AstroPix: A Pixelated HVCMOS Sensor for Space-Based Gamma-Ray Measurement

TL;DR

The paper presents AstroPix, a high-voltage CMOS MAPS detector tailored for space-based MeV gamma-ray measurements within AMEGO-X. It details the AstroPix_v3 design, including a 2×2 cm^2 reticle with a 35×35 pixel matrix, deep N-well HV isolation, and a per-pixel front-end that enables self-triggering and 12‑bit ToT readout at 200 MHz, with a 2 MHz timestamp. Results show a 10.4% FWHM energy resolution at 59.5 keV and depletion depths around 94 μm on a medium-resistivity substrate, along with noise performance enabling 25 keV sensitivity and manageable pixel variance. Heavy-ion radiation tests on AstroPix_v2 indicate no destructive latchup and conservative SEFI rate estimates for space, supporting the platform’s resilience for future missions. The work demonstrates AstroPix as a scalable, low-power, space-qualified detector technology, with first flight demonstrations planned for A-STEP (2026) and large-format AMEGO‑X deployments to follow.

Abstract

A next-generation medium-energy gamma-ray telescope targeting the MeV range would address open questions in astrophysics regarding how extreme conditions accelerate cosmic-ray particles, produce relativistic jet outflows, and more. One concept, AMEGO-X, relies upon the mission-enabling CMOS Monolithic Active Pixel Sensor silicon chip AstroPix. AstroPix is designed for space-based use, featuring low noise, low power consumption, and high scalability. Desired performance of the device include an energy resolution of 5 keV (or 10% FWHM) at 122 keV and a dynamic range per-pixel of 25-700 keV, enabled by the addition of a high-voltage bias to each pixel which supports a depletion depth of 500 um. This work reports on the status of the AstroPix development process with emphasis on the current version under test, version three (v3), and highlights of version two (v2). Version 3 achieves energy resolution of 10.4 +/- 3.2% at 59.5 keV and 94 +/- 6 um depletion in a low-resistivity test silicon substrate.

Figures (13)

• Figure 1: Cartoon of AstroPix_v3 structure, including DNWs (yellow), CMOS components (structures within DNWs), inter-pixel isolation $p$-stop (blue), and DNW guard rings (dark green). HV is delivered on the frontside of the chip. Drawing not to scale. Units displayed in $\mu$m.
• Figure 2: IV curve for AstroPix_v3 wafer 2 chip ($200-400$$\Omega\cdot\mathrm{cm}$).
• Figure 3: Simplified operational schematic illustrating electronics within each pixel and timestamp generation within pixel buffers.
• Figure 4: AstroPix_v3 signal chain with all basic blocks showing the full readout path. Hitbuffer signals interface with a readout control unit (RCU) which includes a finite state machine (FSM).
• Figure 5: (left) One $2\times2$ cm$^2$ AstroPix_v3 chip mounted on a custom carrier board (right) One $\sim4\times4$ cm$^2$ AstroPix_v3 quad chip, comprised of an array of four individual chips. The top two chips are connected via a flex cable glued on top of the chips.