2019 Impact factor 2.176
Hadrons and Nuclei
Eur. Phys. J. A 1, 27-37

Weak e+e- lines from internal pair conversion observed in collisions of 238U with heavy nuclei

S. Heinz1 - E. Berdermann2 - F. Heine1 - O. Joeres1 - P. Kienle1 - I. Koenig2 - W. Koenig2 - C. Kozhuharov2 - U. Leinberger2 - M. Rhein2 - A. Schröter2 - H. Tsertos3
(The ORANGE Collaboration at GSI)

1 Technische Universität München, D-85748 Garching, Germany
2 Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
3 University of Cyprus, CY-1678 Nicosia, Cyprus

Received: 22 July 1997 / Revised version: 15 October 1997

Abstract
We present the results of a Doppler-shift correction to the measured e+e--sum-energy spectra obtained from e+e--coincidence measurements in 238U + 206Pb and 238U + 181Ta collisions at beam energies close to the Coulomb barrier, using an improved experimental setup at the double-Orange spectrometer of GSI. Internal-Pair-Conversion (IPC) e+e- pairs from discrete nuclear transitions of a moving emitter have been observed following Coulomb excitation of the 1.844 MeV (E1) transition in 206Pb and neutron transfer to the 1.770 MeV (M1) transition in 207Pb . In the collision system 238U + 181Ta , IPC transitions were observed from the Ta-like as well as from the U-like nuclei. In all systems the Doppler-shift corrected e+e--sum-energy spectra show weak lines at the energies expected from the corresponding $\gamma$-ray spectra with cross sections being consistent with the measured excitation cross sections of the $\gamma$ lines and the theoretically predicted IPC coefficients. No other than IPC e+e--sum-energy lines were found in the measured spectra. The transfer cross sections show a strong dependence on the distance of closest approach (Rmin), thus signaling also a strong dependence on the bombarding energy close to the Coulomb
barrier.

PACS
14.60.Cd Electrons and positrons - 23.20.Ra Internal pair production - 25.70.Bc Elastic and quasielastic scattering - 25.70.De Coulomb excitation - 25.70.Hi Transfer reaction - 29.30.Aj Charged-particle spectrometers: electric and magnetic


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