  It has been suggested that relativistic shocks in extragalactic sources may
accelerate the most energetic cosmic rays. However, recent theoretical advances
indicating that relativistic shocks are probably unable to accelerate particles
to energies much larger than a PeV cast doubt on this. In the present
contribution we model the radio to X-ray emission in the southern hotspot of
the quasar 4C74.26. The synchrotron radio emission is resolved near the shock
with the MERLIN radio-interferometer, and the rapid decay of this emission
behind the shock is interpreted as the decay of the downstream magnetic field
as expected for small scale turbulence. If our result is confirmed by analyses
of other radiogalaxies, it provides firm observational evidence that
relativistic shocks at the termination region of powerful jets in FR II
radiogalaxies do not accelerate ultra high energy cosmic rays.
