Positron spectra from internal pair conversion observed in $\chem{^{238}U}$ + $\chem{^{181}Ta}$ collisions

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

doi:doi:10.1007/s100500070055

2022 Impact factor 2.7

Hadrons and Nuclei

Eur. Phys. J. A 9, 55-61

Positron spectra from internal pair conversion observed in
$\ensuremath{\mathsf{^{238}U}}$ + $\ensuremath{\mathsf{^{181}Ta}}$ collisions

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: 27 April 2000 / Revised version: 24 August 2000
Communicated by D. Schwalm

Abstract
We present new results from measurements and simulations of positron spectra, originating from ²³⁸U + ¹⁸¹Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e⁺-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed $\gamma$ -transitions ( $E_{\gamma} \sim1250\mbox{--}1600$ keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed.

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 reactions - 29.30.Aj Charged particle spectrometers: electric and magnetic