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This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The goal of this project was to develop a new technique using ground-based measurements to determine the cosmic-ray composition at energies around 1015 eV (the knee in the cosmic-ray spectrum). Cosmic rays are high-energy nuclei that continuously bombard the earth. Though cosmic rays were first detected in the 1870s it wasn't until 1915 that their cosmic origin was established. At present, the authors still do not know the source of cosmic rays. At energies above 50 TeV (1 TeV = 1 trillion electron-volts) they do not know the composition of the cosmic rays. At about 5 PeV (1PeV = 1015 eV) the cosmic ray spectrum steepens. Knowledge of the composition above and below this point can help determine the origin of cosmic rays.
Since the discovery of cosmic rays over one hundred years ago, many experiments have studied their properties. However, a definitive answer to the questions of where cosmic rays originate and how they are produced is still not known. Over the last several decades, a much more detailed understanding of high energy cosmic rays has begun to materialize. In particular, the cosmic-ray energy spectrum, with its transitions at 3 PeV (the "knee") and 3 EeV (the "ankle"), has been extensively investigated. Based on magnetic confinement arguments, it's generally believed that the energy range between the knee and ankle is where the transition from Galactic to extragalactic sources of cosmic rays. The ability to distinguish between high energy cosmic rays of different composition and study the relative mass abundances of cosmic rays in this transition region can provide invaluable insight in answering the open questions surrounding the origins of cosmic rays. This work focuses on measuring the composition-resolved cosmic-ray energy spectrum at and above the all-particle knee using one year of data collected by the IceCube Observatory. Sepcifically, we focus on making a two mass group spectrum measurement from 10^6.4 GeV to 10^7.8 GeV. The first mass group, referred to as the "light" mass group, is modeled using proton and helium cosmic rays, while the second, "heavy" mass group, is modeled using oxygen and iron cosmic rays. We observe a clear softening of the light spectrum near 3 PeV, while the energy spectrum for the heavy mass group follows a power-law like structure with a spectral index of ~2.7 throughout the entire energy range considered. The observed transition from a primarily light to a heavy-dominant spectrum takes place near 10^7.1 GeV. This feature is characteristic of a potential rigidity-dependent cutoff, or Peters cycle. The change in relative mass abundance could also indicate a possible transition in the source population of cosmic rays. In addition, a study to determine whether or not the light, heavy, or all-particle cosmic-ray energy spectra vary as a function of arrival direction is also presented. This marks the first time an analysis of this kind has been conducted using the IceCube Observatory. No statistically significant spectrum deviations were observed. The results from this analysis can be used to set a limit on the range of possible spectral deviations
Without qualification, cosmic rays usually mean the primary cosmic rays of extra-terrestrial origin that continually bombard the earth and consist mostly of high-energy protons, about 9 percent helium and heavier nuclei, a small percentage of electrons, and some gamma rays. The energies of cosmic rays are well in excess of billions of electron volts. Secondary cosmic rays result from interactions between primary rays and atoms in the earth's atmosphere. Most cosmic rays probably originate from the Milky Way galaxy, but a small fraction come from the sun as evidenced by diurnal variations in the cosmic ray flux. This volume presents new research on cosmic rays.
The Pierre Auger Observatory, in Argentina, combines a 3000 $\mathrm{km^2}$ surface array of water Cherenkov detectors with fluorescence telescopes to measure extensive air showers initiated by ultra-high energy cosmic rays. This "hybrid" observatory (in operation since 2004, and completed in 2008) is fully efficient for cosmic rays energies above $10^{18}$ eV, that is, from just below the "ankle" of the energy spectrum up to the highest energies.After the completion of the main observatory, the Auger collaboration has started to deploy new instruments to extend the energy range down to about 0.1 EeV. The planned extensions include two infill surface arrays with 750 and 433 m spacing, with muon detection capabilities, and three additional fluorescence telescopes with a more elevated field of view. The 750 m infill array (covering about 24 $\mathrm{km^2}$) and the new telescopes are now operational. Their aim is the measurement of cosmic rays from below the second knee of the spectrum up to the ankle, where data from the extensions overlap those from the main observatory. The study of the evolutior of the spectrum through the second knee and the ankle, together with the primary mass composition, are crucial to the understanding of the transition from a galactic cosmic ray origin to an extragalactic one.This thesis makes use of data from the 750 m infill array: the objective is the measurement of the cosmic ray energy spectrum in the energy region above $3 \times 10^{17}$ eV, where the array is fully efficient. To get to the energy spectrum, several steps are needed, from the reconstruction of events, through the precise determination of the exposure of the array, up to the determination of the primary energy. The thesis deals with these aspects, before reaching the final result.The first chapter gives a general introduction to cosmic ray physics and detectors. It also summarizes experimental results above the first knee of the spectrum with particular emphasis on those obtained above $10^{17}$ eV. The next two chapters describe the Pierre Auger Observatory and the infill array, respectively. In chapter 2, the main Auger results are summarized too, after a schematic description of th different components of the observatory. Chapter 3 sets the stage for the following chapters. It presents a more detailed description of the characteristics of the infill array, in particular the trigger definitions, event selection and reconstruction. In chapter 4 the performance of the reconstruction of the lateral distribution of observed showers is studied in detail. This is particularly important for the energy spectrum, since the signal at a fixed distance from the shower axis is used as the energy estimator of the event. This signal is estimated by means c the measured lateral distribution of the shower. Chapter 5 presents a comparison between the event reconstruction of the infill and main arrays. Using the set of showers detected by both instruments, the derived geometry and energy estimation are compared, showing a good agreement. In chapter 6, the energy threshold of the array, and hence the set of events to be used, is defined. The methods to obtain the exposure of the array are discussed, as well as related systematic uncertainties. Finally, in chapter 7, the technique to derive the primary energy for each detected shower is presented. The derived energy spectrum is discussed, and the flux is shown to be consistent with that measured by other instruments in the overlapping energy regions.