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

S. Heinz; E. Berdermann; F. Heine; O. Joeres; P. Kienle; I. Koenig; W. Koenig; C. Kozhuharov; U. Leinberger; M. Rhein; A. Schröter; H. Tsertos (The ORANGE Collaboration at GSI)

EPJ

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 (R_min), 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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EPJ

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

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