Quantitative evidence of weak isospin mixing in the low-lying isobaric analog resonances in 209Bi

The excitation functions of 208Pb(p vector, p0)208Pb have been measured in the energy range Esub(p) = 14.2 to 17.4 MeV in 50 keV steps at THETAsub(lab) = 1200, 1400 and 1600. The isobaric analog resonances of the parent states in 209Pb up to Esub(x) = 2.5 MeV and the optical-model background were fit simultaneously at all energies and angles. The spreading widths and the values of a parameter β2, which measures the isospin purity of the IAR, were determined for the gsub(9/2), isub(11/2), jsub(15/2), dsub(5/2), and ssub(1/2) resonances. An average value of the isospin purity of β2 = 66% was found. (orig.)