Forward to backward asymmetry of the photoneutron cross sections of calcium and lead in the energy region of the isovector electric quadrupole resonance

This experiment measured the photoneutron cross sections of Ca at 55°, 90°, and 125° between E')' = 28 to 38.8 MeV for residual excitation energies less than 4 MeV. From the 55° and 125° data a forward-to-backward asymmetry A = du/dfl(55°) -du/dfl(125°) du/dfl(55°) du/dfl(125°) was extracted. These data span the energy in which the Isovector Giant Quadrupole Resonance (IVQR) is expected to lie. Photons for this experiment were derived from an 49.7 MeV electron beam by a bremsstrahlung-tagging monochromator. Neutrons were detected in an array of NE213 liquid scintillator detectors; neutron energy was determined by a time of flight method over a 1.5 m flightpath. The energy resolution was between 1.8 and 4.2 Me V (FWHM), so final states could not be resolved. Instead the integral yield to states with residual excitation energy Ex < 4 and 2 MeV was obtained. The Ca asymmetries show a steady rise over our energy range from low values of ~ 0.2 at 28 MeV to ~ 0.6 at 38.8 MeV. This pattern can be understood as an interference between the tail of the Giant Dipole Resonance and the IVQR. A semiclassical model consisting of a series of Breit-Wigner resonances for the Isoscalar and Isovector E2 and the Giant Dipole Resonance, can be fitted to the data. It yields a width for the IVQR of ~ 23 MeV centered around 29.5 ± 1.5 MeV. The IVQR strength is found to be between 20% and 50% of a sumrule. The IVQR is fairly spread out and so weak compared to the GDR that there is no visible structure in the cross section. In continuation of a previous experiment [Murakami et at., Phys. Rev. C 35 479-494 (1987)] cross sections were also measured and asymmetries extracted for natPb over an energy range of E-y = 28 to 34.2 MeV above the IVQR. Consistent with the previous experiment, the asymmetries remain at a large value of 0.65 0.1. The semiclassical model yields a width for the IVQR of R:: 10 MeV centered at 21 MeV. The IVQR strength is found to 0.36 ±0.18 sum rule units.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Freytag, Asmus

Contributors dc:contributor

• Debevec, Paul T.

• photoneutron cross sections of Ca
• residual excitation energies
• forward to backward assymetry
• isovector electric quadrupole resonance

Statement dc:rights

• 1988 Asmus Freytag

Language dc:language

en

Identifier

1857056

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/23898