Scientific tasks and present status of the GAMMA-400 project

The GAMMA-400 telescope is designed to investigate discrete high-energy gamma-ray sources in the energy range of 0.1–3000 GeV, to measure the energy spectra of galactic and extragalactic diffuse gammaray emissions, and to study gamma-ray bursts and gamma-ray emissions from an active Sun. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. Its special assignment is to measure fluxes of gamma rays, electrons, and positrons that could be associated with the annihilation or decay of dark matter particles.     

Capabilities of the Gamma-400 Gamma-ray Telescope for Observation of Electrons and Positrons in the TeV Energy Range

The space-based GAMMA-400 gamma-ray telescope will measure the fluxes of gamma rays in the energy range from ∼20 MeV to several TeV and cosmic-ray electrons and positrons in the energy range from several GeV to several TeV to investigate the origin of gamma-ray sources, sources and propagation of the Galactic cosmic rays and signatures of dark matter. The instrument consists of an anticoincidence system, a converter-tracker (thickness one radiation length, 1 X₀), a time-of-flight system, an imaging calorimeter (2 X₀) with tracker, a top shower scintillator detector, an electromagnetic calorimeter from CsI(Tl) crystals (16 X₀) with four lateral scintillation detectors and a bottom shower scintillator detector. In this paper, the capability of the GAMMA-400 gamma-ray telescope for electron and positron measurements is analyzed. The bulk of cosmic rays are protons, whereas the contribution of the leptonic component to the total flux is ∼10⁻³ at high energy. The special methods for Monte Carlo simulations are proposed to distinguish electrons and positrons from proton background in the GAMMA-400 gamma-ray telescope. The contribution to the proton rejection from each detector system of the instrument is studied separately. The use of the combined information from all detectors allows us to reach a proton rejection of up to ∼1 × 10⁴.

     

Characteristics of the GAMMA-400 gamma-ray telescope for searching for dark matter signatures

The GAMMA-400 gamma-ray telescope currently under development is designed to measure fluxes of gamma rays and electron-positron cosmic-ray components, which could be associated with the annihilation or decay of dark matter particles, and to survey in detail the celestial sphere in order to search for and investigate discrete gamma-ray sources; to measure the energy spectra of Galactic and extragalactic dif- fuse gamma-ray emissions; and to study gamma-ray bursts and the gamma-ray emissions of active Sun. The GAMMA-400 energy range is 100 MeV to 3000 GeV. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. The GAMMA-400 will be installed on Russia’s Navigator space platform. Observations are planned to commence in 2018.     

Method for reconstructing the gamma-ray arrival direction in the converter + calorimeter system

When designing devices of modern gamma astronomy, one of the most important problems is achieving the maximum possible angular resolution. This study is devoted to the method for reconstructing the arrival direction of primary gamma-rays with energies E _γ > 10 GeV in the GAMMA-400 satellite experiment. By the example of the GAMMA-400 gamma telescope, the possibility of improving the angular resolution of gamma telescopes incorporating a “converter + calorimeter” system is shown. The dependence of the angular resolution on the step of silicon strips used to determine the coordinates of incident particles in the converter and calorimeter and the distance between the converter and calorimeter is analyzed.     

Atmospheric gamma rays in the energy region 40–1000 GeV

A large stack of lead-emulsion sandwich detector assembly was flown over Hyderabad, India. High energy gamma rays at the float altitude were unambiguously identified from the cascades they induced, and their energies reliably determined by improved methods. From an analysis of 163 gamma rays of energy ≳ 30 GeV, it is found that the differential energy spectrum is represented by the power lawJ _r (E)= 129·4E ^{−2·62±0·12} photons m⁻² sr⁻¹sec⁻¹ GeV⁻¹ at an effective atmospheric depth of 14·3 g cm⁻²; this is the first reliable balloon measurement of atmospheric gamma rays in the energy range 40–1000 GeV. After correcting for the gamma rays radiated by the primary cosmic ray electrons, the production spectrum of gamma rays, resulting from the collisions of cosmic ray nuclei with air nuclei, at the top of the atmosphere isP _r (E, 0)=8·2 × 10⁻⁴ E^2.60±0.09 photons g⁻¹sr⁻¹sec⁻¹ GeV⁻¹. The atmospheric propagation of the electromagnetic component due to the cascade process is also derived from the gamma ray production spectrum.     

