  In this model a collimated ultra-relativistic ejecta collides with an
amorphous dense cloud surrounding the central engine, producing gamma-rays via
synchrotron process. The ejecta is taken as a standard candle, while assuming a
gaussian distribution in thickness and density of the surrounding cloud. Due to
the cloud high density, the synchrotron emission would be an instantaneous
phenomenon (fast cooling synchrotron radiation), so a GRB duration corresponds
to the time that the ejecta takes to pass through the cloud. Fitting the model
with the observed bimodal distribution of GRBs' durations, the ejecta's initial
Lorentz factor, and its initial opening angle are obtained as
$\Gamma_{0}\lesssim 10^{3}$, and $\zeta_{0} \approx 10^{-2}$, and the mean
density and mean thickness of the surrounding cloud as $\bar{n} \sim 3 \times
10^{17} cm^{-3}$ and $\bar{L} \sim 2 \times 10^{13} cm$. The clouds maybe
interpreted as the extremely amorphous envelops of Thorne-Zytkow objects. In
this model the two classes of long and short duration GRBs are explained in a
unique frame.
