Why do some studies suggest that very-high-energy cosmic rays include more heavy nuclei such as iron?

Studies suggesting an increased proportion of heavy nuclei, such as iron, among very-high-energy cosmic ray primaries are based on the analysis of extensive air shower (EAS) data.1

Because very-high-energy particles—those with energies above approximately ${10}^{18}$ eV—are extremely rare, they cannot be observed directly. Instead, scientists detect them by studying the massive showers of secondary particles they produce when they collide with the atmosphere.12

Due to the limited number of these high-energy events recorded, researchers currently face challenges in drawing definitive conclusions. Consequently, the suggestion that these primaries include a higher concentration of heavy nuclei remains an alternative interpretation of the shower data rather than a confirmed observation.1