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

AstroPix is a high-voltage CMOS (HV-CMOS) monolithic active pixel sensor originally developed to enable precision gamma-ray imaging and spectroscopy in the medium-energy regime (approximately 100 keV-100 MeV) based on the groundwork laid by ATLASpix and MuPix. It features a 500 um pixel pitch, in-pixel amplification and digitization, and low power consumption (around 3-4 mW/cm^2), making it scalable for large-area, multilayer telescope detector planes. The detectors have a designed dynamic range of 25 keV to 700 keV. With these features, AstroPix meets the requirements of future space-based high-energy telescopes and the imaging layers of the Barrel Imaging Calorimeter (BIC) in the Electron-Proton/Ion Collider (ePIC) detector at the future Electron-Ion Collider (EIC). For the space-based payload, AstroPix is being integrated into sounding rocket and balloon payloads to demonstrate the technical readiness of the devices. For BIC, AstroPix-based imaging layers interleaved within the lead/scintillating-fiber (Pb/SciFi) sampling calorimeter provide granular shower imaging, enabling key performance features such as electron/pion or gamma/neutral-pion separation. As part of the ongoing detector R&D efforts, we have been testing various AstroPix v3 configurations: the single chip, a quad-chip assembly, a three-layer stack of quad chips, and a nine-chip module that represents the smallest prototype unit of the BIC imaging layer. This presentation will highlight recent performance test results from these AstroPix detector configurations.

💡 Research Summary

AstroPix_v3 is a high‑voltage CMOS (HV‑CMOS) monolithic active pixel sensor originally developed for medium‑energy gamma‑ray astronomy (∼100 keV–100 MeV). It features a 500 µm pixel pitch, 35 × 35 pixel matrix (≈2 cm × 2 cm active area), in‑pixel amplification and digitisation, and a low power consumption of 3–4 mW cm⁻². The device is designed for a dynamic range of 25 keV to 700 keV, making it suitable for both space‑based telescopes such as the AMEGO‑X concept and for the imaging layers of the Barrel Imaging Calorimeter (BIC) in the ePIC detector at the future Electron‑Ion Collider (EIC).

The paper reports a systematic R&D program that evaluated several configurations of AstroPix_v3: a single chip, a 2 × 2 quad‑chip assembly, a three‑layer stack of quad‑chips (the A‑STEP sounding‑rocket payload), and a nine‑chip linear module that represents the smallest BIC imaging‑layer unit. All tests were performed with a common bias of –150 V and a global threshold of 200 mV, using the A‑STEP hardware platform and a Cmod A7 FPGA for data acquisition.

Quad‑chip results – Noise scans identified pixels with rates >2 Hz, which were masked. The two bottom chips achieved a 99 % active‑pixel yield, while the two top chips showed a slightly lower yield (≥97 %) due to a digital‑bus‑line noise feature that appears only when multiple chips are powered simultaneously. Injection tests using a uniform voltage pulse demonstrated pixel‑to‑pixel uniformity: the average time‑over‑threshold (ToT) values clustered between 5 µs and 7 µs, with Gaussian‑shaped distributions and a 20–30 % variation, consistent with earlier single‑chip measurements.

Three‑layer quad‑chip stack (A‑STEP) – Twelve chips (three quad‑chip layers) were operated in a daisy‑chain configuration with a shared ToA clock (400 ns tick). Cosmic‑ray events were reconstructed by requiring coincident hits in all three layers within one clock tick, confirming synchronized multi‑layer readout and reliable SPI communication across 12 chips.

Nine‑chip module (BIC prototype) – A tighter noise cut (1 Hz) yielded a 99 % active‑pixel yield for all chips except the last, which achieved 91.9 % (91 noisy pixels). A 10 µCi ⁹⁰Sr source was moved across the module using 3 mm and 5 mm collimators. Hit maps clearly followed the source position, and the ToT distributions for each chip were well described by a Landau function convoluted with a Gaussian. Two distinct ToT groups emerged: chips 2, 6, 7 peaked near 3 µs, while chips 0, 1, 3, 4, 5 peaked near 4 µs. The distributions were independent of collimator size and matched single‑chip performance.

Hit‑rate measurements – By varying the collimator diameter from 0.2 mm to 1.8 mm, the maximum per‑chip hit rate reached ≈1.6 kHz (total ≈7 kHz) before data drop‑out occurred at larger diameters. This exceeds the expected 925 Hz per chip for the BIC imaging layers under EIC beam‑background conditions and the projected 45 Hz per chip for a bright gamma‑ray burst scenario in AMEGO‑X. For typical space‑mission conditions the expected rate is ≈1 Hz cm⁻², well within the demonstrated capability.

The authors conclude that AstroPix_v3 meets all critical performance metrics for both collider and space applications. The quad‑chip and three‑layer stack validate scalability and multi‑layer synchronization, while the nine‑chip prototype demonstrates high pixel yield, stable ToT response, and sufficient hit‑rate handling for realistic operating environments. Future work will focus on large‑scale production with strict quality control (≥99 % good pixels) and extended beam‑test campaigns to further certify long‑term reliability in the EIC and space‑flight contexts.