$R_O$. This behaviour might be improved by allowing particles also to freeze out and fly into the detector at all times of the collision. Especially, if there are fast pions produced during the first hard collisions in UrQMD at the edge of the system they should be able to fly into the detector without being forced into the hydrodynamic evolution.

![$k_T$ dependence of the HBT radii $R_L$, $R_O$, and $R_S$ (at midrapidity) for central HICs at SPS energies ($E_{\rm
lab}=20A$, $30A$, $40A$, $80A$, and $158A$ GeV). The NA49 data are indicated by solid stars [@Alt:2007uj]. The HG-EoS is employed in all calculations but under different freeze-out conditions: 1) without HR, calculations with default hydro-freeze-out criteria (GF, $5\cdot\varepsilon_0$) are depicted by lines. 2) full hybrid model calculations with two different cuts of the energy density (default and $4\cdot \varepsilon_0$) for the GF and with the default cut of the energy density for the IF are depicted by dashed lines with symbols.[]{data-label="HBT-fig3"}](HBT-fig3.eps){width="48.00000%"}

Fig. \[HBT-fig3\] illustrates the $k_T$ dependence of the HBT radii under various freeze-out conditions, which may be divided into two parts: 1) without HR and 2) with HR after the hydrodynamic phase. Here, without HR (lines) means that the evolution 