EXTREMELY HIGH-ENERGY COSMIC-RAYS

Cosmic ray experiments at energies above 10(17) eV attempt to measure the energy spectrum, chemical composition, and anisotropy in order to discover where and how such high energy particles are accelerated. Air shower experiments at Sydney (Australia), Haverah Park (UK), Yakutsk (Russia), Akeno (Japan), and Utah (USA) have employed different detection techniques to study these cosmic rays. The methods range from measuring the longitudinal development of the electromagnetic shower in the atmosphere (Fly's Eye detector in Utah) to measuring the lateral distribution of muons at ground level (Sydney). Comparison of results requires allowance for the uncertainties and possible biases intrinsic to each method. Although results from the different experiments do not blatantly contradict each other, numerous effects of marginal statistical significance are in dispute. These include the existence of air showers with energies in excess of 10(20) eV and a neutral particle flux from Cygnus X-3. We review the cosmic ray issues and results in this energy region as well as the various detection techniques and analysis procedures. Fundamental questions remain to be answered by more sensitive detectors which are now in development.