Composition measurement above 1 EeV with a large array near the high resolution fly's eye

Abstract

The purpose of this workshop is to design a detector which will shed light on the origin of cosmic rays at the highest known energies. The emphasis has been on detecting cosmic rays with energies above 10EeV with the specific goals of testing the Greisen-Zatsepin hypothesis, searching for anisotropy, and inferring composition of the primaries. These are noble goals, but there is value in concentrating on understanding these properties above E = 1EeV. The previous experimental designs (mainly ground arrays) have focused principally on anisotropy, then spectrum, and most poorly, composition. There are practical reasons for this. For one, anisotropy is generally easier to measure than composition. However, barring the detection of point sources, composition needs to be known if propagation and the acceleration mechanism are to be understood. Further, composition information permits highly discriminating anisotropy and spectral change analyses. It is my opinion that focusing on composition is a prudent next step. The proposed detector consists of a ground array in the vicinity of the High Resolution (HiRes) Fly's Eye. Building and operating an array is not only useful in refining experimental techniques for the grander proposal of lO%m2sr, but in conjunction with the HiRes Fly's Eye, the combination will clarify certain outstanding questions regarding EAS phenomenology and cosmic ray acceleration processes. This proposal necessarily fails to satisfy one of the original requirements of the workshop design criteria; namely a time averaged 104km2sr collection area above 100EeV. Arguments can be made that an interesting energy region begins near 1EeV. It may also be possible that a large array similar to those proposed at the workshop[I],[2] situated near the HiRes Fly's Eye will satisfy the necessary design requirements for a useful composition measurement.