enough for both the space telescope and the particle microscope.

Summary: The Verdict

The paper concludes that AstroPix is a success. It is scalable (you can build it big), it is efficient (it doesn't use too much power), and it is smart enough to handle the intense environments of both deep space and high-energy physics labs.

It’s a successful "prototype" that proves we are one step closer to seeing the invisible parts of our universe.

Technical Summary

Technical Summary: Performance of the AstroPix Prototype Module

1. Problem Statement

The paper addresses the need for high-precision, scalable, and low-power imaging detectors capable of operating in two distinct, demanding environments:

• Space-Based Gamma-Ray Astronomy: Missions like AMEGO-X and the ComPair-2 balloon mission require medium-energy (100 keV–100 MeV) gamma-ray telescopes that can identify astrophysical phenomena through precise spectroscopy and imaging.
• High-Energy Particle Physics: The Electron-Ion Collider (EIC) requires the Barrel Imaging Calorimeter (BIC) within the ePIC detector to provide fine-grained 3D shower imaging. This is critical for distinguishing between particles (e.g., electrons/pions or gamma/neutral pions) during Deep Inelastic Scattering (DIS) measurements.

The challenge lies in developing a sensor that offers high granularity (small pixel pitch), a wide dynamic range, low power consumption for large-area scalability, and the ability to handle specific hit-rate requirements in both vacuum/space and collider environments.

2. Methodology

The researchers utilized the AstroPix_v3, a High-Voltage CMOS (HV-CMOS) monolithic active pixel sensor. The technical specifications include:

• Architecture: 500 µm pixel pitch, 35 × 35 pixel matrix per chip, and in-pixel amplification/digitization.
• Readout: Streaming readout with self-triggering at the pixel level, utilizing a 2.5 MHz clock for Time-of-Arrival (ToA) and a 200 MHz clock for Time-over-Threshold (ToT).
• Testing Configurations: To validate scalability, the team tested four progressive configurations:
  1. A single chip.
  2. A quad-chip assembly (2×2 array).
  3. A three-layer stack of quad-chips (simulating the A-STEP sounding rocket payload).
  4. A nine-chip module (a 1×9 linear array representing the basic unit of the BIC imaging layer).

Testing Procedures:

• Injection Tests: Used to evaluate pixel-to-pixel uniformity and response variation.
• Cosmic-Ray Tests: Performed on the three-layer stack to verify synchronized multi-layer operation and data acquisition (DAQ) via a shared ToA clock.
• Source Tests ($^{90}\text{Sr}$): Used a radioactive source and collimators (3 mm and 5 mm) to test hit maps, energy response (ToT distributions), and maximum hit-rate capabilities.

3. Key Contributions

• Development of AstroPix_v3: A versatile HV-CMOS sensor designed for both medium-energy gamma-ray imaging and high-energy calorimeter layers.
• Scalability Validation: Demonstrated that the sensor can be daisy-chained into complex modules (quad-chips and nine-chip arrays) while maintaining reliable communication and synchronized timing across multiple layers.
• Multi-Environment Proof-of-Concept: Provided a unified technological platform that meets the specific requirements of both the A-STEP space mission and the ePIC detector at the EIC.

4. Results

• Pixel Yield: The quad-chip achieved an active pixel yield of $\geq 97\%$. The nine-chip module achieved a $99\%$ yield for most chips (with one chip at $91.9\%$ due to noise).
• Uniformity and Response: Injection tests showed good uniformity across chips, with ToT values generally ranging between 5–7 µs. The $^{90}\text{Sr}$ source tests produced ToT distributions well-described by a Landau function convoluted with a Gaussian.
• Hit-Rate Capability: The nine-chip module maintained reliable operation up to a per-chip hit rate of approximately 1.6 kHz.
• Requirement Matching:
  • EIC/BIC: The measured rates exceed the expected $\sim 925\text{ Hz}$ per chip.
  • Space (A-STEP/AMEGO-X): The performance significantly exceeds the expected $\sim 1\text{ Hz/cm}^2$ and the high-intensity GRB simulation of $\sim 45\text{ Hz}$ per chip.

5. Significance

This work validates the AstroPix technology as a viable candidate for next-generation high-energy physics and astrophysics instrumentation. By demonstrating that a single sensor design can satisfy the divergent needs of space-based telescopes (low power, high sensitivity) and particle colliders (high granularity, high rate capability), the research paves the way for cost-effective and technologically streamlined detector development. While current designs are limited to $\sim 4\text{ Hz}$ per pixel, the successful prototype testing provides a foundation for future iterations aimed at higher hit rates and full depletion.