The SPHERE-2 balloon-borne detector designed for extensive air shower (EAS) observations using EAS optical Vavilov-Cherenkov radiation (“Cherenkov light”), reflected from the snow-covered surface of Lake Baikal is described. We briefly discuss the concept behind the reflected Cherenkov light method, characterize the conditions at the experimental site and overview the construction of the tethered balloon used to lift the SPHERE-2 telescope above the surface. This paper is mainly dedicated to a detailed technical description of the detector, including its optical system, sensitive elements, electronics, and data acquisition system (DAQ). The results of some laboratory and field tests of the optical system are presented.

A compact device lifted over the ground surface might be used to observe optical radiation of extensive air showers (EAS). Here we consider spatial and temporal characteristics of Vavilov-Cherenkov radiation ("Cherenkov light") reflected from the snow surface of Lake Baikal, as registered by the SPHERE-2 detector. We perform detailed full direct Monte Carlo simulations of EAS development and present a dedicated highly modular code intended for detector response simulations. Detector response properties are illustrated by example of several model EAS events. The instrumental acceptance of the SPHERE-2 detector was calculated for a range of observation conditions. We introduce the concept of "composite model quantities", calculated for detector responses averaged over photoelectron count fluctuations, but retaining EAS development fluctuations. The distortions of EAS Cherenkov light lateral distribution function (LDF) introduced by the SPHERE-2 telescope are understood by comparing composite model LDF with the corresponding function as would be recorded by an ideal detector situated at the ground surface. We show that the uncertainty of snow optical properties does not change our conclusions, and, moreover, that the expected performance of the SPHERE experiment in the task of cosmic ray mass composition study in the energy region similar to 10 PeV is comparable with other contemporary experiments. Finally, we compare the reflected Cherenkov light method with other experimental techniques and briefly discuss its prospects. (C) 2019 Elsevier B.V. All rights reserved.

The design for a balloon instrument to study the energy spectrum and mass composition of primary cosmic rays at energies exceeding 1018 eV is presented. It is planned to conduct the experiment during Antarctica’s polar night. The equipment allows the separate registration of fluorescent light (FL) and Cherenkov radiation (CR) in each event. The advantages of the experiment over existing ground-based installations and future orbiting stations are discussed. A way of separating FL from CR with light filters and optical silicon detectors is described.

An absolute calibration method for the PMT mosaic used in the SPHERE-2 experiment is presented. The method is based on the relative calibration of all PMTs in the mosaic to a single stable PMT, incorporated in it, during each measurement event and subsequent absolute calibration of that single PMT using a known stable light source. The results of the SPHERE-2 detector PMTs calibration are presented and are discussed.

Although a large number of experiments were carried out during the last few decades, the uncertainty in the spectrum of all nuclei of primary cosmic rays (PCRs) with superhigh energies is still high, and the results of many experiments on nuclear composition of PCRs are contradictory. An overview of the SPHERE experiment on detecting Vavilov-Cherenkov radiation from extensive air shower (EAS) reflected from a ground snow surface is given. A number of experimental studies implementing this method are presented and their results are analyzed. Some other popular methods of studying PCRs with superhigh energies (E (0) > 10(15) eV) and their main advantages and drawbacks are briefly considered. The detecting equipment of the SPHERE-2 experiment and the technique of its calibration are considered. The optical properties of snow, which are important for experiments on reflected Cherenkov light (CL) from EAS, are discussed and the history of observing reflected EAS CL is described. The algorithm of simulating the detector response and calculating the fiducial acceptance of shower detection is described. The procedure of processing the experimental data with a subsequent reconstruction of the spectrum of all PCR nuclei and analysis of the mass composition is shown. The first results of reconstructing the spectrum and separating groups of cosmic-ray nuclei with high energies in the SPHERE-2 experiment are presented. Main sources of systematic errors are considered. The prospects of developing the technique of observation of reflected EAS CL in future experiments are discussed.

Представлен новый метод оценки типа частиц первичных космических лучей в области энергий 10–1000 ПэВ для индивидуальных событий, зарегистрированных установкой СФЕРА-2. Метод основан на сравнении образов зарегистрированных событий с моделированными событиями в предположении различных типов первичной частицы с учетом погрешностей измерения. Исследование направлено на поиск предела чувствительности в определении химического состава первичных космических лучей сверхвысоких энергий при использовании метода регистрации отраженного черенковского света широких атмосферных ливней.

