It is shown that a macroscopic superposition state of radiation, stronglyinteracting with an ensemble of two-level atoms, is removed, generating a coherentstate describing a classical radiation field, when the thermodynamic limit is takenon the unitary evolution obtained by the Schrödinger equation. Decoherenceappears as a dynamical effect, in agreement with a recent proposal (Frasca 2001Phys. Lett. A 283 271). To prove that this effect is quite general, we show thatthis same behaviour appears when a superposition of two Fock numberstates is also considered. Higher order corrections are computed, showingthat this result tends to become exact in the thermodynamic limit. Itappears as a genuine example of intrinsic collapse of the wavefunction.