  A model-independent, three-dimensional source function for pion pairs has
been extracted from Pb+Pb collisions at $\sqrt s_{NN}=17.3$ AGeV. The extracted
sourc e exhibits long-range non-Gaussian tails in the directions of the
pion-pair net transverse-momentum and of the beam. Comparison with the
Therminator model allow s for an extraction of the pion source proper breakup
time and of emission durat ion in the collisions.
