  We investigate particle production in heavy-ion collisions at RHIC energies
as function of incident energy, and centrality in a three-sources Relativistic
Diffusion Model. Pseudorapidity distributions of produced charged hadrons in Au
+ Au and Cu + Cu collisions at sqrt(s_NN) = 19.6 GeV, 62.4 GeV, 130 GeV and 200
GeV show an almost equilibrated midrapidity source that tends to increase in
size towards higher incident energy, and more central collisions. It may
indicate quark-gluon plasma formation prior to hadronization.
