e+e- pairs from 238U + 206Pb collisions

Abstract

We present the results from e+e- coincidence measurements in 238U + 206Pb collisions at a beam energy of 5.93 MeV/u, using an improved experimental setup at the double-Orange spectrometer of GSI. The capability of our device to detect Internal-Pair-Conversion (IPC) e+e- pairs from discrete nuclear transitions of a moving emitter is demonstrated by investigating the known 1.844 MeV (E1) transition in 206Pb and the 1.770 MeV (M1) transition in 207Pb, excited via Coulomb excitation and a neutron transfer reaction, respectively. The Doppler-shift corrected e+- sum-energy spectra show weak lines at the expected energies with cross sections being consistent with the measured excitation cross sections of the corresponding $γ$ lines and the theoretically predicted IPC coefficients. No other e+e- sum-energy lines were found in the measured spectra. The observed transfer cross sections show a strong dependence on the distance of closest approach (R_min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier.