In the past ten years the ground-based imaging atmospheric Cherenkov telescopes have discovered over 150 sources of gamma rays, mostly in the energy range ~0.1 – 10 TeV. In spite of that great success, it still remains unclear what the galactic cosmic ray accelerators at highest energies, up to the knee (~1015eV) and above, are. We are planning to extend the existing Tunka-133 with HiSCORE detectors, with an additional array of underground muon detectors and with an array of small-size, wide angle imaging telescopes of 8°x8° field of view, operating above the threshold energy of a few TeV. The telescopes are planned operating in coincidence with the HiSCORE wide angle ~1km² size array and at higher energies also with the Tunka-133 array. This novel, worldwide unique detector technique is aiming to become an inexpensive alternative for building very large size, nearly background-free very high energy gamma-ray detectors. This complex cosmic ray detector, dubbed TAIGA (which stands for “Tunka Advanced Instrument for cosmic ray physics and Gamma Astronomy”), will include also underground muon detectors, initially of 100 m² area, with the aim completing it to 1000-2000 m². This shall enable strong suppression of hadron-induced background for energies above 100 TeV. Along with gamma source hunting we plan also studying cosmic rays in the same energy range.
Gamma-radiation of ≥ 100 GeV was detected from more than 150 sources of different types. but gamma-rays with energies higher than 10 TeV are detected only from handful of sources. The highest energy detections are made only up to several tens of TeV. There are no source detections near or above 100 TeV energy range. One can say that the latter energy range remains Terra incognita for present day gamma-astronomy, albeit it is of utmost importance for resolving many fundamental problems of astro-particle physics.
So far most of the knowledge about sources of high energy gamma-rays has been obtained from arrays of Imaging Atmospheric Cherenkov Telescopes (IACT), namely: HEGRA , H.E.S.S., MAGIC, VERITAS, etc. An IACT consists of a tessellated mirror and a ns-fast camera in its focal plane, which shoots photos of extensive airshowers (EAS) in Cherenkov light.The method of image shape and direction analysis proposed by A.M.Hillas in 1985, allows one selecting with high efficiency EAS produced by gamma-rays from those initiated by charged cosmic rays. Typically the effective detection area of a single IACT is about 5 x 104 m². The so-called 4th generation IACT array CTA will have substantially higher sensitivity than the currently operational telescopes in the energy range of up to 100 TeV. The limited collection area of the air shower arrays of (0.1-1 km2) prohibits going into the energy range considerably higher than 100 TeV. For extending the sensitivity of an instrument towards higher energies, due to progressively lower fluxes from sources it is necessary to construct very large size arrays with lots of telescopes, distributed over an area of several square kilometers at least. The estimated cost of such an IACT array can easily become comparable to100 million US dollars. So, it is a sensitive task to find alternative cost-effective experimental solutions, which would allow one to provide an array of an area of the order of 10 - 100 km2 for studying PeVatrons in ourgalaxy.
Below we list the main components of the TAIGA detector:
• Th1. The HiSCORE wide Field of View (FoV; ~0.6 sr,) integrating air Cherenkov detector stations, placed at distances of 75-200m from each other, covering an area of initially ~ 1 km² and ~ 100 km² at the later phase of the experiment. This detectors consists of 4 closely packed bialkali PMTs of size either 20 cm or 25 cm in diameter. The PMTs are given Winston-cone shape light-guides, which increase their light collection area by a factor of four. The PMT outputs are locally summed up. 9 detector stations are operational since fall 2103. In fall 2014 the number of stations will be increased to 33.
• Th2. The TUNKA-133 wide FoV integrating air Cherenkov detector. This full size detector, operational since 2009 with 175 stations, is spread over an area of ~ 3 km². The individual stations consist of single large PMTs of diameter 20 cm.
• Up3. Up to 10 IACTs with reflector area of ~ 10 m² and equipped with imaging cameras of 400 PMT-based pixels. The FoV of the single telescope will be 8x8 deg.². The inter-telescope distances will be optimized in the range of ~ 600 m. The mechanical mount of the 1st telescope is under construction in Dubna, it should become ready in fall 2014. The PMTs for the camera are available, currently we are working on a solution of a low cost readout for the camera pixels. The mirrors should become available in fall 2014.
4444. An array of surface and underground muon stations. It is desirable to cover an area of a few 1000 m² with underground muon detectors for providing good rejection of hadron showers for energies ≥ 100 TeV. Currently we started deployment of the first few undergroud and surface muon detectors.
TAITTAIGA is on the way of becoming a most powerful, hybrid detector which shall help us finding the highest energy gamma sources in our galaxy and thus answering many open questions on the origin and physics of cosmic rays.