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Register a new userThe Elemental Composition of High-Energy Cosmic Rays: Measurements with TRACER
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P.J. Boyle
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TRACER (Transition Radiation Array for Cosmic Energetic Radiation) is a balloon borne instrument that has been developed to directly measure the composition and energy spectra of individual heavy elements up to 10^15 eV per particle. TRACER achieves a large geometric factor (5 m^2 sr) through the use of a Transition Radiation Detector utilizing arrays of single wire proportional tubes. TRACER has measured the energy spectra of the elements O, Ne, Mg, Si, S, Ar, Ca, and Fe. The energy spectra reach energies in excess of 10^14 eV per particle and exhibit nearly the same spectral index (2.65 +/- 0.05) for all elements.
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The KASCADE-Grande air shower experiment [W. Apel, et al. (KASCADE-Grande collaboration), Nucl. Instrum. Methods A 620 (2010) 202] consists of, among others, a large scintillator array for measurements of charged particles, Nch, and of an array of shielded scintillation counters used for muon counting, Nmu. KASCADE-Grande is optimized for cosmic ray measurements in the energy range 10 PeV to about 2000 PeV, where exploring the composition is of fundamental importance for understanding the transition from galactic to extragalactic origin of cosmic rays. Following earlier studies of the all-particle and the elemental spectra reconstructed in the knee energy range from KASCADE data [T. Antoni, et al. (KASCADE collaboration), Astropart. Phys. 24 (2005) 1], we have now extended these measurements to beyond 200 PeV. By analysing the two-dimensional shower size spectrum Nch vs. Nmu for nearly vertical events, we reconstruct the energy spectra of different mass groups by means of unfolding methods over an energy range where the detector is fully efficient. The procedure and its results, which are derived based on the hadronic interaction model QGSJET-II-02 and which yield a strong indication for a dominance of heavy mass groups in the covered energy range and for a knee-like structure in the iron spectrum at around 80 PeV, are presented. This confirms and further refines the results obtained by other analyses of KASCADE-Grande data, which already gave evidence for a knee-like structure in the heavy component of cosmic rays at about 80 PeV [W. Apel, et al. (KASCADE-Grande collaboration), Phys. Rev. Lett. 107 (2011) 171104].
The KASCADE-Grande experiment, located at KIT-Karlsruhe, Germany, consists of a large scintillator array for measurements of charged particles, N_ch, and of an array of shielded scintillation counters used for muon counting, N_mu. KASCADE-Grande is optimized for cosmic ray measurements in the energy range 10 PeV to 1000 PeV, thereby enabling the verification of a knee in the iron spectrum expected at approximately 100 PeV. Exploring the composition in this energy range is of fundamental importance for understanding the transition from galactic to extragalactic cosmic rays. Following earlier studies of elemental spectra reconstructed in the knee energy range from KASCADE data, we have now extended these measurements to beyond 100 PeV. By analysing the two-dimensional shower size spectrum N_ch vs. N_mu, we reconstruct the energy spectra of different mass groups by means of unfolding methods. The procedure and its results, giving evidence for a knee-like structure in the spectrum of iron nuclei, will be presented.
The results on ultra-high-energy cosmic rays (UHECR) mass composition obtained with the Telescope Array surface detector are presented. The analysis employs the boosted decision tree (BDT) multivariate analysis built upon 14 observables related to both the properties of the shower front and the lateral distribution function. The multivariate classifier is trained with Monte-Carlo sets of events induced by the primary protons and iron. An average atomic mass of UHECR is presented for energies $10^{18.0}-10^{20.0} mbox{eV}$. The average atomic mass of primary particles shows no significant energy dependence and corresponds to $langle ln A rangle = 2.0 pm 0.1 (stat.) pm 0.44 (syst.)$. The result is compared to the mass composition obtained by the Telescope Array with $mbox{X}_{mbox{max}}$ technique along with the results of other experiments. Possible systematic errors of the method are discussed.
Using the Auger mass-composition analysis of ultra high energy cosmic rays, based on the shape-fitting of $X_{max}$ distributions, we demonstrate that mass composition and energy spectra measured by Auger, Telescope Array and HiRes can be brought into good agreement. The shape-fitting analysis of $X_{max}$ distributions shows that the measured sum of proton and Helium fractions, for some hadronic-interaction models, can saturate the total flux. Such p+He model, with small admixture of other light nuclei, naturally follows from cosmology with recombination and reheating phases. The most radical assumption of the presented model is the assumed unreliability of the experimental separation of Helium and protons, which allows to consider He/p ratio as a free parameter. The results presented here show that the models with dominant p+He composition explain well the energy spectrum of the dip in the latest (2015 - 2017) data of Auger and Telescope Array, but have some tension at the highest energies with the expected Greisen-Zatsepin-Kuzmin cutoff. The Auger-Prime upgrade experiment has a great potential to reject or confirm this model.
A new family of parameters intended for composition studies in cosmic ray surface array detectors is proposed. The application of this technique to different array layout designs has been analyzed. The parameters make exclusive use of surface data combining the information from the total signal at each triggered detector and the array geometry. They are sensitive to the combined effects of the different muon and electromagnetic components on the lateral distribution function of proton and iron initiated showers at any given primary energy. Analytical and numerical studies have been performed in order to assess the reliability, stability and optimization of these parameters. Experimental uncertainties, the underestimation of the muon component in the shower simulation codes, intrinsic fluctuations and reconstruction errors are considered and discussed in a quantitative way. The potential discrimination power of these parameters, under realistic experimental conditions, is compared on a simplified, albeit quantitative way, with that expected from other surface and fluorescence estimators.
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