M.-H. A. Huang

Detection of a cosmic ray with measured energy well beyond the expected spectral cutoff due to cosmic microwave radiation

We report the detection of a 51-joule (320 +/- 90 EeV) cosmic ray by the Flys Eye air shower detector in Utah. This is substantially greater than the energy of any previously reported cosmic ray. A Greisen-Zatsepin-Kuzmin cutoff of the energy spectrum (due to pion photoproduction energy losses) should occur below this energy unless the highest energy cosmic rays have traveled less than about 30 Mpc. The error box for the arrival direction in galactic coordinates is centered on b=9.6 deg, l=163.4 deg. The particle cascade reached a maximum size near a depth of 815 g/cm^2 in the atmosphere, a depth which does not uniquely identify the type of primary particle.We report the detection of a 51-joule (320 +/- 90 EeV) cosmic ray by the Flys Eye air shower detector in Utah. This is substantially greater than the energy of any previously reported cosmic ray. A Greisen-Zatsepin-Kuzmin cutoff of the energy spectrum (due to pion photoproduction energy losses) should occur below this energy unless the highest energy cosmic rays have traveled less than about 30 Mpc. The error box for the arrival direction in galactic coordinates is centered on b=9.6 deg, l=163.4 deg. The particle cascade reached a maximum size near a depth of 815 g/cm^2 in the atmosphere, a depth which does not uniquely identify the type of primary particle.

The prototype high-resolution Fly's Eye cosmic ray detector

Abstract The High-Resolution Flys Eye (HiRes) is an observatory for the highest energy cosmic rays. It detects the nitrogen fluorescence light induced by the passage of giant cosmic ray extensive air showers through the atmosphere. A two-site prototype of the observatory was operated from September 1994 to November 1996. In this paper we describe the components of that detector, and the procedures used to calibrate the detector and characterise the atmosphere. Data collected by the HiRes prototype are being used for physics studies, including an analysis of the cosmic ray mass composition in the energy range from 10 17 to 10 18 eV .

Study of Broad-Scale Anisotropy of Cosmic-Ray Arrival Directions from 2 x 1017 to 1020 Electron Volts from Fly's Eye Data

We report results on the broad-scale anisotropy of arrival directions of cosmic rays in the energy range from 2×1017 to 1020 eV. The data were taken by the Flys Eye detector in both monocular and stereo modes of operation. We look for dependence on galactic latitude or supergalactic latitude by fitting the data to a Wdowczyk and Wolfendale plane enhancement function and a north-south gradient functional form. We report a small but statistically significant galactic plane enhancement in the energy range between 2×1017 and 3.2×1018 eV. The probability that this anisotropy is due to fluctuations of an isotropic distribution is less than 0.06%. The most significant galactic plane enhancement factor fE=0.104±0.036 is in the energy range (0.4-1.0)×1018 eV. No statistically significant evidence for a north-south gradient is found. There is no sign of significant deviation from isotropic background when the data are analyzed in terms of supergalactic latitude distributions.

Radio-controlled xenon flashers for atmospheric monitoring at the HiRes cosmic ray observatory

Stable, robust ultraviolet light sources for atmospheric monitoring and calibration pose a challenge for experiments that measure air #uorescence from cosmic ray air showers. One type of light source in use at the High Resolution Fly’s Eye (HiRes) cosmic ray observatory features a xenon #ashbulb at the focal point of a spherical mirror to produce a 1 ls pulse of collimated light that includes a strong UV component. A computer-controlled touch tone radio system provides remote operation of bulb triggering and window heating. These devices, dubbed ‘#ashersa, feature stand-alone operation,

Geometrical reconstruction with the High Resolution Fly's Eye prototype cosmic ray detector

5% shot-to-shot stability, weather proof construction and are well suited for long-termeld use. This paper describes the #ashers, the radio control system, and a 12-unit array in operation at the HiRes cosmic ray observatory ( 1999 Elsevier Science B.V. All rights reserved.

Recent Results from the Fly’s Eye Experiment

Abstract The High Resolution Flys Eye EHE cosmic ray detector (HiRes) was operated for over two years (prior to November 1996) in a two-site prototype configuration. This paper describes the development and testing of an event reconstruction method for extensive air showers (EAS) viewed in coincidence by both sites (stereo viewing). The reconstruction accuracy was directly measured through the use of a UV laser mounted on a telescope that generated airshower-like events with known geometries. For events observed with stereo opening angles greater than about 10° (most events) the median error in the reconstructed laser direction was 0.4° with 95% of events being reconstructed with errors of less than 0.9° (which degrade to 0.8° and 1.8°, respectively, for smaller opening angles). A limited investigation of the likely performance of the HiRes Stage 1.0 detector was undertaken. Reconstruction accuracy is likely to be only slightly degraded compared with the prototype results.

The Cosmic ray energy spectrum observed by the Fly's Eye

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.