Cherenkov Telescope Array (CTA)
The next generation of Imaging Atmospheric Cherenkov Telescopes will be represented by the Cherenkov Telescope Array (CTA). The construction of CTA is now being carried out by a consortium of scientists and institutes from all over the world. The technical performances will provide the scientific potential needed to address some of the most important questions in this field, such as the origin of cosmic rays, the strength of intergalactic radiation fields, and the nature of dark matter.
CTA will be an open observatory with two main sites: in La Palma aimed at the observations of the northern sky, and in Chile for the southern sky. The two sites will consist of large arrays of IACTs of different sizes, and they will provide a full-sky coverage in gamma rays from 20 GeV to more than 300 TeV. The sensitivity of the instrument will improve up to an order of magnitude the sensitivity of the existing instruments such as H.E.S.S., MAGIC, and VERITAS.
The energy band that CTA will be able to cover cannot be guaranteed by a single type of telescopes, and thus the array will need different types of telescopes. At the moment, three different sizes of telescopes are planned to be built:
- Large-Sized Telescopes (LST). These telescopes will be the largest of the array with their 23 m diameter mirrors. They will be able to cover the lowest energies and bring the energy threshold down to 20 GeV.
- Medium-Sized Telescopes (MST). They will be telescopes with about 10-12 m diameter mirror surfaces, and they will be responsible of the core sensitivity of CTA between about 100 GeV and 10 TeV.
- Small-Sized Telescopes (SST). Telescopes with 2 - 4 m diameter mirrors that will extend the sensitivity range up to 100 TeV and will be located only in the southern hemisphere.
Our group is developing the Calibration Pipeline and the Quality Pipeline of the Data Processing and Preservation System software of CTAO.
Our group has worked on the creation of a SiPM camera for a prototype of the SST-1M telescope, and is now deeply involved in the commissioning and first data analysis of the first LST.
The Large Size Telescope (LST)
The LSTs, with their wide reflective surface, will collect light coming from the lowest part of the sensitivity of CTA between 20 and 150 GeV. The design plan consists of four LSTs arranged at the centre of both the northern and the southern sites.
The telescope, planned with a unique design, is provided with an alt-azimuthal mount and a 23 m diameter parabolic reflective surface of 400 m2 . The camera is composed of photomultiplier tubes. Although the big size of LST and the weigh around 100 tonnes, the light design will be able to re-position the telescope within 20 seconds, in such a way to contribute to the follow-up observations of transient events.
One of the most important phases of this project – not only from a mediatic point of view – was the inauguration of the LST prototype in La Palma in October 2018. Additionally, in December 2018, the LST prototype recorded its first Cherenkov light, and at the end of 2019 it detected gamma rays coming from the Crab Nebula.
The Advanced SiPM Camera for the LST
The Swiss experience acquired during the development of the FACT and SST-1M telescopes is utilised at its maximum for the development of the future camera for the LST. The University of Geneva, represented by M. Heller, is coordinating this activity within the LST collaboration.
This activity is very relevant for future opportunities for Switzerland to participate in the hardware implementation of the telescopes, as indicated by the declaration of interest signed in June 2021 by the UNIGE Rector, Prof. Y. Flückiger and the Director of ICCR of Tokyo University, Nobel Laureate T. Kajita and the LST spokesperson Prof. M. Teshima.
The main driver of this initiative is to use SiPMs for the LSTs, as they offer twice more sensitivity to Cherenkov light when compared to classical photomultiplier tubes. Their sensitive dimension (limited to 1 cm2 as they are noisy devices and also integrate more background light) is smaller than that of photomultipliers. This implies a potentially improved camera resolution and capability to capture smaller details in the showers (see Fig. 31). These can be fully exploited by modern analysis techniques. This imposes strict requirements on the camera design as four times more pixels are needed to cover the same field of view. The optimization of the focal plane of this camera has been the subject of a PhD work in the group.
Such an increased number of pixels has a dramatic impact on the camera power consumption, a challenge being tackled with innovative low-power application-specific integrated circuits (ASICs) developed in by the DPNC, the AQUA Lab/EPFL led by Prof. E. Charbon and ETHZ in a FLARE project. The ASICs will be general purpose for SiPM signal amplification and digitisation at GHz-speed developed in cooperation with Swiss microelectronics companies.
Additionally, the DPNC is collaborating with UZH on the development of new triggering techniques based on real-time artificial intelligence inference running on hardware accelerators. Prof. N. Serra at UZH has acquired strong know-how in developing deep-learning algorithms that can run on field-programmable gate arrays (FPGAs) in the frame of the LHCb detector of the Large Hadron Collider at CERN. The principle is that thanks to artificial intelligence algorithms, the decision whether an image acquired by a camera is registered or not is not only based on whether the intensity in any region of the camera is higher than a given threshold but it can be also based on more complex information, such as how much the image resembles an extensive air shower. The challenge will be to develop algorithms which are robust enough to deliver a stable outcome at different levels of background light and to do so with the minimum possible processing power.
The overall aim will be to lower the detection threshold of the LSTs and target energy thresholds as low as 10 GeV.
Simulated muon (top) and gamm (event) as recorded by the existing camera (left) and the proposed camera (right).
Optical module of the SST-1M camera. The same pixel shape and size are used as baseline for the realization of the future cameras of the Large-Sized Telescopes of CTA