Study On An Atmospheric Cherenkov Telescope With Wide Field-of-view Based On Lens

The Very High Energy?VHE??-ray astronomy is a new scientific field of astro-physics developed in recent decades,which opened a new window to understand the universe.Observation of the cosmic VHE?-ray plays crucial role?i?in the exploration of particle acceleration and radiation processes under extreme astrophysical conditions;?ii?revealing the origin of galactic and extragalactic Cosmic Rays?CRs?and the non-thermal transient phenomena?such as?ray burst,GRB?;?iii?the indirect measurement of the Extragalactic Background Light?EBL?and search for Dark Matter?DM?signal etc.Wide field-of-view?FoV?and low threshold energy are the most important goals for future ground-based VHE?-ray astronomy,and it is especially crucial for detailed studies of temporal and spectral characteristics of solitary events,such as?-ray bursts?GRBs?,highly variable sources like blazars,or potential VHE electromagnetic coun-terparts of gravitational waves?GWs?.Atmospheric Cherenkov Telescope?ACT?based on refractive optics and operated at an extreme high altitude observatory may be an appropriate solution.The main results of the dissertation are as follows:?1?A novel wide FoV ACT based on refractive optics?water lens?is proposed and developed.Inspired by the Fresnel lens proposals of GAW,JEM-EUSO and the structure of the human eyes,the ideal water lens should be designed in a hemispherical shape,which allows a very large FoV and achieves a uniform image quality on the curved focal surface for all incident angles.?2?As a pathfinder,we designed and manufactured a telescope prototype based on a water lens.The lens with a diameter of 0.9 m is a thin spherical cap shell filled with purified water.The geometric tolerance of the lens is±1 mm,which is assessed by industrial computerized tomography?CT?.A traditional focusing method is used to measure the focal length and spot size,and the experiment result agrees with the simulation result within the statistical errors from ZEMAX.Each element?pixel?of camera is a PMT Hamamatsu R7725 and the full aperture of the camera with 48 PMTs is 15^?×13^?.The front-end electronics?FEE?and DAQ system are mainly built from commercial modules based on Nuclear Instrument Module?NIM?and the electronics of LHAASO-KM2A.?3?The atmospheric Cherenkov light induced by VHE cosmic rays?CRs?have been observed by the telescope prototype.The experiment was conducted and successfully operated at the Yangbajing Observatory during the winters of 2015 and 2016.By using the prototype with a camera consisting of 48 PMTs,CR events were observed in coincidence with a Extensive Air Shower?EAS?array consists of 75 scintillation counters.The coincident time window was set to 400 ns.The coincident rate is much higher than the accidental coincident rate.Therefore,we can conclude that the events are real Cherenkov signals induced by a CRs air shower,and the accidental coincidence can be negligible.A bright star signal and the coincident events were used to calibrate offline the pointing direction of a pixel,it can compensate the missing of the pre-experiment calibration.?4?A reconstruction method for estimating the arrival direction of the air shower was developed by combining the water-lens telescope and the EAS array.Reconstruct-ing the direction of hadronic showers is particularly difficult with a single telescope.Therefore,on the basis of the hybrid measurement of the air shower by Cherenkov light and the electromagnetic component,a novel method is developed to reconstruct the direction of CRs.The direction of the CR can be reconstructed using the pointing direction of the fired PMT with the maximum amplitude and the position of the shower core determined by the EAS array.?5?Thanks to the hybrid measurement of the air shower by Cherenkov light and the electromagnetic component,the characteristics and performance of the telescope prototype are evaluated.The result implies that the telescope prototype is capable of atmospheric Cherenkov imaging and the median angular resolution(?₅₀)is?0.9^?for?10 TeV CRs.?6?An extreme-altitude observatory for multi-messenger and multi-wavelength ob-servation is proposed.Several viable sites which meet the basic requirement were in-vestigated in Tibet and PuMaJiangTang Town close to Langkazi County could be a potential excellent extreme-altitude ground-based candidate site.The brightness of the Night Sky Background?NSB?at the Yangbajing Observatory has been monitored con-tinuously using a Unihedron Sky Quality Meter,and the average value was found to be21.6 mag/arcsec²for clear moonless nights.Recently,MAGIC team reported the detection of VHE emission?>300 GeV?asso-ciated with GRB 190114C,LIGO/Virgo and FERMI/GBM accomplished the first joint detection of gravitational?GW170817?and electromagnetic radiation?GRB 170817A?from a single source,the cubic-kilometer IceCube Neutrino Observatory detected a?290 TeV neutrino from a direction consistent with the flaring?-ray blazar TXS0506+056.The above tremendous observational progresses give an enormous impetus to the wide FoV ACT technical innovation.An even larger dimensional hemispherical water lens and array located at a very high altitude observatory should be considered for the future of?-ray astronomy.The technology for manufacturing larger diameter lenses,nanosecond/high-quantum-efficiency detectors,and large data handling capabil-ities should be given priority.