M. Ave

Measurement of the depth of maximum of extensive air showers above 10(18) eV

We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.

Trigger and aperture of the surface detector array of the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory consists of 1600 water-Cherenkov detectors, for the study of extensive air showers (EAS) generated by ultra-high-energy cosmic rays. We describe the trigger hierarchy, from the identification of candidate showers at the level of a single detector, amongst a large background (mainly random single cosmic ray muons), up to the selection of real events and the rejection of random coincidences. Such trigger makes the surface detector array fully efficient for the detection of EAS with energy above 3 x 10(18) eV, for all zenith angles between 0 degrees and 60 degrees, independently of the position of the impact point and of the mass of the primary particle. In these range of energies and angles, the exposure of the surface array can be determined purely on the basis of the geometrical acceptance

Measurement of the pressure dependence of air fluorescence emission induced by electrons

The fluorescence detection of ultra high energy (>10^18 eV) cosmic rays requires a detailed knowledge of the fluorescence light emission from nitrogen molecules, which are excited by the cosmic ray shower particles along their path in the atmosphere. We have made a precise measurement of the fluorescence light spectrum excited by MeV electrons in dry air. We measured the relative intensities of 34 fluorescence bands in the wavelength range from 284 to 429 nm with a high resolution spectrograph. The pressure dependence of the fluorescence spectrum was also measured from a few hPa up to atmospheric pressure. Relative intensities and collisional quenching reference pressures for bands due to transitions from a common upper level were found in agreement with theoretical expectations. The presence of argon in air was found to have a negligible effect on the fluorescence yield. We estimated that the systematic uncertainty on the cosmic ray shower energy due to the pressure dependence of the fluorescence spectrum is reduced to a level of 1% by the AIRFLY results presented in this paper.

Composition of Primary Cosmic-Ray Nuclei at High Energies

The TRACER instrument (Transition Radiation Array for Cosmic Energetic Radiation) has been developed for direct measurements of the heavier primary cosmic-ray nuclei at high energies. The instrument had a successful long-duration balloon flight in Antarctica in 2003. The detector system and measurement process are described, details of the data analysis are discussed, and the individual energy spectra of the elements O, Ne, Mg, Si, S, Ar, Ca, and Fe (nuclear charge Z = 8-26) are presented. The large geometric factor of TRACER and the use of a transition radiation detector make it possible to determine the spectra up to energies in excess of 1014 eV per particle. A power-law fit to the individual energy spectra above 20 GeV amu−1 exhibits nearly the same spectral index (2.65 ± 0.05) for all elements, without noticeable dependence on the elemental charge Z.

ENERGY SPECTRA OF PRIMARY AND SECONDARY COSMIC-RAY NUCLEI MEASURED WITH TRACER

The Transition Radiation Array for Cosmic Energetic Radiation (TRACER) cosmic-ray detector, first flown on long-duration balloon (LDB) in 2003 for observations of the major primary cosmic-ray nuclei from oxygen (Z = 8) to iron (Z = 26), has been upgraded to also measure the energies of the lighter nuclei, including the secondary species boron (Z = 5). The instrument was used in another LDB flight in 2006. The properties and performance of the modified detector system are described, and the analysis of the data from the 2006 flight is discussed. The energy spectra of the primary nuclei carbon (Z = 6), oxygen, and iron over the range from 1 GeV amu–1 to 2 TeV amu–1 are reported. The data for oxygen and iron are found to be in good agreement with the results of the previous TRACER flight. The measurement of the energy spectrum of boron also extends into the TeV amu–1 region. The relative abundances of the primary nuclei, such as carbon, oxygen, and iron, above ~10 GeV amu–1 are independent of energy, while the boron abundance, i.e., the B/C abundance ratio, decreases with energy as expected. However, there is an indication that the previously reported E –0.6 dependence of the B/C ratio does not continue to the highest energies.

PROPAGATION AND SOURCE ENERGY SPECTRA OF COSMIC RAY NUCLEI AT HIGH ENERGIES

A recent measurement of the TRACER instrument on long-duration balloon has determined the individual energy spectra of the major primary cosmic ray nuclei from oxygen (Z = 8) to iron (Z = 26). The measurements cover a large range of energies and extend to energies beyond 1014 eV. We investigate if the data set can be described by a simple but plausible model for acceleration and propagation of cosmic rays. The model assumes a power-law energy spectrum at the source with a common spectral index ? for all nuclear species, and an energy-dependent propagation path length (? E ?0.6) combined with an energy-independent residual path length ?0. We find that the data can be fitted with a fairly soft source spectrum (? = 2.3-2.4), and with a residual path length ?0 as high as 0.3 g cm?2. We discuss this model in the context of other pertinent information, and we determine the relative abundances of the elements at the cosmic ray source.

A model-independent method of determining energy scale and muon number in cosmic ray surface detectors

Abstract Surface detector arrays are designed to measure the spectrum and composition of high energy cosmic rays by detecting the secondary particle flux of the extensive air showers (EAS) induced by the primary cosmic rays. Electromagnetic particles and muons constitute the dominant contribution to the ground detector signals. In this paper, we show that the ground signal deposit of an EAS can be described in terms of only very few parameters: the primary energy E , the zenith angle θ , the distance of the shower maximum X max to the ground, and a muon flux normalization N μ . This set of physical parameters is sufficient to predict the average particle fluxes at ground level to around 10% accuracy. We show that this is valid for hadronic air showers, using the two standard hadronic interaction models used in cosmic ray physics, QGSJetII and Sibyll, and for hadronic primaries from protons to iron. Based on this model, a new approach to calibrating the energy scale of ground array experiments is developed, which factors out the model dependence inherent in such calibrations up to now. Additionally, the method yields a measurement of the average number of muons in EAS. The measured distribution of N μ of cosmic ray air showers can then be analysed, in conjunction with measurements of X max from fluorescence detectors, to put constraints on the cosmic ray composition and hadronic interaction models.

A novel method for the absolute fluorescence yield measurement by AIRFLY

One of the goals of the AIRFLY (AIR FLuorescence Yield) experiment is to measure the absolute fluorescence yield induced by electrons in air to better than 10% precision. We introduce a new technique for measurement of the absolute fluorescence yield of the 337 nm line that has the advantage of reducing the systematic uncertainty due to the detector calibration. The principle is to compare the measured fluorescence yield to a well known process—the Cherenkov emission. Preliminary measurements taken in the BFT (Beam Test Facility) in Frascati, Italy with 350 MeV electrons are presented. Beam tests in the Argonne Wakefield Accelerator at the Argonne National Laboratory, USA with 14 MeV electrons have also shown that this technique can be applied at lower energies.