  Recently, it has been realized that nonlocal disentanglement may take a
finite time as opposite to the asymptotic decay of local coherences. We find in
this paper that a sudden irreversible death of entanglement takes place in a
two atom optical Stern-Gerlach model. In particular, the one degree non
dissipative environment here considered suddenly destroys the initial
entanglement of any Bell's states $\ket{\phi^{\pm}}$ superposition.
