Upturn observed in heavy nuclei to iron ratios by the ATIC-2 experiment

TL;DR

The ATIC-2 experiment observed an unexpected sharp increase in heavy nuclei to iron ratios above 100 GeV/n, indicating a potential new phenomenon in cosmic ray composition at high energies.

Contribution

This study reports the first observation of an upturn in heavy nuclei to iron ratios at energies above 100 GeV/n, revealing differences in spectral behavior among cosmic ray elements.

Findings

Ratios decrease from 5 to 80 GeV/n as expected.

Sharp upturn observed above 100 GeV/n for Z=16 to Z=24.

Iron spectrum differs significantly from other abundant nuclei.

Abstract

The ratios of fluxes of heavy nuclei from sulfur (Z=16) to chromium (Z=24) to the flux of iron were measured by the ATIC-2 experiment. The ratios are decreasing functions of energy from 5 GeV/n to approximately 80 GeV/n, as expected. However, an unexpected sharp upturn in the ratios are observed for energies above 100 GeV/n for all elements from Z=16 to Z=24. Similar upturn but with lower amplitude was also discovered in the ATIC-2 data for the ratio of fluxes of abundant even nuclei (C, O, Ne, Mg, Si) to the flux of iron. Therefore the spectrum of iron is significantly different from the spectra of other abundant even nuclei.