Study of the N=28 shell closure in the Ar isotopic chain : a SPIRAL experiment for nuclear astrophysics

The structure of the neutron-rich nucleus (47)Ar has been investigated through the d((46)Ar,(47)Ar)p transfer reaction. The radioactive beam of (46)Ar at 10 A . MeV was provided by the SPIRAL facility at GANIL. The protons corresponding to a neutron pick-up on bound or unbound states mechanism in (47)Ar nuclei were detected at backward angles by the position-sensitive Si array-detector MUST. The transfer-like ejectiles were detected in the SPEG spectrometer. Level scheme, spin assignments and spectroscopic factors have been deduced for (47)Ar and compared to shell model predictions. They suggest a slight erosion of the N = 28 shell gap from the weakening of the spin-orbit interaction arising from the f and p orbitals. These spectroscopic information are subsequently used to infer (n, gamma) reaction rates in the Ar isotopic chain to understand the origin of the (48)Ca/(46)Ca abnormal isotopic ratio observed in certain inclusions of meteorites.