  A semi-classical non-Hamiltonian model of a spontaneous collapse of unstable
quantum system is given. The time evolution of the system becomes
non-Hamiltonian at random instants of transition of pure states to reduced
ones, given by a contraction C. The counting trajectories are assumed to
satisfy the Poisson law. A unitary dilation of the concractive stochastic
dynamics is found. In particular, in the limit of frequent detection
corresponding to the large number limit we obtain the Ito-Schroedinger
stochastic unitary evolution for the pure state of unstable quantum system
providing a new stochastic version of the quantum Zeno effect.
