HBT interferometry is a well known technique to study the space-time extent of the particle source in heavy ion collisions. The NA44 experiment measures one and two particle spectra of identified ...pions, kaons and protons around mid-rapidity produced in heavy ion collision at the CERN SPS. We present the relation between charged multiplicity density and the HBT radius parameters in S + Pb collisions at 200 A GeV and Pb + Pb collisions at 158 A GeV. The HBT radius parameter increases as a function of charged multiplicity density in Pb + Pb collisions in a similar way to that found previously in S + Pb collisions. We will discuss what we can learn about the condition at freeze-out using these experimental results.
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