We give an overview of the SPHERE experiment based on detection of reflected Vavilov-Cherenkov radiation (Cherenkov light) from extensive air showers in the energy region E>10^{15} eV. A brief history of the reflected Cherenkov light technique is given; the observations carried out with the SPHERE-2 detector are summarized; the methods of the experimental datasample analysis are described. The first results on the primary cosmic ray all-nuclei energy spectrum and mass composition are presented. Finally, the prospects of the SPHERE experiment and the reflected Cherenkov light technique are given.

We present an event-by-event study of cosmic ray (CR) composition with the reflected Cherenkov light method. The fraction of CR light component above 5 PeV was reconstructed using the 2013 run data of the SPHERE experiment which observed optical Vavilov-Cherenkov radiation of extensive air showers, reflected from snow surface of Lake Baikal. Additionally, we discuss a possibility to improve the elemental groups separability by means of multidimensional criteria.

A new method for assessing the type of particles of primary cosmic rays in the energy range of 10–1000 PeV for individual events recorded by the SPHERE-2 facility is presented. The method is based on comparing images of recorded events and simulated events, while assuming various types of primary particles with allowance for measuring errors. The aim of the study is to find the limits of sensitivity in determining of the chemical composition of ultrahigh-energy primary cosmic rays using the detection of reflected Cherenkov light generated by extensive air showers (EASes).

First preliminary results of the balloon-borne experiment SPHERE-2 on the all-nuclei primary cosmic rays (PCR) spectrum and primary composition are presented. The primary spectrum in the energy range 1016–5 · 1017 eV was reconstructed using characteristics of Vavilov-Cherenkov radiation of extensive air showers (EAS), reflected from a snow surface. Several sources of systematic uncertainties of the spectrum were analysed. A method for separation of the primary nuclei' groups based on the lateral distribution function' (LDF) steepness parameter is presented. Preliminary estimate of the mean light nuclei' fraction f30-150 at energies 3 · 1016–1.5 · 1017 eV was performed and yielded f30-150 = (21±11) %.

Here is presented the current state of the SPHERE-2 balloon-borne experiment. The detector is elevated up to 1 km above the snow surface and registers the reflected Vavilov-Cherenkov radiation from extensive air showers. This method has good sensitivity to the mass-composition of the primary cosmic rays due to its high resolution near the shower axis. The detector consists of a 1500 mm spherical mirror with a 109 PMT cluster in its focus. The electronics record a signal pulse profile in each PMT. In the last 2 years the detector was upgraded: time resolution of pulse registration was enhanced up to 12.5 ns, channel sensitivity was increased by a factor of 3, a new LED-based relative PMT calibration method was introduced, and new hardware and etc. was installed.

The SPHERE-2 detector was lifted above the snow-covered surface of Lake Baikal by a captive balloon several times in 2010. The Vavilov-Cherenkov radiation of extensive air showers was measured. Preliminary results from processing the data of the SPHERE-2 experiment at various altitudes of observation are presented.

В начале 2010 г. проведено несколько подъемов установки СФЕРА-2 с помощью привязного аэростата над заснеженной поверхностью озера Байкал. Были проведены измерения черенковского света широких атмосферных ливней (ШАЛ). Приведены предварительные результаты обработки данных эксперимента СФЕРА-2 на различных высотах наблюдения.

The expedition carried out; in March, 2008 to Lake Baikal became an important stage in the development of the SPHERE experiment. During the expedition the SPHERE-2 installation was hoisted, for the first time, on a tethered balloon, APA, to a height of 700 to over the lake surface covered with ice and snow. A series of test measurements were made. Preliminary results of the data processing are presented. The next plan of the SPHERE experiment is to begin a set of statistics for constructing the CR spectrum in the energy range 10(16) - 10(18) eV.

Criteria for selecting proton events among the total sequence of events from primary nuclei of cosmic rays with zenith angles θ < 20° are analyzed in the energy region of E 0 ≈ 1016 eV. These criteria are concretized for the case of the SPHERE-2 experiment geometry. The QGSJET-I and QGSJET-II model calculations show that the criteria based on the shape of the transverse distribution of Cherenkov light allow detection of more than 10% of proton events and rejection of 99% nuclear events.