E. N. Kouznetsov

Elemental energy spectra of cosmic rays from the data of the ATIC-2 experiment

This paper reports on the results of measurements performed in the course of the ATIC-2 balloon experiment (2002–2003) for the energy spectra of particles (such as protons; He, C, O, Ne, Mg, Si, and Fe nuclei; and some groups of nuclei) and the all-particle energy spectrum in primary cosmic rays at energies ranging from 50 GeV to 200 TeV. The conclusion is drawn that the energy spectra of protons and helium nuclei differ substantially (the spectrum of protons is steeper) and that the shape of the energy spectra of protons and heavy nuclei cannot be described by a power function.

Possible structure in the cosmic ray electron spectrum measured by the ATIC-2 and ATIC-4 experiments

A strong excess in a form of a wide peak in the energy range of 300-800 GeV was discovered in the first measurements of the electron spectrum in the energy range from 20 GeV to 3 TeV by the balloon-borne experiment ATIC (J. Chang et al. Nature, 2008). The experimental data processing and analysis of the electron spectrum with different criteria for selection of electrons, completely independent of the results reported in (J. Chang et al. Nature, 2008) is employed in the present paper. The new independent analysis generally confirms the results of (J. Chang et al. Nature, 2008), but shows that the spectrum in the region of the excess is represented by a number of narrow peaks. The measured spectrum is compared to the spectrum of (J. Chang et al. Nature, 2008) and to the spectrum of the Fermi/LAT experiment.

Upturn observed in heavy nuclei to iron ratios by the ATIC-2 experiment

The ratios of fluxes of heavy nuclei from sulfur (Z=16) to chromium (Z=24) to the flux of iron were measured by the ATIC-2 experiment. The ratios are decreasing functions of energy from 5 GeV/n to approximately 80 GeV/n, as expected. However, an unexpected sharp upturn in the ratios are observed for energies above 100 GeV/n for all elements from Z=16 to Z=24. Similar upturn but with lower amplitude was also discovered in the ATIC-2 data for the ratio of fluxes of abundant even nuclei (C, O, Ne, Mg, Si) to the flux of iron. Therefore the spectrum of iron is significantly different from the spectra of other abundant even nuclei.

Measuring the deposited energy by the scintillation calorimeter in the ATIC experiment

The purpose of the ATIC balloon experiment is to measure the energy spectra of primary cosmic rays with individual charge resolution from protons to iron over the energy range from ∼50 GeV to 200 TeV. The particle energy is measured by a bismuth germanate (BGO) scintillation calorimeter. The procedure of calorimeter calibration is described, in particular, calibration of the temperature dependence of the calorimeter sensitivity using the data of in-flight measurements. A technique for determining the energy deposited in the calorimeter in view of the temperature dependence of its sensitivity is presented. The maximum systematic error in determining the deposited energy by the calorimeter is 10% or less, and the probable error is estimated at 6%.

Fine structure in the cosmic ray electron spectrum measured by the ATIC-2 and ATIC-4 experiments

A strong anomaly in form of a wide peak in the energy range 300–800 GeV was discovered in the first measurements of the electron spectrum in the energy range from 20 GeV to 3 TeV by the balloon-borne experiment ATIC [1]. The experimental data processing and analysis of the electron spectrum with different criteria for selection of electrons completely independent of the results reported in [1] is employed in the present paper. New independent analysis generally confirms the results of [1] but shows that the spectrum in the region of the anomaly is represented by a number of narrow peaks. Measured spectrum is compared to the spectrum of [1] and to the spectrum of the Fermi/LAT experiment.

Fine Structure in the Cosmic Ray Electron Spectrum Measured by ATIC

New analysis to separate electrons from protons in the ATIC experiment has been performed. Five new discriminants were studied by different Monte Carlo programs. New electron spectrum, when compared with the published results [1], show good agreement in the most interesting region of energy (from 90 GeV to 600 GeV). It is argued that there is no disagreement between ATIC’s results and Fermi-LAT ones. Finally, high-resolution electron spectrum is obtained and possible fine structure is found out in it. The ATIC (Advanced Thin Ionization Calorimeter) balloon-borne spectrometer was designed to measure the energy spectra of elements from H to Fe with individual resolution of charges in primary cosmic rays for energy region from 50 GeV to 100 TeV. ATIC has had three successful flights around the South Pole in 2000–2001 (ATIC-1), 2002-2003 (ATIC-2) and 2007-2008 (ATIC-4). ATIC is comprised of a fully active bismuth germanate (BGO) calorimeter, a carbon target with embedded scintillator hodoscopes, and a silicon matrix that is used as the main charge detector. The calorimeter is comprised of 8 layers with 40 BGO crystalls in each for ATIC-1 and ATIC-2 and of 10 layers for ATIC-4. The details of the construction of the apparatus are described in the papers [2–4]. It was shown that it is possible also to measure the spectrum of cosmic ray electrons plus positrons [5] with ATIC (hereinafter we use ’electrons’ for brevity). To separate electrons from the higher background of protons and other nuclei some differences in shower development for incident electrons and for nuclei are used. The spectrum of electrons measured with the ATIC spectrometer by this method was published in the paper [1]. The most notable detail of the electron spectrum reported was an ‘excess’ of electrons between energies of 300–800 GeV. The main purpose of this work is to investigate possible alternate techniques to separate electrons from hadrons and was curried ae-mail: panov@dec1.sinp.msu.ru E, GeV 2 10 3 10 ) 2 G eV -1 st er -1 s -2 (m 3

Albedo in the ATIC experiment: Measurements and simulations

We analyze the characteristics of the albedo, or the backscatter current, which constitutes a background for charge measurements in calorimetric experiments in high-energy cosmic rays. We compare the experimental data obtained in the flights of the ATIC spectrometer with the simulations performed using the GEANT 3.21 code. We discuss the influence of the backscatter on the charge resolution in the ATIC experiment.