Design of the LHAASO detectors

Introduction The Large High Altitude Air Shower Observatory plans to build a hybrid extensive air shower array with an area of about 1 km^(2) at an altitude of 4,410 m a.s.l.in Sichuan province,China,to explore the origin of high-energy cosmic rays.KM2A LHAASO-KM2A will detect gamma ray sources with a sensitivity of about 1%Crab Unit at 100 TeV.It covers an area of 1 km^(2) with a total of 5195 scintillation detectors.Its angular resolution reaches about 0.3 degrees,and the energy resolution is better than 25%.With the help of 1171 muon detectors,cosmic nuclei background will be rejected to a level of 10-4 at 50 TeV.The design and performances of the scintillation detectors and muon detectors are described in detail.WCDA LHAASO-WCDA focuses on surveying the northern sky for steady and transient sources from 100 GeV to 20 TeV,with a very high background rejection power and a good angular resolution.The WCDA consists of three water ponds with a total area of 78,000 m^(2),and the effective water depth is 4 m.Each water pond is divided into 5m×5m cells partitioned by black plastic curtains to prevent penetration of the light from neighboring cells.An 8-inch PMT sits at the bottom center of each cell,looking upward to collect Cherenkov light generated by shower secondary particles in water.WFCTA LHAASO-WFCTA is composed of 12 wide-field-of-view Cherenkov/fluorescence telescopes.Each telescope consists of a spherical light collector of about 4.7 m^(2) and focal plane camera of 32×32 pixels with a pixel size of 0.5 degree.LHAASO prototype arrays A prototype array about 1%of LHAASO has been constructed at Yangbajing Cosmic Ray Observatory and has been in operation for more than 2 years.Its performance fully meets the design requirements.Conclusion The LHAASO detectors are designed to fulfill the physical goals in gamma ray astronomy and cosmic ray physics.One-fourth of LHAASO will be constructed and put into operation to produce physical data by the end of 2018.The whole array will be finished in the beginning of 2021.

Measurement of water quality in LHAASO-WCDA

Introduction The water Cherenkov detector array(WCDA)is an important detector in the Large High Altitude Air Shower Observatory(LHAASO),which is tasked with continuously surveying VHE gamma-rays sky.The WCDA covers an area of 78,000 m2 and contains 350,000 tons of purified water.The water quality and its stability are critical for its performance and long-term operation.Method Light is exponentially attenuated when propagating in water.Water attenuation length can be obtained by comparing light intensities at different optical path lengths,which is the basic principle of direct measurements.Here,a cylindrical device was designed to continuously monitor and measure water quality in the LHAASO-WCDA.Conclusion The systematic error of the cylindrical device was about 4.1%,indicating it is capable of measuring the water attenuation length in WCDA.Furthermore,suspended particle number and turbidity were confirmed as significantly related to the measured attenuation length.

Strong constraints on Lorentz violation using new γ-ray observations around PeV

The tiny modification of dispersion relation induced by Lorentz violation(LV)is an essential topic in quantum gravity(QG)theories,which can be magnified into significant effects when dealing with astrophysical observations at high energies and long propagation distances.LV would lead to photon decay at high energies;therefore,observations of high-energy photons could constrain LV or even QG theories.The Large High Altitude Air Shower Observatory(LHAASO)is the most sensitive gamma-array instrument currently operating above 100 TeV.Recently,LHAASO reported the detection of 12 sources above 100 TeV with maximum photon energy exceeding 1 PeV.According to these observations,the most stringent restriction is achieved in this study,i.e.,limiting the LV energy scale to 1.7×10^(33) eV,which is over 139,000 times that of the Planck energy,and achieving an improvement of approximately 1.9 orders of magnitude over previous limits.