Barrel calorimeter of the CMD-3 detector

The structure of the barrel calorimeter of the CMD-3 detector is presented in this work. The procedure of energy calibration of the calorimeter and the method of photon energy restoration are described. The distinctive feature of this barrel calorimeter is its combined structure; it is composed of two coaxial subsystems: a liquid xenon calorimeter and a crystalline CsI calorimeter. The calorimeter spatial resolution of the photon conversion point is about 2 mm, which corresponds to an angular resolution of ～6 mrad. The energy resolution of the calorimeter is about 8% for photons with energy of 200 MeV and 4% for photons with energy of 1 GeV.     

Hybrid method for detecting cascades produced by ultrahigh-energy cosmic particles using a circumlunar satellite

A hybrid method for detecting cosmic rays and neutrino cascades using the radio method and the conventional method for detecting cascade particles was proposed. Cascades produced in the lunar soil near the surface by ultrahigh-energy cosmic rays and neutrinos in the energy range of 1 GeV–100 TeV, coming from above at different angles, were calculated. The calculated energy and angular distributions were extrapolated to the energy region of 10²⁰ eV. Using these results, the detection threshold was estimated as 10²⁰ eV which is approximately identical to the threshold for the radio detector previously considered by the authors.     

Measurement of Angular Distributions of Gamma Rays from the Inelastic Scattering of 14.1-MeV Neutrons by Carbon and Oxygen Nuclei

The results obtained by measuring the angular and energy distributions of gamma rays originating from the inelastic scattering of 14.1-MeV neutrons by carbon and oxygen nuclei are presented. The measurements in question were performed by the tagged-neutron method in a beam of an ING-27 standard portable neutron generator. The angular distributions of gamma rays emitted by the 2⁺ state of ¹²С at 4.43 MeVand the 3^? state of ¹⁶O at 6.13 MeV were obtained.     

Investigation of Inelastic Neutron Scattering on 27Al Nuclei

The results obtained by measuring the angular and energy distributions of gamma rays produced in reactions induced by the inelastic scattering of 14.1-MeV neutrons on ²⁷Al nuclei are presented. The respective measurements were performed by the tagged-neutron method in a beam from the ING-27 compact neutron generator. The angular distributions were obtained for gamma rays emitted from the 844-keV 1/2⁺, 1015-keV 3/2⁺, 2212-keV 7/2⁺, and 3004-keV 9/2⁺ states of ²⁷ Al nuclei.

     

Spatial distribution of albedo particles on altitudes ∼500 km

Spatial distributions of energetic charged particles, neutrons and gamma rays at altitude 500 km (below the radiation belts of the Earth), obtained by the measurements of two apparatuses on board the Intercosmos-17 satellite, are presented. The latitudinal dependences, [i.e. the variation of flux with vertical cut-off rigidity of the measurement point], for neutrons (E _n = 1 –30 MeV), gamma rays (E =0·15–6 MeV), secondary electrons (E _e > 100 MeV) and for primary protons coming from the west and the east, respectively, are given. The main characteristic, the ratioN _p/N _e of the counting rate of the particles in the polar regionN _p(R_vert< <0·1 GeV/c) and on the equatorN _e(R_vert > 16 GeV/c), is obtained for the various types of particles. This value is 10 for neutrons, 3.7 for gamma rays, 1·8 for electrons, 11 for protons in westward direction, 10 for protons in eastward direction. The latitude profile of neutrons and gamma rays is in a good agreement with calculations assuming their production by nuclear reactions of primary cosmic rays with nuclei of the atmosphere. The weakening of rigidity dependence of protons coming from east in comparison with those coming from the west, is interpreted as the cause of additional proton albedo flux. The equality of albedo electron fluxes (E_e = 100–3500 MeV) from these directions is observed. With the use of the shadowing effect the obtained data on electron-positron component are consistent with the flux of albedo positrons (E_e + > 3·5 GeV) of the value (0·5±0·2) m^–2. s^–1. ster^–1. The possibility of abundance of albedo positrons above electrons at these altitudes for the energy intervalE=0·2÷0·3 GeV is indicated.     

Performance of LHAASO-WCDA and observation of the Crab Nebula as a standard candle

The first Water Cherenkov detector of the LHAASO experiment (WCDA-1) has been operating since April 2019. The data for the first year have been analyzed to test its performance by observing the Crab Nebula as a standard candle. The WCDA-1 achieves a sensitivity of 65 mCU per year, with a statistical threshold of 5 \begin{document}$\sigma$\end{document}. To accomplish this, a 97.7% cosmic-ray background rejection rate around 1 TeV and 99.8% around 6 TeV with an approximate photon acceptance of 50% is achieved after applying an algorithm to separate gamma-induced showers. The angular resolution is measured using the Crab Nebula as a point source to be approximately 0.45° at 1 TeV and better than 0.2° above 6 TeV, with a pointing accuracy better than 0.05°. These values all match the design specifications. The energy resolution is found to be 33% for gamma rays around 6 TeV. The spectral energy distribution of the Crab Nebula in the range from 500 GeV to 15.8 TeV is measured and found to be in agreement with the results from other TeV gamma ray observatories.     

