  We present an evaluation of the non-mesonic decay widths for
Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of
the polarization propagator method. The full Lambda self-energy is evaluated
microscopically in nuclear matter by using the functional approach, which
supplies a theoretically well grounded approximation scheme for the
classification of the relevant diagrams, according to the prescriptions of the
bosonic loop expansion. We employ average Fermi momenta, suitably adapted to
different mass number regions (medium-light, medium and heavy hypernuclei).
Moreover, we study the dependence of the decay rates on the NN and Lambda-N
short range correlations. With a proper choice of the parameters which control
these correlations in the new approximation scheme, it is possible to reproduce
the experimental decay widths for A > 10 hypernuclei.
