Todor Stanev

This cosmic ray expert has the following to say about UHECR.

One important result from the studies of the composition of UHECR is the strict limit of the fraction of γ-rays which is set to 2% for all particles above 10¹⁹ eV . This limit shows that at least 98% of the UHECR particles are nuclei accelerated in astrophysical objects.

The reference is:

Ultrahigh Energy Cosmic Rays and Neutrinos