Internal pair decay of giant resonances in hot 200Pb

Abstract

Electron-positron pairs with transition energies of <or=25 MeV emitted during the deexcitation of highly excited 200Pb produced in the 19F+181Ta reaction were detected in the Stony Brook pair detector in order to study internal pair conversion from giant resonances in a hot nucleus. The observed kinematic e+-e-variables show that the pair decay is overwhelmingly of E1 character. The extracted transition spectrum confirms the statistical model prediction that the giant dipole resonance in the hot (Ex=57 MeV) 200Pb nucleus dominates internal pair conversion from 4 to 20 MeV transition energy. E1 pair conversion coefficients extracted from a comparison with available gamma spectra obtained in the same reaction show that the point nucleus and Born approximation can be used for energies above 6.5 MeV even in this high-Z nucleus. Results from a search for e+-e-pairs from giant monopole resonances are presented.