Isovector giant quadrupole resonance observed in Y-89 (p -->, gamma) Zr-90
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6 pages
Published in:
- Phys.Rev.C 50 (1994) 1528-1534
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Abstract: (APS)
The cross section and vector analyzing power for transitions to the ground and low-lying states in the Y89(p→,γ)90Zr reaction have been studied over the angular range θγ=38.8°–130° at Ep=22.5 MeV and over the energy range Ep=18.5–28.5 MeV at θγ=90°. Angular distributions for the γ0 and γ1+2+3 transitions (i.e., to the first three strong single-particle excited states) reveal large asymmetries about θγ=90° in both σ(θ) and σ(θ)Ay(θ), suggesting E1/E2 interference effects. Legendre polynomial fits to these data yield nonzero coefficients of order k=3,4, thus confirming interference between the dominant E1 radiation and opposite parity E2 radiation. A transition-matrix element analysis of the γ0 angular distribution data, including E1 and E2 radiation, suggests a 28% E2 contribution, while a pure direct capture calculation predicts only 12% E2 strength. The energy dependence of Ay(90°) for the γ123 data is adequately described using the direct-semidirect model by including isovector quadropole resonances at EGQR=27.5 MeV, built on each of the three states, with widths of ΓGQR=7.0 MeV and strengths which exhaust 100% of the isovector E2 energy-weighted sum rule for each state. These results provide strong evidence for collective E2 strength built on excited states in Zr90, approximately at the energy predicted for the isovector giant quadrupole resonance, but are inconclusive concerning the ground-state transition.- 24.70.+s
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