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High-Resolution Measurement of the 4He(g,n) Reaction in the Giant Resonance Region

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Author

  • Björn Nilsson

Summary, in English

A comprehensive near-threshold 4He(g,n) absolute cross section measurement has been performed at the high-resolution tagged-photon facility MAX-lab located in Lund, Sweden. The 20 < Eg < 45 MeV tagged photons (covering the Giant Dipole Resonance energy region) were directed towards a liquid 4He target, and knocked-out neutrons were detected in a pair of 60 cm x 60 cm vetoed NE213A liquid scintillator arrays. The intense and varying charge-neutral experimental backgrounds were carefully quantified and removed from the data using a precision fitting procedure. Eight average laboratory angles (30, 45, 60, 75, 90, 105, 120, and 135 deg) were investigated for eight photon energy bins (25, 27, 29, 31, 35, 36, 39, and 41 MeV), resulting in 64 differential cross sections. These angular distributions were integrated to produce total cross sections as a function of photon energy. The resulting cross sections peak at 1.9 mb at a photon energy of 27 MeV, and fall off to a near-constant value of 1.1 mb by 36 MeV. Further, they are in excellent agreement with those measured by Sims et al. using tagged photons in the Quasi-Deuteron energy region.



Overall, the results favor modern theoretical models which are based upon a charge-symmetric nucleon-nucleon force, in marked contrast to the recommendations made by Calarco et al. in 1983 based on the sparse 4He(g,n) data available at the time.

Department/s

Publishing year

2003

Language

English

Document type

Dissertation

Publisher

Department of Physics, Lund University

Topic

  • Subatomic Physics

Keywords

  • tagged photons
  • photonuclear reactions
  • liquid helium
  • neutron detectors
  • liquid scintillators
  • giant resonances
  • angular distributions
  • differential cross sections
  • total cross sections
  • Nuclear physics
  • Kärnfysik
  • Fysicumarkivet A:2003:Nilsson

Status

Published

Supervisor

  • [unknown] [unknown]

ISBN/ISSN/Other

  • ISBN: 91-628-5615-4
  • ISRN: LUNFD6/(NFFR-1020)/1-238/(2003)

Defence date

11 April 2003

Defence time

13:15

Defence place

Föreläsningssal B, Fysiska institutionen

Opponent

  • Gerald Feldman (Prof)