Elsevier

Nuclear Physics A

Volume 439, Issue 1, 3 June 1985, Pages 117-175
Nuclear Physics A

The decay of isoscalar giant resonances in 24Mg and 40Ca

https://doi.org/10.1016/0375-9474(85)90211-8Get rights and content

Abstract

In this paper we present data on the charged-particle decay of the isoscalar 2+ strength between 10 and 20 MeV excitation energy (Ex) in 24Mg and 40Ca. The isoscalar strength was excited by inelastic scattering of 120MeV α-particles at 14° and 12.5° for 24Mg and 40Ca, respectively. The charged particles originating from the decay were detected in coincidence with the α′ particles at several angles in the scattering plane. Jπ assignments of the decaying states were made on the basis of the angular correlation pattern of the α0 decay to the ground state of 20Ne and 36Ar, respectively, using a DWBA calculation for the m-state population of the decaying state.

For 40Ca, about 40% of the E2 EWSR is found to be located in the interval Ex = 13.5 ± 1.5 MeV, which is similar to what has been found from previous inelastic scattering experiments at Ex = 18 ± 2 MeV, but much more than such experiments located in the region Ex = 12–15 MeV. The difference for the region Ex < 16 MeV is due to the fact that from our α0 angular correlation pattern we conclude that virtually no continuum is excited in the (α, α′) process up to Ex = 16 MeV while all previous inelastic hadron scattering experiments assumed such a continuum to be present. The E2 strength distribution for 40Ca thus obtained is very different from what previous theoretical calculations predict. For 24Mg about 30% of the E2 EWSR is present in the interval 12.5 ⩽ Ex ⩽ experiments. 16.5 MeV which again is about twice as much as deduced from previous inelastic scattering The observed branching ratios are compared with calculated ones assuming statistical decay. Reasonable agreement was obtained for 40Ca, but for 24Mg especially the α0-decay branch and to a lesser extent also the p1 one are much stronger than the statistical calculations predict, indicating that especially the α0 decay occurs mainly in a non-statistical way.

A similar conclusion can be drawn from the behaviour of the forward-backward asymmetry in the angular correlations of the decay particles as a function of the excitation energy FBA(Ex). For 40Ca, FBA(Ex) for all decay channels increases smoothly on the average once Ex is above a well-defined threshold, which is due to the onset of knock-out processes. For 24Mg, however, the FBA(Ex) for the α0 shows a large fluctuation as a function of Ex, indicating an interference process between semi-direct decay and knock-out processes.

References (60)

  • K.T. Knöpfle et al.

    Phys. Rev. Lett.

    (1981)
  • K.T. Hecht et al.

    Nucl. Phys.

    (1978)
  • O.S. van Roosmalen, KVI annual report 1979, p. 112,...
  • A. Moalem

    Nucl. Phys.

    (1977)
  • D.H. Youngblood et al.

    Phys. Rev.

    (1977)
  • P.F. Bortignon et al.

    Nucl. Phys.

    (1981)
  • F. Zwarts et al.

    Phys. Lett.

    (1983)
  • A.G. Drentje et al.

    Nucl. Instr. Meth.

    (1974)
  • K. van der Borg et al.

    Nucl. Phys.

    (1981)
  • F. Zwarts

    Thesis

    (1983)
  • R.S. Mackintosh

    Nucl. Phys.

    (1976)
    M.N. Harakeh

    KVI internal report BEL, KVI77i

    (1981)
  • A.M. Bernstein, Advances in nuclear physics, ed. M. Baranger and E. Vogt, vol. 3 (Plenum, New York) p....
  • J. Arvieux et al.

    Phys. Rev. Lett.

    (1979)
  • Y. Torizuka et al.

    Phys. Rev.

    (1975)
  • T. Hoshino et al.

    Phys. Rev. Lett.

    (1976)
  • R. A. Broglia and P.F. Bortignon, private...
  • R. De Leo et al.

    Nucl. Phys.

    (1982)
    J. van der Plicht et al.

    Nucl. Phys.

    (1980)
  • F. Pühlhofer

    Nucl. Phys.

    (1977)
  • H.L. Harney et al.

    Phys. Rev.

    (1977)
  • H.T. Fortune et al.

    Phys. Rev.

    (1978)
  • E. Krämer et al.

    Nucl. Phys.

    (1971)
  • T. Yamaya et al.

    Phys. Lett.

    (1980)
  • P. Doll et al.

    Nucl. Phys.

    (1976)
  • M.N. Harakeh et al.

    Nucl. Phys.

    (1976)
  • J. Speth et al.

    Rep. Prog. Phys.

    (1981)
    A. van der Woude

    Lecture Notes in Physics

    (1982)
  • F.E. Bertrand

    Nucl. Phys.

    (1981)
  • D.R. Brown et al.

    Phys. Rev.

    (1976)
  • G.J. Wagner
  • K.T. Knöpfle

    Lecture Notes in Physics

    (1979)
  • K.T. Knöpfle et al.

    Phys. Lett.

    (1978)
  • Cited by (0)

    View full text