[Paper Review] The ASTRI SST-2M Prototype: Camera and Electronics

O. Catalano, S. Giarrusso+16 more|arXiv (Cornell University)|Jul 19, 2013

Particle Detector Development and Performance2 references16 citations

TL;DR

This paper presents the design and lab validation of the ASTRI SST-2M prototype's camera and front-end electronics, featuring 37 Photon Detection Modules (PDMs) with Hamamatsu S11828-3344M SiPMs and custom EASIROC ASICs. Key results confirm high photon detection efficiency (PDE > 50% at 400 nm), low crosstalk (<20%), and stable gain control, validating the system for Cherenkov telescope applications in the 1–100 TeV energy range.

ABSTRACT

ASTRI is a Flagship Project financed by the Italian Ministry of Education, University and Research, and led by INAF, the Italian National Institute of Astrophysics. The primary goal of the ASTRI project is the realization of an end-to-end prototype of a Small Size Telescope for the Cherenkov Telescope Array. The prototype, named ASTRI SST-2M, is based on a completely new double mirror optics design and will be equipped with a camera made of a matrix of SiPM detectors. Here we describe the ASTRI SST-2M camera concept: basic idea, detectors, electronics, current status and some results coming from experiments in lab.

Motivation & Objective

• Develop a compact, high-performance camera for the ASTRI SST-2M prototype telescope to enable detection of very high-energy gamma rays (1–100 TeV).
• Address the challenges of low-light, fast-responding Cherenkov photon detection using novel SiPM technology in a compact, lightweight, and thermally stable system.
• Ensure robust signal processing and calibration in the camera electronics to maintain dynamic range, low noise, and high linearity across varying light levels.
• Validate the camera system’s performance in lab conditions prior to field deployment at the Serra La Nave observatory.
• Establish a scalable, modular design for future deployment in the ASTRI mini-array and the Cherenkov Telescope Array (CTA).

Proposed method

• Adopt a dual-mirror Schwarzschild-Couder optical design with f/0.5 and a 9.6° field of view to enable a compact, curved focal plane for the camera.
• Implement a modular camera architecture using 37 independent PDMs, each housing 16 SiPM sensor units (4×4 pixels), forming 8×8 logical pixels per PDM.
• Use Hamamatsu S11828-3344M SiPMs with 50 µm pitch, 62% fill factor, and 70 V bias, operating in Geiger mode for high gain (~10⁶) and single-photon sensitivity.
• Integrate EASIROC ASICs as front-end electronics: 64-channel, low-noise, low-power (≤5 mW/channel), with programmable gain (high/low), threshold, and dynamic range (1–1000 pe).
• Implement a thermoelectric cooling system using Peltier cells to stabilize SiPM gain at 16°C, minimizing temperature-induced gain drift.
• Incorporate an internal LED-fiber calibration system to enable relative gain calibration across all SiPM channels.

Experimental results

Research questions

• RQ1 Can SiPMs with controlled over-voltage and temperature stabilization achieve sufficient photon detection efficiency (PDE) and low crosstalk for Cherenkov telescope applications?
• RQ2 Can the EASIROC ASIC achieve sufficient linearity, dynamic range (1–1000 pe), and low noise for accurate single-photon counting in high-background environments?
• RQ3 How do dark count rates and optical crosstalk in SiPMs scale with operating voltage, and can they be kept below system performance thresholds?
• RQ4 Does the modular PDM design enable reliable maintenance and thermal stability on a curved focal surface?
• RQ5 Can the combined SiPM–EASIROC system achieve stable, calibrated performance under lab conditions simulating real Cherenkov flash signals?

Key findings

• The SiPMs achieved a photon detection efficiency (PDE) exceeding 50% at 400 nm, with optimal performance at 71.98 V bias, balancing PDE and dark count rates.
• Optical crosstalk was measured at 16% at 70 V bias, well below the 20% threshold, and increased linearly with over-voltage, confirming predictable behavior.
• The EASIROC ASIC demonstrated excellent linearity: 22 ADC units per photoelectron (pe) in high-gain mode, with linearity up to 50 pe (HG) and 1000 pe (LG).
• The staircase curve analysis revealed a total dark count rate of ~600 kHz per pixel, with crosstalk contribution estimated at 16% of the total dark rate.
• Pulse height distributions clearly resolved 1st and 2nd pe peaks, separated by ~22 ADC units, confirming high single-photon resolution.
• The thermal control system maintained a stable 16°C environment, ensuring gain stability and minimizing temperature-induced drift in SiPM response.