With the start of the BES program at RHIC, the studies of the phase structure of QCD at non-vanishing baryon density became of major interest. It is clear that any meaningful interpretation of data requires good knowledge of the net baryon density and thus both the net baryon number and the volume these baryons occupy in the configuration space. While the net baryon number in the central rapidity region, y cm ≈ 0, is directly accessible in experiment, this is of course not the case for the

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... e. Moreover, as we have argued in our recent paper [1], the theoretical evaluation of this volume is by no means straightforward, as it depends on the adopted picture of the nucleus-nucleus and nucleon-nucleon collisions. Although for the central collisions the transverse dimension of the interaction volume is relatively well-determined, the longitudinal dimension is strongly model-dependent. Two limiting pictures may be considered: (i) The colliding nucleons transfer most of their energy into produced particles very soon after collision and thus stop at a very short distance from the collision point. In this case, the resulting longitudinal (z) distance between them is of the order of nuclear radius reduced by Lorentz contraction, i.e. very small. (ii) The colliding nucleons continue their motion for some time after collision, loosing gradually their energy at some (approximately constant) rate, σ, often called string tension. The best-known example of realization of the picture is the Lund model [2] . In this case, the right-moving * Presented at the XIII Workshop on Particle Correlations and Femtoscopy, Kraków, Poland, May 22-26, 2018. (165)