Messung einiger (n, γ)-Spektren mit einem Paarspektrometer hoher Empfindlichkeit

A pairspectrometer of the 180°-focussing type with high efficiency and good resolution is described. The efficiency has been determined in function of the energy, both theoretically and experimentally and it is possible, therefore, to use this spectrometer for absolute calibration of gamma rays above 2·5 MeV. The evaluation methods of the pair spectra allow the single line to be determined with an error as little as ±8 keV (from 2·5 to 9 MeV). We have measured the neutron capture gamma spectrum of J¹²⁷, Cs¹³³, Ir^191,193, Bi²⁰⁰ and given the energy and intensity of the resolved gamma rays. From these results it is possible to compute the binding energy of the last neutron and also some unknown levels. The experiments, which have been performed in connection with the pairspectrometer, have shown an interesting possibility for research work with reactors. With an appropriate experimental arrangement and some special neutron capture gamma spectra, monochromatic gamma rays in the energy range between 3 and 11 MeV are obtained, which are better and much more intense as corresponding gamma rays from (p, γ) reactions.     

Scission gamma rays

Gamma rays probably emitted by the fissioning nucleus ²³⁶U* at the instant of the break of the neck or within the time of about 10⁻²¹ s after or before this were discovered in the experiment devoted to searches for the effect of rotation of the fissioning nucleus in the process ²³⁵U($ \vec n $ \vec n , γf) and performed in a polarized beam of cold neutrons from the MEPHISTO Guideline at the FRM II Munich reactor. Detailed investigations revealed that the angular distribution of these gamma rays is compatible with the assumption of the dipole character of the radiation and that their energy spectrum differs substantially from the spectrum of prompt fission gamma rays. In the measured interval 250–600 keV, this spectrum can be described by an exponential function at the exponent value of α = −5 × 10⁻³ keV⁻¹. The mechanism of radiation of such gamma rays is not known at the present time. Theoretical models based on the phenomenon of the electric giant dipole resonance in a strongly deformed fissioning nucleus or in a fission fragment predict harder radiation whose spectrum differs substantially from the spectrum measured in the present study.     

Comparison of Lu3Al5O12:Pr and Bi4Ge3O12 scintillators for gamma-ray detection

The scintillation properties of Lu₃Al₅O₁₂:Pr³⁺ (LuAG:Pr) single crystal grown by the Czochralski method with praseodymium concentration of 0.19 mol% were investigated. For a comparison, a good quality Bi₄Ge₃O₁₂ (BGO) single crystal grown by Bridgman method was also studied. The light yield and energy resolution were measured using photomultiplier tube (XP5200B PMT) readout. Moderate light yield of 15,900 photons per MeV was measured for the LuAG:Pr(0.19%) crystal. For 662 keV gamma rays (¹³⁷Cs source), an energy resolution of 6.5% obtained for LuAG:Pr(0.19%) is much better than that of 9.0% obtained for BGO. The light yield non-proportionality and energy resolution versus energy of gamma rays were measured and the intrinsic resolution of the crystals was determined after correcting the measured energy resolution for PMT statistics. The LuAG:Pr(0.19%) showed a good proportionality of the light yield within 5% over the energy range from 1274.5 keV down to 32 keV, which is much better than that of 14% for BGO. The photofraction was determined at 320 and 662 keV for both crystals and compared with the ratio of the cross-sections for the photoelectric effect to the total one calculated using WinXCOM program.     

Positrons and electrons in primary cosmic rays as measured in the PAMELA experiment

The PAMELA experiment is being carried out on board the Russian satellite Resurs DK1 placed in the near-earth near-polar orbit on June 15, 2006. The apparatus comprising a silicon-strip magnetic spectrometer and an electromagnetic calorimeter allows measurement of electron and positron fluxes in cosmic rays in a wide energy interval from ～100 MeV to hundreds of GeV. The high-energy electron and positron separation technique is discussed and the data on positron-to-electron ratio in primary cosmic rays up to E ≈ 10 GeV from the 2006–2007 measurements are reported in this work.     

