ELASTIC-SCATTERING CROSS-SECTIONS OF 11.4 MEV PHOTONS FROM PB-206, PB-207, PB-208, BI-209, AND TA-181

Monoenergetic photons of 11.4 MeV, originating from the Ni(n, gamma) r eaction were elastically scattered from three isotopically enriched le ad targets, a Bi-209 target, and a Ta-181 target. Cross sections for t he lead isotopes at theta = 90-degrees and 140-degrees differ signific antly from predictions based on known giant dipole resonance (GDR) par ameters showing marked structures in this region of energy. The cross sections for Bi-209 and Ta-181 are in agreement with a previous measur ement. The Ta-181 cross section is the only case where agreement is fo und with calculations based on known GDR parameters. New GDR parameter s for Bi-209 were deduced by combining the present elastic scattering with those from previous measurements, leading to kappa(GDR) = 0.35 +/ - 0.06.