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The measurement of thermal neutrons generated by cosmic ray extensive air showers (EAS) on the Earth’s surface provides a new method for studying the composition and energy of cosmic rays with energy in the so-called “knee” region. Given the success of the PRISMA-YBJ array, a new type of thermal neutron detector array was installed at the high-altitude cosmic ray observatory in Yangbajing, Tibet. The array consists of 16 EN-detectors and utilizes a new type scintillator based on a compound alloy of ZnS(Ag) and B2O3 with natural boron. This report describes the deployment of the detector array and its operational results.

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 new results were obtained by the array of EN-detectors (Electron and Neutron detectors) developed in the frame of the PRISMA (PRImary Spectrum Measurement Array) project for Extensive Air Showers detection. Our EN-detectors running both on the Earth surface and underground are continuously measuring the environmental thermal neutron flux. Neutrons are partially produced by radioactive gas radon and its daughter decays through (α,n)-reactions in soil close to the detectors. Then neutrons thermalize in media and, being in equilibrium with it, they are sensitive to many geo-dynamic phenomena including earthquakes. In this work the EN-detectors were measuring the variations of an environmental neutron flux in Tibet (30.11 N, 90.53 E, 4300 m a.s.l) at a distance of ∼600 km from the collision zone of the Asian-Indian plates subduction zone (Nepal region). We have observed some anomalies in the dynamics of the neutron flux around the time of the catastrophic earthquakes of magnitude M = 7.8 happened in Gorkha (Nepal) on 25.04.2015 followed by a series of aftershocks of M > 6. The use of nuclear physics methods can provide novel results in geophysics and this work demonstrates the sensitivity of the environmental thermal neutron flux to changes in tense-deformed crust conditions caused by earthquakes with epicentral distances greater than 500 km.

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.

The Experimental complex (EC) NEVOD includes a number of unique experimental facilities for studies of main components of cosmic rays on the Earth's surface. The complex is used for basic research of CR flux characteristics and their interactions in the energy range 1015 – 1019 eV. To extend the experimental capabilities nowadays new large-scale detectors are being deployed around the EC NEVOD: an array for the EAS registration (NEVOD-EAS), a detector of atmospheric neutrons (URAN) and a large-area coordinate-tracking detector (TREK). The description of experimental capabilities of the complex is given, and first results of simultaneous detection of various EAS components are discussed.

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.

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.

The results from a 29-month investigation into the EAS electromagnetic component using the PRISMA-32 array are presented, notably for the central density spectrum and energy deposit spectrum. The array consists of 32 en-detectors in which specialized inorganic scintillators (SL6-5 phosphorus composition) based on silver-activated zinc sulfide (ZnS) with LiF enriched to 90% 6Li (ZnS(Ag) + 6LiF) are used as detecting layers. The possibility of detecting the electromagnetic component using a thin scintillation neutron detector with the multiple passage of charged particles is shown. The experimental data are compared to the results from simulations.

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

All astrophysical explanations of the knee in cosmic ray spectrum accept the hypothesis of its existence {\it a priori} without any doubt. But, there exist experimental evidences against this hypothesis. Experimental data on the knee in various secondary cosmic ray components obtained by KASCADE array as well as by many other arrays reveal the existence of the knee ($\Delta\gamma\approx0.4\div0.5$) in electromagnetic component (e and $\gamma$ including Cherenkov light) and the absence of it or a very small knee ($\Delta\gamma\approx0.1\div0.2$) in muonic and hadronic components. But, one could expect the inverse relationship in case of the astrophysical knee origin. A brief review of experimental data concerning this problem is given., Comment: 4 pages, 1 table. Submitted to Nucl. Phys. B (Proc. Suppl.)

The problem of the knee in primary cosmic ray at energy about 3–5 PeV is the most exciting problem in cosmic ray physics. Since 1958, physicists have been trying to solve this problem. In our opinion, the problem could be solved from the experimental point of view, whereas the primary spectrum would follow a pure power law. A key to the "knee" problem lies in the hadronic structure of EAS and its propagation in the Earth's atmosphere. Neither exotic processes nor new physics are used. An explanation of the approach and some results of Monte Carlo simulations are given below.

The origin of the neutron bursts in Extensive Air Showers (EAS) is explained using results of the experiments and CORSIKA based Mont–Carlo simulations. It is shown that events with very high neutron multiplicity observed last years in neutron monitors as well as in surrounding detectors, are caused by the usual EAS core with primary energies >1 PeV . No exotic processes were needed for the explanation.

A novel type of Extensive Air Shower (EAS) array is proposed and described. It is shown that only new approaches to the so called "knee problem" could solve this complicated and old problem., Comment: 4 pages, 2 figures, 1 table. A talk given to ISVHECRI2008. To be published in Nucl. Phys. B (Proc. Suppl.)