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The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane3–6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators. Observations of γ-rays with energies up to 1.4 PeV find that 12 sources in the Galaxy are PeVatrons, one of which is the Crab Nebula.

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

We report the discovery of a new unidentified extended $\gamma$-ray source in the Galactic plane named LHAASO J0341+5258 with a pre-trial significance of 8.2 standard deviations above 25 TeV. The best fit position is R.A.$=55.34^{\circ}\pm0.11^{\circ}$ and Dec$=52.97^{\circ}\pm0.07^{\circ}$. The angular size of LHAASO J0341+5258 is $0.29^\circ \pm 0.06^\circ_{stat} \pm0.02^\circ_{sys}$. The flux above 25 TeV is about $20\%$ of the flux of Crab Nebula. Although a power-law fit of the spectrum from 10 TeV to 200 TeV with the photon index $\alpha=2.98 \pm 0.19_{stat} \pm 0.02_{sys}$ is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum %($\alpha \leq 1.75$) spectrum with a cutoff at $\approx 50$ TeV. We briefly discuss the origin of UHE gamma-rays. The lack of an energetic pulsar and a young SNR inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude both the leptonic and hadronic scenarios of gamma-ray production., Comment: Accepted by APJL

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.

The LHAASO-ENDA (Electron Neutron Detector array) array will consist of 400 special electron-neutron detectors (en-detectors) developed at Institute for Nuclear Research of Russian Academy of Science (INR RAS) for the PRISMA project. The main feature of the array is capability to simultaneously measure both number of EAS electrons and number of secondary neutrons produced by high energy hadrons in surrounding matter. Electron/neutron ratio was found to be a good parameter to separate different primary nuclei and also to search for gammainduced showers. Detailed Monte-Carlo simulations for the first-step array of 64 en-detectors is now performing. Mass composition analysis with machine learning techniques for Monte-Carlo simulated events is presented

Some unexpected sporadic increases of an environmental radioactive background have been recorded at mountain level at Baksan Neutrino Observatory (BNO, 1700 m above sea level) using electron-neutron detectors (en-detectors), which could be explained by radioactive aerosol enhancements. The large area inorganic scintillator en-detectors developed for cosmic ray study are continuously monitoring environmental thermal neutron fluxes at various geophysical conditions. Application of the pulse shape discrimination method allows us to select and separately measure both thermal neutrons and radioactive beta-decay nuclides being products of radon decays in air (mostly Rn-222 and Rn-220). There are two en-detector setups running now at BNO, one deep underground while another one at surface. Both installations had recorded some strange sporadic increases of radioactive nuclides in air. In this paper, we present results and the most probable explanation of the significant increases joint by radioactive aerosols production but caused by different reasons: Baksan river floods or nearby underground experiment with powerful Cr-51 radioactive source.

Primary cosmic ray energy spectrum above 1 PeV has been measured with PRISMA-YBJ being a prototype of PRISMA array at altitude of 4300 m a.s.l. It realized a novel type of EAS recording method measuring hadronic EAS component over the total array area through thermal neutron detection with a specially developed so-called en-detectors sensitive to electron and thermal neutron EAS components. Primary c. r. spectrum was recovered in 2 ways: through usual Ne procedure as well as through measurements of EAS size spectrum in thermal neutrons. Monte-Carlo simulations of the experiments allowed us recover the primary spectrum above 1 PeV. The both obtained spectra follow power law function with index close that measured by direct measurements below 1 PeV.

Long-term variations in the natural thermal neutron flux in Tibet at an altitude of 4300 m above sea level are studied using scintillation electron-neutron detectors (en-detectors) developed at the Institute for Nuclear Research, Russian Academy of Sciences. Substantial growth (on the level of several percent each year over the last 3.5 years) in the thermal neutron flux recorded by the en-detectors is observed. The effect is explained by an increase in the low-energy cosmic ray flux, due to decreasing solar activity in the current solar cycle. Seasonal variations in anti-correlation with the rain season were also established.

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.

Primary cosmic ray mass composition above 1 PeV has been measured with PRISMA-YBJ being a prototype of PRISMA array at altitude of 4300 m a.s.l. It realized a novel type of EAS recording method measuring hadronic EAS component over the total array area through thermal neutron detection with a specially developed so-called en-detectors sensitive to electron and thermal neutron EAS components. Primary c. r. mass composition was recovered through measurements of two EAS components with the en-detectors: electromagnetic (energy deposit) and the number of secondary thermal neutrons produced locally by high energy hadrons. Monte-Carlo simulations of the experiments allowed us to find a parameter highly sensitive to the primary particle mass (atomic number) A above 1 PeV. The preliminary obtained mass composition is consistent with light composition and does not show any significant change at higher energies.

Some exotic geophysical events are observed by a global net of electron-neutron detectors (en-detectors) developed in the framework of the PRISMA EAS project. Our en-detectors running both on the Earth's surface and underground are continuously measuring the environmental thermal neutron flux. Thermal neutrons are in equilibrium with media and are therefore sensitive to many geophysical phenomena, which are exotic for people studying ultra high-energy cosmic rays or carrying out low background experiments deep underground.

An Extensive Air Shower array of a novel type has been constructed in Tibet at an altitude of 4300 m above sea level. It consists of specially designed scintillator en-detectors capable of measuring two main EAS components: hadrons (n) and electrons (e). Results of the experiment are presented. It is demonstrated that the EAS integral distribution in the number of thermal neutrons can be well fitted by a single power law function. Comparison with detailed Monte-Carlo simulations using CORSIKA6.9 and GEANT4.10 codes are also shown.

The status of and main features of the PRISMA-LHAASO project are presented. We discuss here the science project and some preliminary results obtained with the PRISMA prototype.

Air radon concentration measurement is useful for research on geophysical effects, but it is strongly sensitive to site geology and many geophysical and microclimatic processes such as wind, ventilation, air humidity and so on that induce very big fluctuations on the concentration of radon in air. On the contrary, monitoring the radon concentration in soil by measuring the thermal neutron flux reduces environmental effects. In this paper we report some experimental results on the natural thermal neutron flux as well as the concentration of air radon and its variations at 4300 m a.s.l. These results were obtained with unshielded thermal neutron scintillation detectors (en-detectors) and radon monitors located inside the ARGO-YBJ experimental hall. The correlation of these variations with the lunar month and 1-year period is undoubtedly confirmed. A method for earthquakes prediction provided by a global net of the en-detectors is currently under study., 16 pages, 7 figures

Some results on the EAS neutron component measurements by means of the PRISMA-32 array are presented. The array consists of 32 electron-neutron detectors (en-detectors) capable to detect two main EAS components: electromagnetic one consisting of charged particles, and hadronic one by measuring delayed thermal neutrons accompanying the showers. For thermal neutrons detection, a compound of a well-known inorganic scintillator ZnS(Ag) and LiF, enriched to 90 % with 6Li isotope is used. The setup allows us to record neutron component over the whole array area.

EAS array of novel type have been constructed on the base of ARGO-YBJ experiment (Tibet, China). It consists of the four specially designed scintillator en-detectors capable to measure two main EAS components: hadrons through thermal neutrons (n) and electrons (e). The results of simulation for these arrays using CORSIKA and GEANT4 codes are presented. Simulated thermal neutron and electron lateral distributions are compared with experimental data. Obtained distributions are compared with those obtained by other arrays.