J. K. K. Tang

Measurement of the cosmic-ray energy spectrum and composition from 1017 to 1018.3 eV using a hybrid technique

We study the spectrum and average mass composition of cosmic rays with primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector consisting of the High Resolution Flys Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum as a function of energy. A complete Monte Carlo simulation of the detector response and comparisons with shower simulations leads to the conclusion that the cosmic ray intensity is changing f rom a heavier to a lighter composition in this energy range. The spectrum is consistent with earlier Flys Eye measurements and supports the previously found steepening near 4 \times 10^{17} eV .

Measurement of the Cosmic Ray Energy Spectrum and Composition from 10^{17} to 10^{18.3} eV Using a Hybrid Fluorescence Technique

We study the spectrum and average mass composition of cosmic rays with primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector consisting of the High Resolution Flys Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum as a function of energy. A complete Monte Carlo simulation of the detector response and comparisons with shower simulations leads to the conclusion that the cosmic ray intensity is changing f rom a heavier to a lighter composition in this energy range. The spectrum is consistent with earlier Flys Eye measurements and supports the previously found steepening near 4 \times 10^{17} eV .

HiRes, a High Resolution Fly's Eye detector

Abstract The High Resolution Eye detector (HiRes), based on the Flys Eye technique [1], is being developed at Dugway, Utah. Detector parameters and some preliminary test results will be given.

Reexamination of Cosmic-Ray Composition around 1018 eV from Fly's Eye Data

The cosmic-ray composition around 1018 eV is reexamined using the Chou-Yang model for the simulation of air showers and comparing with Flys Eye data. This model is quite different from models previously used. Using this model leads to the same conclusion that the primary cosmic-ray composition gets lighter in the region from 3 × 1017 eV to 1019 eV. This conclusion of a changing composition is not sensitive to the choice of model because it results from the measured elongation rate being greater than that predicted for a constant composition by any reasonable model that can account for the measured depths of shower maxima at the Flys Eye threshold energy. That constraint on elongation rate for a constant composition also implies that the increased rate of energy dissipation that appears in hadron-nucleus interactions near 1014 eV may continue up to the region of 1019 eV.

The calibration of the absolute sensitivity of photomultiplier tubes in the high resolution Fly's eye detector

Abstract The High Resolution Flys Eye Experiment (HiRes) measures cosmic rays from 1017 eV to beyond 1020 eV by the atmospheric fluorescence light produced in their cascade showers. The light is detected with photomultiplier tubes (PMTs). The full HiRes project proposes to use about 30000 PMTs in two sites. More than four thousand tubes are already in operation. Accurate calibrations of the absolute sensitivity of the PMTs are essential to this experiment. We have developed at the University of Utah a facility to calibrate the absolute gain and quantum efficiency of the tubes to the accuracy of better than three percent. This facility has proved to be capable of testing a large number of photomultiplier tubes efficiently and economically. In addition, it can be used to map the gain and the quantum efficiency of a PMT as a function of where the incident photons hit the tubes window. Although this facility is developed for HiRes, it is applicable to other experiments that utilise PMTs. We shall describe in this report the operation principles, the apparatus and some typical results.

Results from the fly’s eye experiment

We report recently analyzed results on the energy spectrum, and composition of cosmic rays above 0.3 EeV. We observe a break in the spectrum at 3 EeV and a changing composition. The results can be explained by a simple two component model: galactic cosmic rays dominated by heavy primaries and an extragalactic component dominated by light primaries. The observed isotropic arrival direction distribution is consistent with the predictions of this model. A 320 EeV event was also recorded.

Recent Results from the Fly’s Eye Experiment

We report on recent results from the Fly’s Eye experiment up to July 1992. The detector consists of FE1, running from 1981 to 1992, and FE2 running from 1986 till now. The monocular data comes from a single eye detector, and the stereo data are detected by both detectors and reconstructed by stereo geometry. This report includes results on the spectrum, composition, anisotropy, and a 320 EeV event. The stereo spectrum shows a dip around 1018.5eV. The composition changes from predominately heavy (below 1018eV) to predominately light (above 1019eV). The anisotropy shows no significant deviation from random fluctuation in both a small scale and large scale anisotropy search. A 320 -40 +35 × 1018eV event was detected by FE1 on Oct. 15, 1991.