The space-based gamma-ray telescope must effectively separate photons from charged particles of instrumental background and cosmic rays. It requires that the anticoincidence system of the telescope must have high detection efficiency, large dynamic range and good enough energy and time resolution for charged particles. The main results obtained using 246 MeV secondary positron beam of synchrotron S-25R “PAKHRA” of Lebedev Physical Institute with prototype of system of anticoincidence detectors of space-based gamma-ray telescope GAMMA-400 are presented. The amplitude resolution, time resolution and charged particles detection efficiency are adduced. All measurements were performed using “fast” output of silicon photomultipliers of prototype scintillation detectors sensors. Fractal dimensions of temporal profiles registered during measurements using positron beam and atmospheric muons are discussed.

The future space-based GAMMA-400 scientific observatory is being developed under the Federal Space Program of Russian Federation in the period from 2016-2025. This observatory includes a γ-ray telescope, which will detect gamma rays in the energy range from ~20 MeV to several TeV with high angular and energy resolutions and cosmic-ray electrons + positrons up to several tens of TeV using main and lateral appertures. Recent experimental observations indicate a possible existence of a break in the electrons + positrons spectrum at around TeV energies. A number of speculations to explain this phenomenon have arisen. This article presents the method of high-energy electrons detection from the lateral aperture of the GAMMA-400 gamma-ray telescope. This method implements the machine learning analysis and provides the high-energy proton background rejection at the level of 10$^4$ in the energy range from 100 GeV to 10 TeV. The effective acceptance for electron detection with such proton rejection is about 0.52 m$^2$×sr for the four lateral sides of the GAMMA-400 gamma-ray telescope. This effective acceptance exceeds by several times that of CALET and DAMPE experiments. This capability of our instrument will allow to improve significantly the measurements of electron + positron flux above 1 TeV and, hence, understand better the debatable spectrum break.

The GAMMA-400 gamma-ray telescope is the successor of Soviet and Russian gamma-ray telescopes. GAMMA-400 is being developed for cosmophysical research in accordance with the Russian Federal Space Program 2016–2025. The GAMMA-400 experiment will be implemented aboard the Russian astrophysical space observatory in a highly elliptic orbit during 7 years to provide new data on gamma-ray emission mainly from the Galactic plane, Galactic Center, the Sun and cosmic-ray electron + positron fluxes. The main mode of observations will be the continuous point-source mode with the duration of up to ∼ 100 days. The GAMMA-400 gamma-ray telescope will study high-energy gamma-ray emission up to several TeV and cosmic-ray electrons + positrons up to 20 TeV. GAMMA-400 will have the never-achieved angular resolution, the high-energy and time resolutions, as well as very good separation efficiency of gamma rays from cosmic-ray background and of electrons + positrons from protons. The distinctive features of GAMMA-400 are the excellent angular resolution of ∼0.01^∘ at E_γ=100 GeV that exceeds resolutions of the space-based and ground-based gamma-ray telescopes by a factor of 5–10, as well as high-energy resolution of ∼2% at E_γ=100 GeV. GAMMA-400 studies can discover gamma-ray emission from annihilation or decay of dark matter particles, identify many unassociated discrete sources, explore the structure of extended sources, search for gamma-ray bursts and solar gamma-ray flares, improve the data on cosmic-ray electron + positron spectra for energies of >50 GeV.

The space observatory GAMMA-400 is processed currently in accordance with the Federal Space Program of the Russian Federation for 2016–2025. The observatory includes a gamma-ray telescope for experimental studies of gamma rays in the energy range from ~20 MeV to ~1 TeV with high angular and energy resolution, as well as for research of electrons + positrons at energies above 100 GeV in both the main (top-down), and lateral apertures. At present time, there are experimental indications concerning the possibility of existing of spectrum break in electrons + positrons intensities about TeV energies. This point stimulates several speculations to explain such phenomena. In this paper we examined capabilities of GAMMA-400 telescope to explore this problem. The methods for electron detection in the energy range from 100 GeV up to 10 TeV from the lateral aperture of a gamma-ray telescope are presented. Also, the results of calculation for proton rejection factor and for electron acceptance are revealed.

