Hanburry-Brown-Twiss (HBT) correlation functions and radii of negatively charged pions from C+C, Si+Si, Cu+Cu, and In+In at lower RHIC/SPS energies are calculated with the UrQMD transport model and the CRAB analyzing program. We find a minimum in the excitation function of the pion freeze-out volume at low transverse momenta and around E lab ∼ 20–30AGeV which can be related to the change from initial string emission to bulk emission from the created resonance matter. For small systems, we predict a local minimum in the excitation function of the HBT (freeze-out) volume which is explained by the competition of two mechanisms of the particle production, namely resonance decays and string formation/fragmentation.
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