[Paper Review] The Photodetector Plane of the 4m Davies Cotton Small Size Telescope for the Cherenkov Telescope Array

V. Boccone, J. A. Aguilar+5 more|arXiv (Cornell University)|Jul 10, 2013

Astrophysics and Cosmic Phenomena2 references3 citations

TL;DR

This paper presents the design and characterization of a large-area hexagonal Geiger-mode avalanche photodiode (G-APD) photodetector plane (PDP) for the 4m Davies-Cotton Small Size Telescope (SST) in the Cherenkov Telescope Array (CTA). The PDP uses 1296 G-APDs with 93.6 mm² active area, segmented into four channels to reduce capacitance, achieving a peak photon detection efficiency (PDE) of 31±2% at 470 nm, with crosstalk reduced to under 10% at optimal overvoltage, enabling moonlight operation and improved performance over PMTs.

ABSTRACT

Photomultipliers (PMTs) are currently adopted for the photodetector plane of Imaging Atmospheric Cherenkov Telescopes (IACTs). Even though PMT quantum efficiency has improved impressively in the recent years, one of the main limitation for their application in the gamma-astronomy field - the impossibility to operate with moon light - still remains. As a matter of fact, the light excess would lead to significant and faster camera ageing. Solid state detectors, in particular Geiger-mode avalanche photo-diodes (G-APDs) represent a valuable alternative solution to overcome this limitation as demonstrated in the field by the FACT experiment (The First G- APD Cherenkov Telescope). They can be regarded as a more promising long term approach, which can be easily adopted for the new generation of cameras and for the Cherenkov Telescope Array (CTA). We describe here the Photo-Detector Plane (PDP) of the camera for the 4 m Davies Cotton CTA Small Size Telescopes, for which large area G-APD coupled to non-imaging light concentrators are planned. The PDP includes 1296 photosensors, the biasing and pre-amplification stages, the control electronics as well as the mechanical support and the water- tight enclosure. We developed with Hamamatsu a new large area hexagonal shaped G-APD with an area of 93.6 mm^2. This G-APD is divided into 4 channels which will be summed after the pre-amplification stage to maintain an acceptable time characteristic of the signal. The characterization of this device for 50 um and 100 um micro-cell sizes will be discussed and compared to other non-custom photodetectors.

Motivation & Objective

• To develop a high-performance photodetector plane (PDP) for the 4m Davies-Cotton Small Size Telescope (SST) in the Cherenkov Telescope Array (CTA) using solid-state detectors.
• To overcome the limitation of photomultiplier tubes (PMTs) in moonlight conditions by adopting Geiger-mode avalanche photodiodes (G-APDs).
• To design a large-area, hexagonally shaped G-APD with 93.6 mm² active area, segmented into four channels for improved signal timing and reduced capacitance.
• To characterize key performance metrics—photon detection efficiency (PDE), crosstalk, and dark count rate—of custom G-APDs and compare them with commercial alternatives.
• To enable long-duration observations at high energies (1–100 TeV) by achieving stable, low-noise performance under moonlight conditions.

Proposed method

• Development of a custom 93.6 mm² hexagonal G-APD with 50 µm and 100 µm micro-cell sizes, fabricated in collaboration with Hamamatsu.
• Segmentation of the G-APD into four channels to minimize capacitance effects and maintain signal timing characteristics.
• Use of non-imaging light concentrators to couple the G-APDs to the telescope optics, maximizing light collection efficiency.
• Implementation of pre-amplification and biasing stages integrated into the PDP for signal conditioning and control.
• Employment of a CAEN V814 low-threshold discriminator and V560 scaler to measure dark count rates and crosstalk by analyzing pulse height distributions at different thresholds.
• Application of a shaping amplifier (Ortec NIM) to reduce pulse width to 15–20 ns, enabling operation at high trigger rates (up to 15–20 MHz).

Experimental results

Research questions

• RQ1 Can a large-area, hexagonal G-APD with 93.6 mm² active area achieve sufficient photon detection efficiency (PDE) for high-energy gamma-ray detection in the CTA SST?
• RQ2 How does crosstalk in the new G-APD compare to commercial alternatives like SenSL and Hamamatsu MPPCs, especially at optimal overvoltage?
• RQ3 Can the G-APD-based PDP enable stable data acquisition under moonlight conditions, overcoming a key limitation of PMTs?
• RQ4 What is the dark count rate per unit area of the new G-APD, and how does it scale with device size for high dynamic range operation?
• RQ5 Does the four-channel segmentation strategy effectively reduce signal distortion and maintain timing performance in a high-channel-count camera system?

Key findings

• The S12516-050 G-APD achieved a peak photon detection efficiency (PDE) of (31±2)% at 470 nm, comparable to commercial Hamamatsu S10985-050 and µB30035-X13-E15 devices.
• The S12516-100 G-APD showed a higher PDE of (37.7±1.2)% but exhibited twice the crosstalk of the 50 µm version, reaching 14–22% at 1.6 V overvoltage.
• At optimal overvoltage (2 V), the crosstalk in the 50 µm G-APD dropped below 10%, significantly lower than the 100 µm version and approaching the performance of the SenSL sensor.
• The SenSL sensor had a very low crosstalk (<10% at 2 V), but its dark count rate of 0.6 MHz/mm² would result in an estimated 50 MHz dark count rate per 93.6 mm² device, making it impractical for this application.
• The dark count rate per mm² was normalized and found to scale with device area, with the S12516-050 showing a lower dark count rate than the SenSL sensor at comparable overvoltage levels.
• Laboratory measurements confirmed that the new G-APD design supports high-rate operation (up to 15–20 MHz) with proper pulse shaping, enabling robust performance in high-background conditions such as moonlight.