The upcoming GAMMA-400 experiment will be implemented aboard the Russian astrophysical space observatory, which will be operating in a highly elliptical orbit over a period of 7 years to provide new data on gamma-ray emissions and cosmic-ray electron + positron fluxes, mainly from the galactic plane, the Galactic Center, and the Sun. The main observation mode will be a continuous point-source mode, with a duration of up to ~100 days. The GAMMA-400 gamma-ray telescope will study high-energy gamma-ray emissions of up to several TeV and cosmic-ray electrons + positrons up to 20 TeV. The GAMMA-400 telescope will have a high angular resolution, high energy and time resolutions, and a very good separation efficiency for separating gamma rays from the cosmic-ray background and the electrons + positrons from protons. A distinctive feature of the GAMMA-400 gamma-ray telescope is its wonderful angular resolution for energies of >30 GeV (0.01° for Eγ = 100 GeV), which exceeds the resolutions of space-based and ground-based gamma-ray telescopes by a factor of 5–10. GAMMA-400 studies can reveal gamma-ray emissions from dark matter particles’ annihilation or decay, identify many unassociated, discrete sources, explore the extended sources’ structures, and improve the cosmic-ray electron + positron spectra data for energies of >30 GeV.

The characteristics of the prototype of the scintillation detecting segment of time-of-flight and anticoincidence systems of being developed space-based GAMMA-400 gamma-ray telescope is studied. The amplitude resolution, time resolution and charged particle detection efficiency of the prototype with silicon photomultipliers readout obtained using ∼ 250 MeV positron beam of synchrotron C-25P ‘‘PAKHRA’’ of P.N. Lebedev Physical Institute are presented. The comparison of applying both ‘‘standard’’ and ‘‘fast’’ outputs of silicon photomultipliers type ON Semiconductor MICROFC-60035-SMT used in the prototype is featured.

The currently developing space-based gamma-ray telescope GAMMA-400 will measure the gamma-ray and electron + positron fluxes using the main top-down aperture in the energy range from ~20 MeV to several TeV in a highly elliptic orbit (without shading the telescope by the Earth and outside the radiation belts) continuously for a long time. The instrument will provide fundamentally new data on discrete gamma-ray sources, gamma-ray bursts (GRBs), sources and propagation of Galactic cosmic rays and signatures of dark matter due to its unique angular and energy resolutions in the wide energy range. The gamma-ray telescope consists of the anticoincidence system (AC), the converter-tracker (C), the time-of-flight system (S1 and S2), the position-sensitive and electromagnetic calorimeters (CC1 and CC2), scintillation detectors (S3 and S4) located above and behind the CC2 calorimeter and lateral detectors (LD) located around the CC2 calorimeter. In this paper, the capabilities of the GAMMA-400 gamma-ray telescope to measure fluxes of GRBs from lateral directions of CC2 are analyzed using Monte-Carlo simulations. The analysis is based on off-line second-level trigger construction using signals from S3, CC2, S4 and LD detectors. For checking the numerical algorithm the data from space-based GBM and LAT instruments of the Fermi experiment are used, namely, three long bursts: GRB 080916C, GRB 090902B, GRB 090926A and one short burst GRB 090510A. The obtained results allow us to conclude that from lateral directions the GAMMA-400 space-based gamma-ray telescope will reliably measure the spectra of bright GRBs in the energy range from ~10 to ~100 MeV with the on-axis effective area of about 0.13 m2 for each of the four sides of CC2 and total field of view of about 6 sr.

