Cosmic Particles

Our Universe is full of powerful particle accelerators: pulsars, supernova remnants, gamma-ray bursts (GRBs) and AGN. Many experiments were designed to identify these acceleration sites, to understand the acceleration mechanisms and the role that accelerated particles play in star formation and galaxy evolution. A direct way to identify sources of cosmic particles is to find fluxes of photons (gamma rays – E>105 eV) arriving from a single direction. Such photons are produced by π0 decays which suggest the existence of high-energy hadrons that cause the production of π0 mesons at or near the source. Charged hadrons cannot be used to identify the source because their trajectory is deflected by magnetic fields. Observation of gamma rays became possible in the 1960s. For the detection of very energetic gamma rays (E>30 GeV), the space-based instruments are not enough. Their flux is too low and detectors with a bigger area located on the ground are necessary. Charged particles moving through the atmosphere with a velocity larger than the local speed of light (the vacuum speed of light divided by the refractive index of the air) produce Cherenkov light. This light is emitted on a narrow cone around the direction of the particle. From each part of the particle track, the Cherenkov light arrives on a ring on the ground. In an air shower, the initial particle interacts with the atoms in the atmosphere and produce many new particles. The ones which are faster than the local speed of light emit Cherenkov light. At the ground, the imprint of the shower will be the overlapping of the light produced by each particle. Imaging atmospheric Cherenkov telescopes (IACTs) provide the most powerful tool for observing gamma rays and probing the high-energy universe in the TeV (1012 eV) regime. They have an excellent point source sensitivity and angular resolution, together with a large collection area. Large arrays of imaging telescopes are the natural progression of the IACT technique. Simplistic scaling of the number of telescopes suggests that sensitivity improves with √N (N=number of telescopes).

CTA (Cherenkov Telescope Array) and its telescopes

CTA will be the world’s biggest observatory for very high energy gamma-ray astronomy. Its scientific goals range from understanding the role of relativistic cosmic particles through to the search for dark matter. CTA will be able to study photons with energies from 20 GeV (109 eV) to 300 TeV and will survey hundreds of times faster than previous TeV telescopes, thanks to its wider field of view and improved sensitivity. Large arrays of IACTs will be installed in both the southern (European Southern Observatory (ESO) Paranal site in Chile) and northern (Instituto de Astrofisica de Canarias (IAC) Roque de los Muchachos Observatory site in La Palma, Spain) hemispheres to provide full-sky coverage. IACTs are instruments capable of imaging very short light flashes, which are the imprint of the Cherenkov light emitted when a very high energy gamma ray crosses the atmosphere. Analyzing the image collected by the IACT camera, information on the primary photons such as the arrival direction and energy, are obtained. The best compromise between wide energy range and cost constraints has been achieved by planning a graded array of telescopes of different sizes: the lowest energies will be covered by four large-sized telescopes (LSTs - D~ 23 m) to detect gamma rays down to 20 GeV; an array of 25 (south) or 15 (north) medium-sized telescopes (MSTs - D~ 12 m) will observe the core energy range (100 GeV - 10 TeV), and finally the highest energies will be covered by a several km2 array of 70 small-sized telescopes (SSTs - D~ 9.5 m).

FIG. 1: Layouts for the baseline arrays for CTA North (left) and CTA South (right).
FIG. 1: Layouts for the baseline arrays for CTA North (left) and CTA South (right).
FIG. 2: Photo of R11920-100-05/R12992-100-05 PMT used in the larger telescope of this experiment (LST and MST)
FIG. 2: Photo of R11920-100-05/R12992-100-05 PMT used in the larger telescope of this experiment (LST and MST)

During the last years, extensive work has been done to define the prototype for all three telescope types. The experience gained through the current generation of IACT has been very important, but new techniques have also been tested. For example, in telescope design, two approaches are being developed: the single mirror approach based on the traditional Davies-Cotton - DC design and the dual mirror based on the Schwarzschild-Couder - SC design. Furthermore, both photomultiplier tubes (PMTs) and Silicon photomultipliers (SiPMs) are being evaluated as camera photosensors. The larger telescope of this experiment (LST and MST) will use PMTs as the detection element. The four types of PMT that will be used are the R11920-100-20, R11920-100-05, R12992-100-20 and R12992-100-05 which differ in gain and the integration of HV circuit in the assembly (FIG. 2). In particular, in MST the PMTs R11920-100-05 and R12992-100-05 will be integrated in optical cameras called NectarCAM and FlashCAM.

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) telescope (FIG. 3 - left) is one of three proposed SST designs developed by the Italian National Institute for Astrophysics (INAF). It is a two-mirror telescope based on the SC design. The goal of this design is to eliminate much of the optical aberration across the field of view. The ASTRI SST-2M prototype was inaugurated in September 2014 and is currently being tested under field conditions at the Serra La Nave observing station on Mount Etna in Sicily. It is composed of a 4.3 m diameter primary mirror and a 1.8 m monolithic secondary mirror. It has a focal length F=2.15 m, a field of view FoV~ 9.6°, for a ratio F/D1 =0.5 (FIG. 3 - right).

FIG. 3: The ASTRI telescope prototype located in Serra La Nave (Mt. Etna), Italy (left). Optical layout for the telescope (right).
FIG. 3: The ASTRI telescope prototype located in Serra La Nave (Mt. Etna), Italy (left). Optical layout for the telescope (right).

The ASTRI camera is extremely compact (~ 50 cm × 50 cm × 50 cm) and light (~ 70 kg). It is located in the curved focal plane of the telescope, which has a radius of curvature of ~ 1 m. The final camera will incorporate 2368 channels divided into 37 PDM. A PDM (Photon-Detection-Module) is composed of 64 SiPM pixels organized in an 8x8 matrix. Each PDM consists of 4 MPPC (Multi-Pixel Photon Counter) array produced by Hamamatsu. Custom made MPPCs, which have higher sensitivity for Cherenkov light, have been chosen for the ASTRI telescopes. The output of an MPPC may contain spurious pulses, namely afterpulse and crosstalk, which are separate from the output pulses of the incident photons. Crosstalk is output from other pixels at the same time as the detection of light. These MPCC series have been optimized to reduce the crosstalk.

An ASTRI SST-2M camera image for an on-axis simulated primary gamma-ray event is shown on the left of FIG. 4. On the right, the photon distributions obtained after the gain calibration for each pixel of a PDM are represented.

FIG. 4: An ASTRI SST-2M camera image for an on-axis simulated gamma-ray event (left). Photon distributions obtained after the gain calibration for each pixel of a PDM (right).
FIG. 4: An ASTRI SST-2M camera image for an on-axis simulated gamma-ray event (left). Photon distributions obtained after the gain calibration for each pixel of a PDM (right).

Finally, FIG. 5 depicts Polaris observed by a CCD arranged in different zones of the ASTRI camera. Each image, taken with different offsets from the optical axis of the telescope, has approximately the same angular size. Therefore, the new approach used for the ASTRI telescope guarantee that there are no optical aberrations across the full field of view and the morphology of the shower imprint can be “well” detected. This is very important to reconstruct the direction of gamma-ray photons responsible for the Cherenkov light emission and identify their celestial sources.

FIG. 5: Polaris as observed by ASTRI with different offsets from the optical axis of the telescope.
FIG. 5: Polaris as observed by ASTRI with different offsets from the optical axis of the telescope.
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