  Three-dimensional analyses of the pion source revealed a heavy, non-Gaussian
tail in the direction of the pair transverse momentum. The interpretation of
these pion sources in terms of pure hydrodynamical evolution is, however,
complicated by the strong contribution of feed-down from long-lived resonances
to the source. On the other hand, kaons provide a much cleaner probe of the
expanding fireball. Here we present a recent three-dimensional kaon correlation
analysis using Cartesian harmonics decomposition technique. In contrary to
pions, the three-dimensional source function of kaons is largely Gaussian.
Comparison with thermal simulations and a hydrokinetic model show that
resonance decays, as well as non-zero emission duration and/or rescattering in
the hadronic phase play an important role. The analysis of the three
dimensional extent of the kaon source w.r.t. the pair transverse momentum
favors the hydrokinetic model over the exact mT-scaling featured by perfect
hydrodynamical models.
