The nuclear deformation versus the spin-flip like excitations and the suppression of the 2 nu beta beta decay amplitude

The suppression mechanism of the Gamow-Teller double beta decay amplitude M-GT is studied using a many body Hamiltonian which describes a composite system of protons and neutrons moving in a projected spherical single particle basis, Alike nucleons interact through pairing while protons and neutrons by a separable dipole-dipole force both in the particle-hole (ph) and particle-particle (pp) channels. The spin-Rip and non-spin-flip components of the QRPA phonons have a differents contribution to the M-GT values. The relative magnitudes and phases depend both on the strength of the particle-particle interaction (g(pp)) and on the nuclear deformation. The deformation yields a fragmentation of the M-GT value on one hand and washes out the separation of states of pure spin-hip and non-spin-flip structures. Due to this effect M-GT has only one fragmented resonance structure in the low part of the spectrum. (C) 1997 Elsevier Science B.V.