  We investigate the dynamical Casimir effect for a one-dimensional resonant
cavity, with one oscillating mirror. Specifically, we study the discrete
spectrum of created particles in a region of frequencies above the oscillation
frequency $\omega_0$ of the mirror. We focus our investigation on an
oscillation time equal to $2L_0/c$, where $L_0$ is the initial and final length
of the cavity, and $c$ is the speed of light. For this oscillation duration, a
field mode, after perturbed by the moving mirror, never meets this mirror in
motion again, which allows us to exclude this effect of re-interaction on the
particle creation process. Then, we describe the evolution of the particle
creation with frequencies above $\omega_0$ only as a function of the
relativistic aspect of the mirror's velocity. In other words, we analyze the
formation of relativistic bands in a discrete spectrum of created particles.
