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A description is given of the ENDA (electron–neutron detector array) setup launched as part of the international LHAASO project in December 2019 in Tibet at an altitude of 4410 m above sea level. A unique feature of the ENDA is its simultaneous recording of two main components of extensive air showers (hadronic and electronic) over whole array using the same detectors. The main parameters of the array obtained from calculations and measurements are presented.

Hadrons are the “skeleton” of extensive air shower (EAS). They possess favorable information concerning the cosmic ray components and energy. The electron-neutron detector (EN-detector) can detect both electrons and thermal neutrons generated by EAS hadrons in surrounding matter. The electron-neutron detector array (ENDA) was proposed to add into the LHAASO project to improve its capability of EAS hybrid detection. Up to present 64 EN-detectors have been produced and are running in China. In 2018, a cluster (16 EN-detectors) was installed at Yangbajing (YBJ), Tibet. In 2019, another cluster, so called ENDA-16-HZS, was installed in LHAASO at Haizishan (HZS), Daocheng, Sichuan. Besides, 2 clusters are tested at Hebei Normal University (HNU). ENDA-16-HZS is running and get amount of EAS events at energy above 100 TeV. Moreover, a number of coincident events between ENDA and the LHAASO electron detector (ED) and muon detector (MD) arrays composed the KM2A, as well as Cherenkov detectors WFCTA and WCDA are obtained. The events with cores falling into ENDA are selected. The ED array and ENDA accurately offers the EAS directions and the core positions respectively. Both the lateral distributions of neutrons, electrons and muons and the longitudinal development of atmospheric Cherenkov lights are effectively sampled. A hybrid detection of EAS including thermal neutrons, electrons, muons and Cherenkov lights can provide a strong capability of cosmic nuclei discrimination as well as energy measurement with high resolution. In this talk, the status of the clusters at the different places are summarized, and the preliminary results of coincident events between ENDA and the LHAASO array are presented.

The electron and neutron detector (EN-detector) can detect the thermal neutrons produced by hadronic components in the secondary particles of cosmic rays. According to our previous paper about the performance of EN-detectors in Yangbajing, Tibet, during the period from August 2019 to January 2020, the number of detected neutrons in rain season is significantly (∼ 10%) lower than that in periods of dry season. In order to further study the correlation between the detector performance and humidity in soil, five soil moisture meters have been installed at different depths inside ENDA-16-HZS in August 2020. By analyzing the data in September 2020, it is demonstrated that both counting rate of EN-detector and neutrons in triggered events are negatively correlated with humidity

Ground extensive air shower experiment is powerless for detecting cosmic ray particles of tens-GeV energy in the GRBs (Gamma Ray Burst) so far, because of its threshold energy. The experimental altitude needs to be increased in order to achieve more effective observation. In this study a water Cherenkov detector prototype based on the single-particle counting technology was constructed to detect GRBs at an altitude of 3,650 m. The preliminary test results show that the detector made in this study has stable performance and can detect cosmic rays. If it is placed at an altitude of approximately 5,200 m, the GRBs with tens-GeV of energy can hopefully be detected.

EN-detectors (Electron-Neutron Detector) can provide detection of both electrons and thermal neutrons generated in ground by secondary hadrons of extensive air shower (EAS). The 400 EN-detector array, so called ENDA (Electron-Neutron Detector Array), was proposed to add into the LHAASO (Large High Altitude Air Shower Observatory) project to improve its capability of EAS hybrid detection. ENDA will be located at the center of the first 1/4 of the KM2A surface scintillator array. Supported by Tibet University, Institute of High Energy Physics of Chinese Academy of Sciences (IHEP CAS) and Institute for Nuclear Research of Russian Academy of Science (INR RAS), 64 EN-detectors of so called ENDA-64 have been produced at Hebei Normal University and pulse shapes are sampled with FADCs made by Sichuan University, networked with the White Rabbit precise clock system. We expect to deploy ENDA-64 (64 EN-detectors) at LHAASO site in 2019. In this paper, we report the results of ENDA-16 (16 EN-detectors) testing run at Yangbajing (YBJ) together with YBJHA offering electrons and muons. It is the first time to obtain the high altitude hybrid detection results between neutrons, electrons and muons in EAS.