The future space-based GAMMA-400 gamma-ray telescope will operate onboard the Russian astrophysical observatory in a highly elliptic orbit during 7 years to observe Galactic plane, Galactic Center, Fermi Bubbles, Crab, Vela, Cygnus X, Geminga, Sun, and other regions and measure gamma- and cosmic-ray fluxes. Observations will be performed in the point-source mode continuously for a long time (~100 days). GAMMA-400 will measure gamma rays in the energy range from ~20 MeV to several TeV and cosmic-ray electrons + positrons up to several tens TeV. GAMMA-400 instrument will have very good angle and energy resolutions, high separation efficiency of gamma rays from cosmic-ray background, as well as electrons + positrons from protons. The main feature of GAMMA-400 is the unprecedented angular resolution for energies >30 GeV better than the space-based and ground-based gamma-ray telescopes by a factor of 5-10. GAMMA-400 observations will permit to resolve gamma rays from annihilation or decay of dark matter particles, identify many discrete sources, clarify the structure of extended sources, specify the data on cosmic-ray electron + positron spectra.

The future space-based GAMMA-400 $$\gamma$$ -ray telescope will operate onboard the Russian astrophysical observatory in a highly elliptic orbit during 7 years. Observing $$\gamma$$ -ray sources from Galactic plane, $$\gamma$$ -ray bursts, $$\gamma$$ -ray diffuse emission, $$\gamma$$ rays from the Sun, and $$\gamma$$ rays from dark matter particles will be performed uninterruptedly for a long time ( $${\sim}$$ 100 days) in point-source mode in contrast to scanning mode for Fermi-LAT and other space- and ground-based instruments. GAMMA-400 will measure $$\gamma$$ rays in the energy range from $${\sim}$$ 20 MeV to several TeV units, have the unprecedented angular ( $${\sim}0.01^{\circ}$$ at $$E_{\gamma}=100$$ GeV) and energy ( $${\sim}2{\%}$$ at $$E_{\gamma}=100$$ GeV) resolutions better than for Fermi-LAT, as well as ground-based $$\gamma$$ -ray facilities, by a factor of 5–10, and perfectly separate $$\gamma$$ rays from cosmic-ray background.

The Spin Physics Detector, a universal facility for studying the nucleon spin structure and other spin-related phenomena with polarized proton and deuteron beams, is proposed to be placed in one of the two interaction points of the NICA collider that is under construction at the Joint Institute for Nuclear Research (Dubna, Russia). At the heart of the project there is huge experience with polarized beams at JINR. The main objective of the proposed experiment is the comprehensive study of the unpolarized and polarized gluon content of the nucleon. Spin measurements at the Spin Physics Detector at the NICA collider have bright perspectives to make a unique contribution and challenge our understanding of the spin structure of the nucleon. In this document the Conceptual Design of the Spin Physics Detector is presented.

A prototype of the electromagnetic calorimeter for the GAMMA-400 γ-ray telescope has been calibrated at the Pakhra S-25R electron synchrotron of the Lebedev Physical Institute. The measured energy resolution of the GAMMA-400 calorimeter is consistent with the results of the Monte Carlo simulation. The applicability of the Pakhra S-25R accelerator for calibrating detectors in various experiments has been confirmed.

The GAMMA-400 gamma-ray telescope is planned for the launch at the end of this decade on the Navigator service platform designed by Lavochkin Association on an elliptical orbit with following initial parameters: an apogee ~300000, a perigee ~500 km, a rotation period ~7 days and inclination of 51.4°. The apparatus is expected to operate more than 5 years, reaching an unprecedented sensitivity for the search of dark matter signatures and the study of the unresolved and so far unidentified gamma-ray sources. An electronics system, which consists of 14 front-end multichannel electronics modules and the main processing unit with a total power consumption of about 400 W (74W for main processing unit), has been developed for providing fast trigger and veto for the data taking to the experiment. The communication between front-end modules, main processing unit and scientific data acquisition system of the gamma-ray telescope is performed via high-speed SPACEWIRE network. To assure the long-term reliability in space environment, a series of critical issues such as the radiation hardness, thermal design, components and board level quality control, warm and cold redundancy are taken into consideration. The main design concepts for the system, measurements setups together with some test results are presented.

