Direct proton decay of the isoscalar giant dipole resonance in 208Pb

The excitation and subsequent proton decay of the isoscalar giant dipole resonance (ISGDR) in 208Pb have been investigated via the Pb208(α,α′p)Tl207 reaction at 400 MeV. Excitation of the ISGDR has been identified by the difference-of-spectra method. The enhancement of the ISGDR strength at high excitation energies observed in the multipole-decomposition-analysis of the singles Pb208(α,α′) spectra is not present in the excitation energy spectrum obtained in coincidence measurement. The partial branching ratios for direct proton decay of ISGDR to low-lying states of 207Tl have been determined and the results are compared with predictions of continuum random-phase-approximation (CRPA) calculations.