2018 Impact factor 2.481
Hadrons and Nuclei
Eur. Phys. J. A 1, 249-256

First energy and angle differential measurements of \ensuremath{\mathrm{e}^+\mathrm{e}^-}-pairs emitted by internal pair conversion of excited heavy nuclei

U. Leinberger1 - E. Berdermann1 - F. Heine2 - S. Heinz2 - O. Joeres2 - P. Kienle2 - I. Koenig1 - W. Koenig1 - C. Kozhuharov1 - A. Schröter1 - H. Tsertos3,a - (ORANGE Collaboration at GSI) - C. Hofmann4 - G. Soff4

1 Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
2 Technical University of Munich, D-85748 Garching, Germany
3 University of Cyprus, CY-1678 Nicosia, Cyprus
4 Institut für Theoretische Physik, Technical University Dresden, D-01062 Dresden, Germany

Received: 22 April 1997 / Revised version: 16 December 1997 Communicated by P. Armbruster

Abstract
We present the first energy and angle resolved measurements of \ensuremath{\mathrm{e}^+\mathrm{e}^-}-pairs emitted from heavy nuclei ($Z\ge 40$) at rest by internal pair conversion (IPC) of transitions with energies of less than 2MeV as well as recent theoretical results using the DWBA method, which takes full account of relativistic effects, magnetic substates and finite size of the nucleus. The 1.76MeV E0 transition in \ensuremath{^{90}\mathrm{Zr}} (\ensuremath{^{90}\mathrm{Sr}} source) and the 1.77MeV M1 transition in \ensuremath{^{207}\mathrm{Pb}} (\ensuremath{^{207}\mathrm{Bi}} source) have been investigated experimentally using the essentially improved setup at the double-ORANGE $\beta$-spectrometer of GSI. The measurements prove the capability of the setup to cleanly identify the IPC pairs in the presence of five orders of magnitude higher $\beta^-$ and $\gamma$ background from the same source and to yield essentially background-free sum spectra despite the large background. Using the ability of the ORANGE setup to directly determine the opening angle of the \ensuremath{\mathrm{e}^+\mathrm{e}^-}-pairs ($\Theta_{e^+e^-}$), the angular correlation of the emitted pairs was measured within the range covered experimentally ($40^{\circ} \leq \Theta_{e^+e^-} \leq 180^{\circ}$). In the \ensuremath{^{90}\mathrm{Zr}} case the correlation could be deduced for a wide range of energy differences \ensuremath{E_{\Delta}} of the pairs (-530 keV $\leq \ensuremath{E_{\Delta}}\leq 530$ keV). The \ensuremath{^{90}\mathrm{Zr}} results are in good agreement with recent theory. The angular correlation deduced for the M1 transition in \ensuremath{^{207}\mathrm{Pb}} is in strong disagreement with theoretical predictions derived within the Born approximation and shows almost isotropic character. This is again in agreement with the new theoretical results.

PACS
14.60.Cd Electrons (including positrons) - 23.20.En Angular distributions and correlation measurements - 23.20.Ra Internal pair production - 29.25.Rm Sources of radioactive nuclei - 29.30.Aj Charged-particle spectrometers: electric and magnetic


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