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

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1988

OSTI ID:5243024

Freytag, A

This experiment measured the photoneutron cross sections of Ca at 55, 90 and 125{degree} between E{sub {gamma}} = 28 to 38.8 MeV for residual excitation energies less than 4 MeV. 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 bremsstralung-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 MeV (FWHM), so final states could not be resolved. Instead the integral yield to states with residual excitation energy E{sub x} < 4 and 2 MeV was obtained. The Ca asymmetries show a steady rise over our energy range from low values of {approx} 0.2 at 28 MeV to {approx} 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 {approx} 23 MeV centered around 29.5 {plus minus} 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 cross sections were also measured and asymmetries extracted for {sup nat}Pb over an energy range of E{sub {gamma}} = 28 to 34.2 MeV above the IVQR.

• Research Organization: Illinois Univ., Urbana, IL (USA)
• OSTI ID: 5243024
• Country of Publication: United States
• Language: English

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73 NUCLEAR PHYSICS AND RADIATION PHYSICS
ALKALINE EARTH METALS
ASYMMETRY
BARYONS
CALCIUM
CROSS SECTIONS
ELEMENTARY PARTICLES
ELEMENTS
ENERGY RANGE
FERMIONS
GIANT RESONANCE
HADRONS
LEAD
MASSLESS PARTICLES
METALS
MEV RANGE
MEV RANGE 10-100
NEUTRONS
NUCLEAR QUADRUPOLE RESONANCE
NUCLEAR REACTIONS
NUCLEONS
PHOTONEUTRONS
PHOTONS
PHOTONUCLEAR REACTIONS
PHOTONUCLEONS
RESONANCE
SEMICLASSICAL APPROXIMATION