The energy spectrum of cosmic rays measured with the HEAT extension at the Pierre Auger Observatory

Autor: Scharf, Nils Sven Sebastian
Přispěvatelé: Hebbeker, Thomas
Jazyk: angličtina
Rok vydání: 2013
Předmět:
Zdroj: Aachen IX, 136 S. : Ill., graph. Darst. (2013). = Aachen, Techn. Hochsch., Diss., 2013
Popis: This thesis describes the calculation of the energy spectrum of cosmic rays, that is the absolute flux of cosmic rays as a function of energy, from data of air showers observed with the HEAT (High Elevation Auger Telescopes) extension and the fluorescence detector of the Pierre Auger Observatory. The Pierre Auger Observatory is the largest observatory for the study of cosmic rays. The Pierre Auger Observatory observes air showers, that are cascades of particles that were instigated by cosmic rays hitting the Earth's atmosphere, with two different detection concepts. The surface detector samples the secondary particles of air showers that hit the ground with an array of surface detector stations, whereas the fluorescence detector measures the energy loss profile of air showers by detecting fluorescence light, produced by the air showers when they travel through the atmosphere, with optical telescopes. The properties of the cosmic rays are not directly measurable but have to be reconstructed from the observed air shower parameters. Properties of particular interest are the type of the primary cosmic ray particle, its energy and its arrival direction. HEAT is an extension to the fluorescence detector of the Pierre Auger Observatory. It is designed to lower the energy threshold by one order of magnitude down to 1017 eV or lower. HEAT is taking data since 2010. The calculation of the absolute flux of cosmic rays needs two ingredients: the number of detected air showers as a function of shower energy and the exposure of the detector as a function of energy. The studied air shower class are hybrid events, which are events that have been detected by a fluorescence detector and at least one surface detector station. The used air showers were observed in a time period of fifteen month starting from June 2010. A first step of the analysis is the reconstruction of air showers and cosmic ray parameters from raw data. To calculate the exposure, the uptime, that is the integral time the detector was taking data, and the aperture, that is a value to describe the observational capabilities of a cosmic ray detector, have to be ascertained. The uptime is calculated with information from the detector monitoring of the Pierre Auger Observatory. The calculation of the aperture is done by calculating the detection efficiency and the effective area of the detector. For the calculation of the efficiency and effective area a large number of Monte Carlo simulated air showers was generated. The simulated air showers were produced with 2 different primary cosmic ray particles (proton or iron) and three different hadronic interaction models. This is needed, because the actual primary composition in this energy range is only known to a certain degree, and the physics of hadronic interactions at these high energies is only known by extrapolations from lower energy data. The simulated air showers are reconstructed with the same process that is used for real data. After the simulation and reconstruction of the air showers, a data selection procedure with several quality cuts is used to remove events with a bad detection quality from simulated and real data alike. From the resulting high quality simulated and real data, the aperture, the exposure and the cosmic ray flux is calculated. A mean flux is calculated from the single flux values for the six combinations of primary and hadronic model. A conservative systematic error approximation from the unknown choice of primary and hadronic model is given for the mean flux. The energy measured by the detector is of course smeared with the finite energy resolution of the detector. To quantify this effect, the energy resolution for this new detector configuration is studied with the same simulated air showers that were used for the exposure calculation. To allow a comparison of the calculated flux values with those from other experiments or analyses, a bin-by-bin unfolding of the energy resolution on the flux values is performed. This unfolding changes the mean flux values between 17 % and 8 %. A fit of the spectral index of the unfolded cosmic ray flux gives a value of γ = -3.23 ± 0.05, which is in good agreement to the spectral index measured at other experiments. The measured spectrum of cosmic rays is in good agreement to that of other experiments, and to the published energy spectrum of the Pierre Auger Observatory, that was calculated from an independent data set.
Databáze: OpenAIRE
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