  We consider the asymptotic evolution of a relativistic spin-1/2-particle.
i.e. a particle whose wavefunction satisfies the Dirac equation with external
static potential. We prove that the probability for the particle crossing a
(detector) surface converges to the probability, that the direction of the
momentum of the particle lies within the solid angle defined by the (detector)
surface, as the distance of the surface goes to infinity. This generalizes
earlier non relativistic results, known as flux across surfaces theorems, to
the relativistic regime.
