Studies of the Giant Resonances of NEON-20 and ZIRCONIUM-90 with Polarized Proton Capture.

Polarized and unpolarized angular distributions of the ('19)F(p,(gamma)(,0))('20)Ne and ('19)F(p,(gamma)(,1))('20)Ne* reactions were measured in small steps at energies from E(,x) = 16.1 to 23.0 MeV. This range encompasses the region of the giant dipole resonance (GDR) built upon the ground state, in which very pronounced intermediate structure has been previously reported. The use of a polarized proton beam enabled solutions for the El and E2 T-matrix elements for the ground state reaction to be extracted. The intermediate structure in the cross section is found to be due primarily to the ('1)P E1 partial wave amplitude, although the ('3)P (spin flip) amplitude also resonates with the cross section. The E2 partial wave amplitudes are found to be rather constant over the range of the GDR. The total measured ground state E2 cross section exhausts 40 to 50 percent of the isoscalar E2 sum rule. The E1 cross section in the ('19)F(p,(gamma)(,0))('20)Ne reaction can be fitted to a series of nearly isolated Breit -Wigner resonances, suggesting that the reaction proceeds through a series of essentially nonoverlapping 1('-) levels in the GDR. The ('19)F(p,(gamma)(,1)) reaction is found to proceed in large part through 2('-) and 3('-) intermediate levels. These findings are in agreement with recently published microscopic calculations. Polarized and unpolarized angular distributions of the ('89)Y(p,(gamma)(,0))('90)Zr reaction were measured in small steps at energies from E(,x) = 10.4 to 24.6 MeV, with particular attention paid to the Isobaric Analogue Resonances (IAR's) at E(,x) = 14.4 and 16.3 MeV. Solutions were extracted for the E1 T-matrix elements. The E(,x) = 14.4 MeV IAR is found to be predominantly s-wave in character, while the E(,x) = 16.3 MeV IAR is comprised of approximately equal contributions from s-wave and d-wave incident proton amplitudes. The data are in agreement with recent suggestions that a statistical reaction mechanism dominates below the GDR, while a direct-semidirect reaction mechanism is dominant at and above the GDR.