Local muon density spectra (LMDS) at various zenith angles have been reconstructed from the data of two detectors of the Experimental complex NEVOD. The inclined muon bundles at the ground level were detected with the coordinate detector DECOR, and for the near-vertical direction with the calibration telescope system (CTS) of the Cherenkov water detector. In comparison with the earlier DECOR results, the experimental statistics has been increased by 2-3 times for different ranges of zenith angle and muon bundle multiplicity and is now based on about 40,000 h of the setup operation. The live time of measurements with CTS is about 12,000 h. It is found with both setups that the slope of LMDS is increasing above the primary energy of about 1017 eV. The details of the experiment and data analysis are presented.

Experimental data on the energy deposits of muon bundles in inclined extensive air showers (EASes), obtained at the NEVOD–DECOR installation over a long period of time, are presented. The experimental data are compared to calculations based on simulating an EAS’s muon component using the CORSIKA code.

The results from measuring EAS local muon density spectra at ultrahigh energies of primary particles are reported. These spectra are recorded by two instruments at the NEVOD experimental complex (MEPhI, Moscow). Muon bundles in inclined EAS at zenith angles exceeding 55° are registered by the DECOR coordinate detector. Mostly near-vertical muon bundles are registered at the bottom plane of scintillation counters in the calibration telescope system positioned underwater at a depth of approximately 9 m. The results from fitting the local muon density spectra measured by both instruments at primary particle energies below and above 10 17 eV confirm the presence of the second knee (an increase in the slope) in the energy spectrum of cosmic rays in this region.

A coordinate tracking unit based on drift chambers (CTUDC) for registering single muons and muon bundles at large zenith angles is created at the MEPhI. The unit consists of multi-wire drift chambers (DCs) with large drift gaps. These DCs were used earlier for the neutrino channel on the U-70 accelerator. The CTUDC consists of two coordinate planes with 8 drift chambers in each, assembled on the opposite sides of the NEVOD Cherenkov water detector (CWD). The effective area of the unit is 29.6 m 2 . The CWD trigger system and DECOR coordinate-tracking detector provide timestamps for the drift chambers. The first results from the registration of single muons and muon bundles by the unit in combination with other detectors of the NEVOD experimental complex are presented.

Experimental data on the energy deposits of muon bundles in inclined extensive air showers (EASes), obtained at the NEVOD–DECOR installation over a long period of time, are presented. The experimental data are compared to calculations based on simulating an EAS’s muon component using the CORSIKA code.

Change of the energy characteristics of muon bundles with an increase of the primary cosmic ray particles energy can be a key to solving the problem of muon excess in the extensive air showers (EAS) observed in a number of experiments. In this work the data on the energy deposit of multi-muon events in a wide range of zenith angles (and as a consequence in a wide range of primary particles energies) obtained with NEVOD-DECOR setup over a long time period are presented. The experimental data are compared with the results of simulations of EAS muon component performed using CORSIKA code.

Investigations of the energy characteristics of muon component with the increase of the primary cosmic rays energy can be a key to solving `muon puzzle' the problem of excess of EAS muons (observed in several experiments at high ALEPH, DELPHI and ultrahigh energies DECOR, Pierre Auger Observatory) in comparison with the expected flux. The measurements results of the energy deposit of inclined muon bundles in water depending on the zenith angle and the local density of muons are presented. As a measure of the energy deposit, the total number of photoelectrons registered by PMTS of the Cherenkov water calorimeter NEVOD was used. The local density of muons, which gives an estimate of the energy of primary particles was obtained from the data of coordinate-tracking detector DECOR. The experimental data are compared with the results of calculations based on simulations of the muon component of EAS by means of the CORSIKA code.

In several UHECR experiments, an excess of muons in comparison with simulations performed in frame of the widely used hadron interaction models was found. In order to solve this puzzle, investigations of muon energy characteristics in EAS are required. An experiment on the measurement of the energy deposit of muon bundles in water has been started with the NEVOD-DECOR experimental complex. Experimental results based on about 20,000 muon bundles detected during first 3,250 h of the experiment are presented. It has been found that the average specific energy deposit in the Cherenkov calorimeter (normalized to the local muon density estimated from the coordinate detector data) only slightly depends on the muon density; however, an appreciable dependence on the zenith angle has been revealed.

The first results from studying the energy deposits of inclined muon bundles in a Cherenkov water detector on the Earth’s surface are presented. The general response of the NEVOD Cherenkov calorimeter’s PMT is used as a measure of the energy deposited (proportional to the muon energy losses in the detector’s matter), while the local muon density at the point of observation is estimated using data from the DECOR coordinate-tracking detector. It is found that the energy deposit normalized to the muon density depends largely on the zenith angle. A comparison of the experimental results and those from EAS muon component simulations using the CORSIKA package shows that the average energy of muons detected in the bundles grows rapidly as the zenith angle increases, reaching ∼500 GeV near the horizon.

В НИЯУ МИФИ ведется разработка крупномасштабного координатно-трекового детектора для регистрации окологоризонтального потока мюонов космических лучей сверхвысоких энергий. Основа установки – дрейфовые камеры нейтринного детектора ускорителя У-70 ИФВЭ, которые имеют большую эффективную площадь (1.85 м2), хорошее координатное и угловое разрешение при относительно малом количестве измерительных каналов.

In several cosmic ray experiments, an excess of multi-muon events in comparison with calculations performed in frame of the widely used hadron interaction models was found. In order to solve this puzzle, investigations of muon energy characteristics in EAS are required. An experiment on the measurement of the energy deposit of muon bundles in water has been started with the NEVOD-DECOR experimental complex. The results of the analysis of the first experimental data are discussed. It has been found that the average specific energy deposit in the Cherenkov calorimeter appreciably increases with zenith angle, thus reflecting the increase of the mean muon energy in the bundles. A possible evidence for an increase of the energy deposit at primary energies above 1017 eV is observed.

A technique fot the reconstruction of cascade profiles by means of Cherenkov radiation in the water of the NEVOD detector is discussed. NEVOD is equipped with a dense spatial lattice of optical modules. The analyzed cascades have been generated either along near-horizontal muons (zenith angles between 85 and 90 degrees), which's tracks are reconstructed by means of the tracking detector DECOR, or by muons with unknown tracks over a wider zenith angle range of 50-90 degrees. Mean cascade profiles and energy spectra of cascades measured during the experimental series of about 7950 hours of 'live time' are presented.

The large-scale coordinate detector for registration of near-horizontal muon flux of ultrahigh energy cosmic rays is being developed in MEPhI. Detector is based on the drift chambers from the neutrino experiment at the IHEP accelerator U-70, their key advantages are the large effective area (1.85 m2), good coordinate and angular resolution with a small number of measuring channels. Detector will be operated as a part of the experimental complex NEVOD, in particular, jointly with Cherenkov water detector with volume of 2000 m3. The current status of the project and results of studies of drift chamber characteristics are discussed.

First results of investigations of the energy deposits of inclined muon bundles in the ground-based Cherenkov water detector NEVOD are presented. As a measure of the muon bundle energy deposit, the total number of photoelectrons detected by PMTs of the Cherenkov calorimeter is used. For each event, the local muon density at the observation point and the muon bundle arrival direction are estimated from the data of the coordinate-tracking detector DECOR. Registration of the bundles in a wide range of zenith angles allows to explore the interval of primary particle energies from ∼ 1016 to ∼ 1018 eV. Measurement results are compared with CORSIKA based simulations of EAS muon component. It is found that the mean energy of muons detected in the bundles rapidly increases with the zenith angle and reaches about 500 GeV near the horizon.