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ERING DESIGN OF NUCLEAR PHYSICS EQUIPMENT

ALEGRO: A New-Generation Cherenkov Gamma Observatory E. E. Kholupenko, P. N. Aruev, D. A. Baiko, A. A. Bogdanov, G. I. Vasilyev, V. V. Zabrodskii, A. M. Krasil’shchikov*, Yu. V. Tuboltsev, and Yu. V. Chichagov Ioffe Physical-Technical Institute, Russian Academy of Sciences, ul. Politekhnicheskaya 26, St. Petersburg, 194021 Russia *e-mail: [email protected] Received May 4, 2016

Abstract—The concept of a new-generation terrestrial Cherenkov gamma observatory is proposed on the basis of the results of numerical modeling and research and development work. The key parameters of this observatory are estimated. Its primary objective should be the observation of cosmic gamma-ray sources in the 5–50 GeV energy range. Neither ground-based Cherenkov gamma observatories nor orbital gamma telescopes are presently able to perform such observations efficiently. Keywords: gamma-ray astronomy, Cherenkov radiation, Geiger avalanche photodiodes DOI: 10.1134/S1063778816130020

INTRODUCTION The observable spectrum of cosmic gamma radiation presently has a gap in the region of 5–50 GeV, where neither orbital gamma observatories nor terrestrial (Cherenkov) gamma telescopes are able to perform efficient observations of the majority of known cosmic gamma-ray sources. At lower energies (30– 500 MeV), the efficiency of current orbital gamma telescopes is sufficiently high to detect radiation from a considerable number of pulsars, supernova remnants, microquasars, and active galactic nuclei [1]. The range of 0.1–30 TeV is open to inspection with third-generation Cherenkov gamma telescopes [2]. At the same time, the interval of 5–50 GeV is crucial for understanding the nature of certain fundamental cosmic gamma sources (see, for example, [3, 4]). In order to fill the gap in data on the properties of gamma spectra of cosmic objects in the region of 5–50 GeV and enhance considerably the statistics of observations at higher energies, we propose to construct a new highaltitude Cherenkov gamma observatory (Atmospheric Low Energy Gamma-Ray Observatory, ALEGRO). BASIC TECHNIQUES AND APPROACHES TO BE USED IN THE ALEGRO PROJECT The following innovative approaches should enable observations at the energy threshold of 5 GeV, which is close to the theoretical lower limit for Cherenkov gamma astronomy in the terrestrial atmosphere [5]. High-voltage vacuum photomultipliers are used in detector units of current Cherenkov gamma telescopes to detect Cherenkov flares from extensive air showers initiated in the atmosphere of the Earth by high-

energy gamma quanta. This limits the overall duration of observation intervals: the yearly exposure time of these telescopes is approximately 1000 h. Therefore, Geiger avalanche photodiodes (see, for example, [6]), which are also known as silicon photomultipliers (SiPM), are proposed to be used in the detector units of ALEGRO telescopes. SiPM-based detectors allow one to perform observations even on moonlit nights, thus increasing considerably the exposure times for the studied c