TAIGA (Tunka Advanced Instrument for Cosmic Ray Physics and Gamma Astronomy)
The TAIGA observatory addresses ground-based gamma-ray astronomy at energies from a few TeV to several PeV, as well as cosmic ray physics from 100 TeV to several EeV.
In the past, the Tunka Astrophysical Center in the Tunka Valley (50 km from Lake Baikal) was solely devoted to the charged cosmic rays. Three arrays to study charged CRs are in operation at the Tunka site: Tunka-133, Tunka-Rex and Tunka-Grande. Their measurement of the energy spectrum and mass composition is important in order to understand the acceleration limit of the Galactic CR sources and the transition from Galactic to extragalactic CR.
New gamma ray experiment TAIGA is designed to study gamma-radiation and charged cosmic rays in the energy range of 1013 eV – 1018 eV. For this energy range, there are many fundamental questions, which need to be answered. The first and foremost one is the question of the sources of Galactic cosmic rays with energies of about few PeV, the most likely limit of the acceleration of protons in the galactic accelerators. The energy of gamma rays originated in such accelerators could extend up to 300 TeV, however so far there was no detection of a single photon with energy greater than 100 TeV. The combination of the wide angle Cherenkov detectors of the TAIGA-HiSCORE array and the 4-m Imaging Atmospheric Cherenkov Telescopes of the TAIGA-IACT array and underground muon detectors offers a very cost effective way to construct a 5 km2 array for gamma-ray astronomy.
Cosmic ray studies from 100 TeV to several 100’s of PeV
The purpose of this detectors is to reconstruct the energy spectrum and the mass composition of cosmic rays.
Reconstruction resolution: arrival direction ~ 0.1°-0.3°,
Axis position ~ 5-10 m,
Xmax ~ 28g/cm2
The Tunka-133 detector is a non-imaging, shower front sampling array consisting of 175 wide-angle Cherenkov detectors, combined in 25 clusters of 7 modules in each and distributed over an area of 3 km2. The cluster has a hexagonal structure with detectors at the tops and center, where the electronics cluster module is also located. Each detector consists of a PMT (Emi 9359 or Hamamatsu R1408) with a hemispherical photocathode of 20 cm diameter, an entrance window heating system and a case with a UV-transparent entrance window and remote-controlled lid. Tunka-133 runs during clear moon-less nights from September to April. Thus, the effective time of measurements is about 250-500 hours per year. The total number of events with Epr > 1017 eV is in order of 103 per year. After three years of data acquisition several features in energy and mass composition spectra were found, confirming results by other experiments: a hardening at 3 ∙ 1016 eV and a softening at about 1017 eV (the second knee).
Tunka-Grande is a particle detector array for recording secondary particles generated by cosmic rays with energies of 1016-1018 eV. The array is located on an area of 1 km2 and consists of 19 scintillation stations. The geometry of the Tunka-Grande replicates the geometry of the Tunka-133 so that the Tunka-Grande stations are located next to the Tunka-133 cluster centers. Each station is composed of two parts: one at surface and one underground. The surface part, which consists of 12 counters, covers a total area of about 8m2 and detects all EAS charged particles at the level of the array. The second part, which consists of 8 counters with a total area of around 5m2, is under a soil layer of 1.5m and is designed to separate the EAS muon component. Both parts of the array are in the immediate vicinity of each other. This arrangement of the modules makes it possible to obtain the ratio of the electronic component of the components. Unlike Tunka-133 the Tunka-Grande can operate full duty cycle in all weather conditions (except thunderstorms).
