  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.
