Design of the LHAASO detectors

Abstract

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. 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. 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 $$\times $$ 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. 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 $$\times $$ 32 pixels with a pixel size of 0.5 degree. 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. 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.