Elsevier

Nuclear Physics A

Volume 108, Issue 1, 29 January 1968, Pages 180-208
Nuclear Physics A

The giant dipole resonance excited by α-capture

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Abstract

We have studied the reactions 24Mg(α,γ)28Si from Eα = 5.3 MeV to 14.5 MeV, 26Mg(α,γ)30Si from 4.0 MeV to 13.5 MeV and 28Si(α,γ)32S from 7.0 MeV to 12.0 MeV. These α-energies lead into the region of the giant dipole resonance in the compound nucleus. The yield of γ-rays leading to the ground state of the final nucleus, γ0, was studied for each of the three targets; and for 24Mg(α,γ) the radiation γ1 going to the first excited state was also studied with resonable accuracy. The angular distributions of γ0, which were measured in 100 keV steps over a wide range of energies for the 24Mg and 26Mg targets and at two energies for the 28Si target, showed a dominant electric-dipole transition. This indicates that the (α,γ0) reactions lead predominantly through the giant dipole resonance in 28Si and 30Si and most likely also in 32S. The gamma-ray yield, which was measured in steps of 30 or 100 keV over the energy ranges studied, exhibited strong fluctuations in each case. For α-capture both in 24Mg and 26Mg, statistical analysis of the fluctuations showed that the reactions proceed nearly 100% through compound-nucleus formation and that the average width of the compound-nucleus resonances is about 60 keV. The integrated cross sections of these three alpha-capture reactions led to the conclusion that formation of the giant resonance by alpha capture is strongly inhibited - even in cases in which the alpha capture is isospin allowed. This is expected since the giant dipole resonance is supposed to consist predominantly of particle-hole states. Earlier experiments of proton capture by 27Al had shown that only a small part of the giant dipole resonance in 28Si leaks into the more complicated nucleon configurations of the compound nucleus. The observation that this part of the giant dipole resonance in 28Si decays with comparable strength by α-particle and proton emission indicates that it has a strong admixture of isospin T = 0.

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†††

Work performed under the auspices of the U.S. Atomic Energy Commission.

Present address: Weizmann Institute of Science, Rehovoth, Israel.

††

Present address: Indiana University, Bloomington, Indiana.

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