[Paper Review] The SST-1M camera for the Cherenkov Telescope Array

E. J. Schioppa, F. Cadoux+49 more|arXiv (Cornell University)|Aug 26, 2015

Astrophysics and Cosmic Phenomena5 citations

TL;DR

The SST-1M camera for the Cherenkov Telescope Array (CTA) features a compact, hexagonal array of custom large-area silicon photomultipliers (SiPMs) with integrated light concentrators and a fully digital, high-throughput readout system (DigiCam), enabling high-performance gamma-ray detection over 30 years. It achieves sub-10% charge resolution at 22 MHz night sky background and demonstrates <0.5% crosstalk at high light levels, validating its suitability for large-scale deployment in the high-energy gamma-ray regime (5–300 TeV).

ABSTRACT

The prototype camera of the single-mirror Small Size Telescopes (SST-1M) proposed for the Cherenkov Telescope Array (CTA) project has been designed to be very compact and to deliver high performance over thirty years of operation. The camera is composed of an hexagonal photo-detection plane made of custom designed large area hexagonal silicon photomultipliers and a high throughput, highly configurable, fully digital readout and trigger system (DigiCam). The camera will be installed on the telescope structure at the H. Niewodnicza{ń}ski institute of Nuclear Physics in Krakow in fall 2015. In this contribution, we review the steps that led to the development of the innovative photo-detection plane and readout electronics, and we describe the test and calibration strategy adopted.

Motivation & Objective

• Develop a low-cost, scalable, and durable camera system for the Small Size Telescopes (SST) sub-array of the Cherenkov Telescope Array (CTA).
• Enable high-performance detection of high-energy gamma rays (5–300 TeV) through optimized photo-detection and readout electronics.
• Ensure long-term stability and robustness under extreme environmental conditions and high background light levels.
• Achieve high uniformity and reliability in mass production for future large-scale deployment across the CTA array.
• Validate the performance of key components—SiPMs, light concentrators, and digital electronics—through extensive testing and calibration.

Proposed method

• Design a hexagonal photo-detection plane (PDP) using 1296 custom hexagonal SiPMs arranged in 108 modules of 12 pixels each, with mechanical stability provided by an aluminum backplate.
• Integrate plastic Winston cones as light concentrators to focus light from hexagonal pixels onto 3×3 mm² square SiPMs, achieving 9° full width at half maximum (FWHM) angular resolution.
• Implement a fully digital, high-throughput, and configurable readout system (DigiCam) with front-end electronics, trigger logic, and slow control, mounted separately from the PDP.
• Use a hybrid cooling system: water-cooled heat pipes connected to the DigiCam boards via heat exchangers, with the PDP backplate acting as a thermal spreader.
• Apply a 45° tilt to DigiCam minicrates to ensure reliable heat pipe operation at any telescope inclination.
• Perform component-level testing using portable, factory-integrated test setups for SiPMs, PreAmp, and Slow Control Boards (SCB), including optical characterization with 470 nm LED sources and fiber-optic illumination.

Experimental results

Research questions

• RQ1 Can a SiPM-based camera achieve stable, high-performance operation over 30 years in the harsh conditions of a ground-based gamma-ray observatory?
• RQ2 How does the use of hexagonal pixel geometry and light concentrators affect the trigger efficiency and angular resolution in the SST-1M camera?
• RQ3 What is the crosstalk level in the SiPM array under high background light conditions (e.g., half-moon), and can it be kept below 1%?
• RQ4 Can the digital DigiCam readout system maintain charge resolution below 10% across a wide dynamic range, even at high night sky background (NSB) levels?
• RQ5 Is the modular, scalable design of the camera suitable for large-scale production with high uniformity and low defect rates?

Key findings

• The SiPM-based PDP achieves a single photoelectron resolution with clear separation between photo-peaks, even with common cathode biasing across four channels.
• Charge resolution for a single sensor is below 10% across the full dynamic range under dark night conditions (22 MHz NSB), meeting the performance goal.
• At half-moon conditions (660 MHz NSB), charge resolution remains below 10% for light levels above 10 photo-electrons and below the goal threshold for levels above 60 photo-electrons.
• Electronic crosstalk is negligible, with induced signals only detectable at light levels of ~3000 photons, and remains below 0.5% in magnitude.
• The PDP cooling system, validated on a 1:10 mock-up and via FEM simulations, effectively manages heat dissipation with a stable thermal gradient across the board.
• Component-level testing revealed excellent uniformity in SiPM response, PreAmp gain, and SCB calibration, enabling reliable flat-fielding and data calibration in future operations.