ULTRA-HIGH ENERGY COSMIC RAYS

We describe the current status of the High Resolution Fly's Eye detector. Recent results indicate that the UHE cosmic ray spectrum exhibits significant structure near 10¹⁹ eV. A few events are seen beyond 10²⁰ eV in contradiction to the AGASA ground array claim of no cut-off. The composition of the cosmic rays is found to change from a predominantly heavy to a predominantly light mixture between and 10¹⁷ and 10¹⁸ eV. No evidence for anisotropy, on either small scales or large scales is found, in contradiction to AGASA. Systematic errors and absolute energy scale issues are now being carefully considered to see how to partially resolve this discrepancy. A new experiment(FLASH) at the Stanford Linear Accelerator Center (SLAC) to measure the Nitrogen fluorescence efficiency more precisely is described.