EAS thermal neutrons measurement gives additional valuable advantages to study energy and mass composition of primary cosmic rays especially above the knee region. After success of the PRISMA-YBJ experiment we built a new EAS thermal neutron detection array at Tibet University, Lhasa, China (3700m a.s.l.) in March, 2017. This prototype array consists of 16 EAS en-detectors measuring two main EAS components: hadronic and electromagnetic ones. These detectors use a thin layer of a novel type of ZnS(Ag) scintillator alloyed with natural boron compound for thermal neutron capture. In this report, we introduce principle of the detection technique, deployment of the array, and the test results obtained with the array.

The muonic component of the extensive air showers (EAS) is of great importance for the astroparticle physics. It carries the information about the properties of primary cosmic ray (CR) particles, such as their mass, and electromagnetic and hadronic nature. It provides a sensitive test for the hadronic interaction models, which are inevitable for describing the cascade shower development of cosmic rays in EAS experiments. The YangBaJing Hybrid Array (YBJ-HA) experiment has been in operation since the end of 2016. Surface detectors are used for the measurements of primary energy, angular direction and core position of a shower event, while underground muon detectors are used for measuring the density of muons at various locations. Using the data obtained by the YBJ-HA experiment, this work reports the first measurement of the lateral muon distribution for the primary cosmic ray energy in the 100 TeV region. The punch-through effect is evaluated via MC simulation.

Aiming at the observation of cosmic-ray chemical composition at the "knee" energy region, we have been developinga new type air-shower core detector (YAC, Yangbajing Air shower Core detector array) to be set up at Yangbajing (90.522$^\circ$ E, 30.102$^\circ$ N, 4300 m above sea level, atmospheric depth: 606 g/m$^2$) in Tibet, China. YAC works together with the Tibet air-shower array (Tibet-III) and an underground water cherenkov muon detector array (MD) as a hybrid experiment. Each YAC detector unit consists of lead plates of 3.5 cm thick and a scintillation counter which detects the burst size induced by high energy particles in the air-shower cores. The burst size can be measured from 1 MIP (Minimum Ionization Particle) to $10^{6}$ MIPs. The first phase of this experiment, named "YAC-I", consists of 16 YAC detectors each having the size 40 cm $\times$ 50 cm and distributing in a grid with an effective area of 10 m$^{2}$. YAC-I is used to check hadronic interaction models. The second phase of the experiment, called "YAC-II", consists of 124 YAC detectors with coverage about 500 m$^2$. The inner 100 detectors of 80 cm $\times $ 50 cm each are deployed in a 10 $\times$ 10 matrix from with a 1.9 m separation and the outer 24 detectors of 100 cm $\times$ 50 cm each are distributed around them to reject non-core events whose shower cores are far from the YAC-II array. YAC-II is used to study the primary cosmic-ray composition, in particular, to obtain the energy spectra of proton, helium and iron nuclei between 5$\times$$10^{13}$ eV and $10^{16}$ eV covering the "knee" and also being connected with direct observations at energies around 100 TeV. We present the design and performance of YAC-II in this paper., Comment: 6 pages, 9 figures, Submitted to Chinese Physics C

In order to observe gamma rays in the 100 TeV energy region, the 4500 m(2) underground muon detector array using water Cherenkov technique is constructed, forming the TIBET III+MD hybrid array. Because the showers induced by primary gamma rays contain much fewer muons than those induced by primary hadrons, significant improvement of the gamma ray sensitivity for TIBET III+MD array is expected. In this paper, the design and performance of the MD-A detector with large Tyvek bag is reported.