Project of a large superconductor detector involving directed diffusion of hot electrons and microcalorimeter

In recent years, radically new detectors for soft x rays and gamma rays were developed on the basis of superconducting tunnel junctions. These detectors made it possible to attain a very high resolution, but their largest area is overly small for employing them in nuclear spectroscopy. This study is devoted to the problem of designing a superconductor detector whose dimensions are sufficiently large for detecting gamma rays and which is suitable for applications in various fields of science. The detector consists of three units: an absorber, a hot-electron calorimeter, and a tunnel-junction (normal metal-insulator-superconductor) thermometer. The absorber has a multilayer structure consisting of thin superconductor layers arranged in the order of variation of the superconductor energy gap. This structure specifies the direction of hot-electron diffusion. Since quasiparticles diffuse in a specific direction, the diffusion time becomes shorter than that in the case of conventional diffusion. It is necessary that this time be shorter than the time of electron-phonon interaction. Calculations of the diffusion time for the particular structure in question and data from the literature on electron-phonon interaction show that the operating area of the detector can be about 3 to 4 mm² and that its thickness can be about 1 mm. These dimensions can be considerably increased in the case of especially pure superconductors.     

Gamma ray and π0 production from hydrogen and complex nuclei by 270–375 MeV gamma rays

Measurements are reported on the differential cross section for secondary gamma ray production on hydrogen and nuclear targets at 90° in the laboratory through the interactions of primary gamma rays in the energy range 270–375 MeV. A difference method using bremsstrahlung beams at different end point energies was employed. The gamma ray detector was a high resolution Nal(Tl) spectrometer and time of flight discrimination was used to reject neutrons. For incident gamma rays in the range 270–375 MeV the first pion nucleon resonance is strongly excited and the observed gamma ray yields are consistent with those expected due to an admixture of coherent and incoherent π⁰ photoproduction in the resonance region. The experiment marks the first successful use of a large Nal(Tl) crystals as a gamma ray spectrometer at a high energy electron linear accelerator, despite the low duty cycle and its accompanying difficulties for such detectors.     

Multipole mixtures in gamma transitions in the (<Emphasis Type="Italic">n,n′γ</Emphasis>) reaction on <Superscript>160</Superscript>Gd

The spectrum of gamma rays from the reaction ¹⁶⁰Gd(n, n′γ) and their angular distribution with respect to the neutron-beam axis are measured by using fast reactor neutrons. The scheme of energy levels and gamma transitions is composed for ¹⁶⁰Gd. Multipole-mixture parameters δ are found for many gamma transitions.     

Boron Isotopes in the PAMELA Experiment

Analysis of the isotope composition of nuclei in galactic cosmic rays (GCR) in the PAMELA orbital international experiment allows studying the problems of cosmic-ray origin and propagation in our Galaxy. PAMELA magnetic spectrometer data provided the significant progress in the study of the light nuclei isotope composition of GCR from H to Be in the energy range ∼0.1–1 GeV/nucleon. This makes it possible to estimate the contribution of local (∼100 pc) young (∼10⁶ years) interstellar sources (LISS) into GCR fluxes from supernova explosions. The analysis of boron (B) isotope fluxes in the GCR has so far been carried out only in the energy range ∼0.08–0.17 GeV/nucleon in the space experiments Voyager, Ulysses, ACE. In the present contribution the attempt was done to determine the ¹¹B/¹⁰B ratio in the energy range ∼0.1–1.0 GeV/nucleon for the first time on the base of 2006–2014 PAMELA data using the measurements of the detected nuclei rigidities, velocities and ionization losses in a multilayer calorimeter. The new PAMELA results are consistent with existing as experimental data and those expected from simulations. However the statistical and systematic measurement uncertainties do not allow to separate the local boron source contributions into GCR fluxes. The preliminary results of the boron isotope flux analysis in GCR (¹⁰B, ¹¹B spectra and ¹¹B/¹⁰B ratio dependences on the rigidity and energy) are presented as well as the existing measurement data and simulation results.

     

Levels in124Te populated in the decay of124Sb

The levels in¹²⁴Te have been investigated by studying the gamma rays emitted following the decay of 60d¹²⁴Sb. Gamma-gamma coincidence spectra have been taken with gates set at 603, 646, 709, 714 and 723 keV gamma rays by using 105 cm³ Ge(Li)-110 cm³ HPGe coincidence system. 69 gamma rays have been observed and nine of them are reported for the first time. Four new levels in¹²⁴Te at 2512, 2550.2, 2808 and 2814.5 keV are proposed. Levels reported in literatures at 2412 and 2733 keV are excluded from the level scheme of¹²⁴Te based on the present study. The experimentally determined energy spectrum of low-lying positive parity states, B(E2) ratios and electric quadrupole moment of the first excited state are compared with the IBM-1 predictions.