Meson-exchange enhancement in first-forbidden β transitions: The case of 50K and 38Ca

P. Baumann, M. Bounajma, F. Didierjean, A. Huck, A. Knipper, M. Ramdhane, G. Walter, G. Marguier, C. Richard-Serre, and B. A. Brown
Phys. Rev. C 58, 1970 – Published 1 October 1998
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Abstract

The β decay of 50K and 38Ca has been investigated with the main motive of determining more accurately the first-forbidden β branches, in particular the rank-0, ΔJ=0,β transitions. 50K and 38Ca have been produced by fragmentation of U and Ti targets, respectively, with a 1 GeV proton beam and subsequent on-line mass separation. For 50K,γ-ray spectroscopy, as well as delayed neutron spectroscopy by time of flight, was carried out to obtain a detailed decay scheme to 20 (bound and unbound) levels in 50Ca. The level structure of 50Ca can be compared to recent calculations which incorporate one-particle–one-hole excitations from the f7/2 shell. The first-forbidden β transition 50K(0)50Ca(0+) ground state has been evaluated for the first time by a direct measurement of β and γ activities. Its strength (61.0±7.4%) is interpreted as an effect of the meson-exchange current (MEC) leading to an enhancement factor of 62(5)% in comparison with the value predicted by shell-model calculations using the impulse approximation. For the 38Ca38K decay, chemical selective production was obtained through separation of the molecular ion CaF+ without contamination by isobars. In these conditions, the measurement of very weak β branches, at a level of 103 per 100 decays, could be made and a limit, at the 2σ confidence level, has been obtained for the 0+0 branch to the level at Ex=2993keV(Iβ<0.0046%). Implications of these results on the general trend of meson-exchange enhancements of first-forbidden transitions within the framework of the spherical shell model are discussed.

  • Received 30 March 1998

DOI:https://doi.org/10.1103/PhysRevC.58.1970

©1998 American Physical Society

Authors & Affiliations

P. Baumann, M. Bounajma, F. Didierjean*, A. Huck, A. Knipper, M. Ramdhane, and G. Walter

  • Institut de Recherches Subatomiques, Université Louis Pasteur, 67037 Strasbourg, France
  • the ISOLDE Collaboration

G. Marguier

  • Institut de Physique Nucléaire, Université Claude Bernard, 69622 Villeurbanne, France
  • the ISOLDE Collaboration

C. Richard-Serre

  • IN2P3 and CERN, CH - 1211 Genève 23, Switzerland

B. A. Brown

  • NSCL and Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824

  • *Present address: Eurisys, Strasbourg-Lingolsheim, France.
  • Present address: University of Constantine, Constantine, Algeria.

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Vol. 58, Iss. 4 — October 1998

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