Dipole strength distribution in Pb206 for the evaluation of the neutron capture cross section of Pb205

Abstract

The dipole strength distribution of $^{206}\mathrm{Pb}$ was investigated via a nuclear resonance fluorescence experiment using bremsstrahlung produced with an electron beam at a kinetic energy of 10.5 MeV at the linear accelerator ELBE. We identified 88 states resonantly excited at energies from 3.7 to 8.2 MeV. The analysis of the measured $\ensuremath{\gamma}$-ray spectra includes the quasicontinuum of levels at high energy. Monte Carlo simulation of $\ensuremath{\gamma}$-ray cascades were performed to obtain the intensities of inelastic transitions and branching ratios of the ground-state transitions. The extracted photoabsorption cross section shows enhanced dipole strength at the excitation energies around 5.5 and 7 MeV, which may related to a pygmy dipole resonance. The present $(\ensuremath{\gamma},\ensuremath{\gamma}\ensuremath{'})$ data combined with $(\ensuremath{\gamma},n)$ data from the literature were used for confining input parameters of the statistical calculation code CCONE to derive the neutron-capture cross section of the unstable $^{205}\mathrm{Pb}$ nucleus.