NUCLEAR PHYSICS A, cilt.970, ss.379-387, 2018 (SCI İndekslerine Giren Dergi)
The nuclear matrix element for 2 nu beta(-)beta(-) decay is obtained by including the virtual contributions coming from the first-forbidden excitations in intermediate nuclei. The forbidden contributions are calculated within proton neutron quasi-particle random phase approximation (pnQRPA) with the separable schematic effective interaction. We try to reproduce available experimental data concerning the first-forbidden excitations in intermediate nuclei. A comparison of the first-forbidden contributions with the allowed ones is given here. (C) 2018 Elsevier B.V. All rights reserved.