Excitation and decay of the Gamow-Teller giant resonance in 90Nb

The proton decay of spin-isospin strength in Nb-90 was investigated after excitation via the reaction Zr-90(Li-6:He-6)Nb-90 at 156 MeV bombarding energy. The Gamow-Teller (GT) strength distribution and corresponding B(GT) values were extracted from singles spectra measured at extreme forward angles including zero degree. Protons in coincidence with He-6 ejectiles were detected using a multidetector arrangement of semi-conductor strip detectors. Relative branching ratios and He-6-p angular correlations have been measured for a proton decay to well defined low lying single hole states in Zr-89. Evidence for a dominant statistical decay has been found in the whole observed region of excitation energy; a small direct decay component, however, cannot be ruled out. In the region of the Gamow-Teller giant resonance between 8.0 and 12.0 MeV excitation energy additional strength with higher mulitpolarities L greater than or equal to 1 was identified from a comparison with predictions in the statistical decay model. At higher excitation energies above the giant resonance region the decay characteristics confirm the presence of a flat distribution of Gamow-Teller strength.