Particle production in a Robertson-Walker space with a de Sitter phase of finite extension

We investigate the phenomenon of particle production in a Friedmann-Robertson-Walker universe which contains a phase of de Sitter expansion for a finite interval, outside which it reduces to the flat Minkowski spacetime. We compute the particle number density for a massive scalar and a spinorial field and point out differences between the two cases. We find that the resulting particle density approaches a constant value at the scale of the Hubble time and that for a certain choice of the parameters the spectrum is precisely thermal for the spinorial field, and almost thermal for the scalar field.