  We compute the spectral index for scalar perturbations generated in a
primordial inflationary model. In this model, the transition of the
inflationary phase to the radiative era is achieved through the decay of the
cosmological term leading a second order phase transition and the
characteristics of the model allow to implement a set of initial conditions
where the perturbations display a thermal spectrum when they emerge from the
horizon. The obtained value for the spectral index is equal to 2, a result that
depends very weakly on the various parameters of the model and on the initial
conditions used.
