In high energy and astroparticle physics energies for cosmic ray particles are given in GeV/n. But how much is that in term of speed?
First of all we must remember that these energies are kinetic energies (k).
The total energy E of a particle is the sum of its kinetic energy k and its mass m: E = m + k with c=1 and energy and mass measured with the same unit.
In special relativity: E = g m where g is the Lorentz factor that is equal to:
is the ratio between the speed of a particle and the speed of light c.
Assuming that mass is linear with number of nucleons in the nucleus, the same calculation applies to any ion using the kinetic energy per nucleon. We see that for energies bigger than 2 GeV/n particles travel almost at light speed (> 95 %).