Studies of the emission of δ electrons in binary heavy-ion collisions at the Coulomb barrier

The differential emission probability of δ electrons was measured in the collisional systems 238U+170Er, 238U+108Pd, and 238U+58Ni at incident energies of 6.1 MeV/u at the UNILAC of the GSI in Darmstadt. Especially the dependence of the δ electron spectra on the impact parameter as well as on the total charge of the system was determined. The experimental arrangement consisted of two 90x90 cm2 large position-resolving particle counters, an orange β spectrometer with plastic ring counters for the detection of the backward emitted electrons, and a diameter 5x3 cm2 BGO γ detector. The electrons were taken up in coincidence to binary scattering events, which were in the analysis separated in scattering-angle dependent reaction classes, which reach up to the grazing angle. In the analysis of the data much made about the careful correction of the spectra regarding the background contributions from internal-pair and electron conversion of excited nuclear levels, because the conversion electrons in the systems with small Zu represent a large contribution to the whole electron yield. The measured δ electron yield agrees for the systems U+Er and U+Pd in the purely elastic scattering range with results of coupled-channel calculations. Against that an increased yield in the measured spectra against the calculation in close collision results, which is presumably caused by nuclear interaction. The deviation from the theory amounts up to about 2 orders of magnitude in the system U+Pd and occurs still augmentedly in the lighter system U+Ni. The causes for this deviation are discussed. (orig.)