Quenched Spectroscopic Factors for Low-Lying Positive Parity States in 31Mg

The excitation function of the proton resonance elastic scattering with a ^30Mg beam of 2.92 MeV/nucleon was measured around 0 degrees in the laboratory frame. Three resonances corresponding to high excited states in ^31Al were successfully observed. They correspond to the isobaric analog resonances of the first three bound states in ^31Mg, which is considered to be located at the border of the island of inversion. An R-matrix analysis on the excitation function revealed that the spectroscopic factors for the first two positive parity resonances were quenched compared to those for other N=19 nuclei, ^35S and ^37Al, clearly indicating that the shell structure changes between ^30Mg and ^31Mg.