Tunka-Rex is an antenna array for the detection of EAS radio emission. It works jointly with the non-imaging air-Cherenkov light detector Tunka-133 and the scintillators of Tunka-Grande and receives triggers from both of them. At the present moment Tunka-Rex consists of 63 antennas covering an area of about 3 km2. Each Tunka-Rex antenna station consists of two perpendicularly aligned SALLAs (Short Aperiodic Loaded Loop Antenna) with 120 cm diameter mounted on a wooden pole on a height of about 2.5 m. The registration of radio emission is conducted in the frequency range of 30-80 MHz. According to data collected over several years of operation, the installation has shown good results in the reconstruction of such parameters as the depth of shower maximum and the energy of the primary particle. Also the Tunka-Rex proved to be a good tool for cross-calibration of the detectors of different types.
Ground-Based Gamma-Ray Astronomy from a Few TeV to Several PeV
Reconstruction resolution: arrival direction ~ 0.1°,
Axis position ~ 5-6 m,
Xmax ~ 20-25g/cm2
The TAIGA-HiSCORE (High Sensitive Cosmic ORigin Explorer) is a wide-angle optical Cherenkov detector of cosmic rays and gamma rays. Like for Tunka-133 TAIGA-HiSCORE method is based on the sampling of the Cherenkov light front from air showers, but with lower energy threshold and higher time-amplitude resolution. TAIGA-HiSCORE will consist of an array of 500 wide-angle (field of view is of 0.6 sr) optical detectors, distributed with a spacing of 75–200 m over an area of 5 km2 during the first stage. The energy threshold for gamma-ray induced showers is about 50 TeV, for cosmic rays 100 TeV. Although according to the TAIGA-HiSCORE data it is impossible to accurately distinguish between gamma quanta and cosmic rays, an increased number of events from the side of the Crab Nebula indirectly indicates the registration of gamma rays. In the future, the development of the TAIGA-HiSCORE will be aimed at conducting hybrid measurements in conjunction with the TAIGA-IACT telescope and scintillation detectors TAIGA-Muon. This will open the possibility of distinguishing showers caused by gamma rays and hadrons.
The TAIGA-IACT detector will consist of 16 IACTs distributed over an area of 5 km2 with a spacing of up to 600–1000 m. The telescope has a composite reflector of Davis-Cotton design consisting of 34 spherical glass mirrors with 60 cm diameter each. The reflector has a total diameter of 4.3 m. Its focal length is 4.75 m. The camera of TAIGA-IACT includes a matrix of photomultipliers (PMTs), high voltage system, signal processing and readout electronics and power supply controller. Construction of the camera provides its reliable operation in the cold winter conditions. To protect PMTs from day light the input window is remotely shut tight using ordinary blinds. The camera consists of 560 hexagonally packed PMTs grouped into 28 clusters. It provides a FoV of ~10°x10°. The angular resolution of the Cherenkov camera is 0.36° per pixel.
The first telescope measurements were started at the end of January 2017. The full count rate of the telescope triggers was about 30 Hz, while the count rate of EAS events was about 2 Hz, and the count rate of the events, combined with the TAIGA-HiSCORE, was ~ 0.3 Hz. For the joint events, the directions for the position of the EAS axis according to the TAIGA-HiSCORE data are in good agreement with the directions of the ellipses of the EAS images in the telescope chamber, which indicates the correspondence of the images registered by the telescope to the EAS data.
With the help of Tunka-Grande it is possible to study the mass composition of CR above 1017 eV. For gamma-astronomy purposes the total area of muon detectors should be increased by 10 times at least. It is planned to develop efficient, low-cost, not served muon counters for the future TAIGA-Muon array, which are capable to operate for a long time under the two-meter layer of the wet soil. The TAIGA-Muon counter will have 4 scintillator sectors in the form of an isosceles triangle. The length of sector hypotenuse is 100 cm. Scintillator plates are located between wavelength shifting bars and have crossection of ~ 10×160 mm2. It allows one to use PMTs with a small photocathode, and reduce the scintillator total volume. The cross section of the bar of the TAIGA-Muon counters is 5×20 mm2. The light from the bars is detected by FEU-85 PMT (QE = 18% at 500 nm). It is assumed that TAIGA-Muon will ensure 100% detection efficiency of a cosmic muon and will have a long lifetime of the detectors.