The future GAMMA-400 γ-ray telescope will provide fundamentally new data on discrete sources and spectra of γ-ray emissions and electrons + positrons due to its unique angular and energy resolutions in the wide energy range from 20 MeV up to several TeV. The γ-ray telescope consists of the anticoincidence system (AC), the converter-tracker (C), the time-of-flight system (S1 and S2), the position-sensitive and electromagnetic calorimeters (CC1 and CC2), the scintillation detectors of the calorimeter (S3 and S4) and lateral anticoincidence detectors of the calorimeter (LD). To extend the GAMMA-400 capabilities to measure γ-ray bursts, Monte-Carlo simulations were performed for lateral aperture of the one of the versions of GAMMA-400. Second-level trigger based on signals from CC2, LD, S3, and S4 allows us to detect γ-ray bursts in the energy range of ~10-300 MeV with high effective area about 1 m 2 .

Our paper reviews the planned space-based gamma-ray telescope GAMMA-400 and evaluates in details its opportunities in the field of dark matter (DM) indirect searches. We estimated the GAMMA-400 mean sensitivity to the diphoton DM annihilation cross section in the Galactic center for DM particle masses in the range of 1–500 GeV. We obtained the sensitivity gain at least by 1.2–1.5 times (depending on DM particle mass) with respect to the expected constraints from 12 years of observations by Fermi-LAT for the case of Einasto DM density profile. The joint analysis of the data from both telescopes may yield the gain up to 1.8–2.3 times. Thus the sensitivity reaches the level of annihilation cross section ⟨ σ v ⟩γγ(mχ = 100 GeV) ≈ 10−28 cm3/s. This will allow us to test the hypothesized narrow lines predicted by specific DM models, particularly the recently proposed pseudo-Goldstone boson DM model. We also considered the decaying DM—in this case the joint analysis may yield the sensitivity gain up to 1.1–2.0 times reaching the level of DM lifetime τγν(mχ = 100 GeV) ≈ 2 ⋅ 1029 s. We estimated the GAMMA-400 sensitivity to axion-like particle (ALP) parameters by a potential observation of the supernova explosion in the Local Group. This is very sensitive probe of ALPs reaching the level of ALP-photon coupling constant gaγ ∼ 10−13 GeV−1 for ALP masses ma ≲ 1 neV. We also calculated the sensitivity to ALPs by constraining the modulations in the spectra of the Galactic gamma-ray pulsars due to possible ALP-photon conversion. GAMMA-400 is expected to be more sensitive than the CAST helioscope for ALP masses ma ≈ (1−−10) neV reaching gaγmin ≈ 2 ⋅ 10−11 GeV−1. Other potentially interesting targets and candidates are briefly considered too.

The GAMMA-400 gamma-ray telescope is planned for the launch at the end of 2026 on the Navigator service platform designed by Lavochkin Association on an elliptical orbit with following initial parameters: an apogee $${\sim}$$300 000, a perigee $${\sim}$$500 km, a rotation period $${\sim}$$7 days and inclination of 51.4$${}^{\circ}$$. The apparatus is expected to operate for more than 5 years, reaching an unprecedented sensitivity for the search of dark matter signatures and the study of the unresolved and so far unidentified gamma-ray sources. The segmented anticoincidence counters surround the converter-tracker and calorimeter of the telescope with the purpose of vetoing to assure a clean track reconstruction and charged particle background suppression. The anticoincidence detector prototype based on long BC-408 scintillator with silicon photomultipliers readout was tested using 300-MeV positron beam of synchrotron C-25P ‘‘PAKHRA’’ of Lebedev Physical Institute. The measurement setup, design concepts for the prototype detector together with test results are discussed.