  Diffusive shock acceleration is invoked to explain non-thermal particle
acceleration in Supernova Remnants, Active Galactic Nuclei (AGN) Jets, Gamma
ray Bursts (GRBs) and various large scale cosmic structures. The importance of
achieving the highest observed particle energies by such a mechanism in a given
astrophysical situation is a recurring theme. In this work, shock acceleration
in relativistic shocks is discussed, mostly focusing on a numerical study
concerning proton acceleration efficiency by subluminal and superluminal
shocks, emphasising on the dependence of the scattering model, bulk Lorentz
factor and the angle between the magnetic field and the shock flow. We
developed a diffuse cosmic ray model based on the study of different shock
boost factors, which shows that spectra from AGN fit current observations of
ultra high energy cosmic rays, above 5.7 x 10^10 GeV, much better than GRBs,
indicating that AGN are the primary candidates to explain the UHECR flux.
Recent Fermi observations of GRB090816c indicate very flat spectra which are
expected within our model predictions and support evidence that GRB particle
spectra can be flat, when the shock Lorentz factor is of order ~1000.
