Are topological defects responsible for the 300 EeV cosmic rays?
  We use of a hybrid matrix--Monte Carlo method to simulate the cascade through the cosmic background radiation initiated by UHE particles and radiation emitted by topological defects. We follow the cascade over cosmological distances and calculate the intensities of hadrons, gamma-rays and neutrinos produced. We compare our results with the observed cosmic ray intensity at 300 EeV and lower energies, and conclude that topological defects are most unlikely to be the origin of the most energetic cosmic ray events. 
R.J. Protheroe and P.A. Johnson (University of Adelaide)