Dipole Absorption by Many-Particle Many-Hole States: a Study in the Nuclei OXYGEN-16 and CALCIUM-40

A search has been conducted for the giant E1 resonances (GDR's) built on the prolate deformed, four-particle four -hole first excited states of ^{16} O (E_{x} = 6.05 MeV, J^{pi} = 0 ^{+}) and ^ {40}Ca (E_{x}=3.35 MeV, J^pi=0^+) in order to study the dependence of the properties of excited state GDR's on the structure of the underlying states. To examine the ^{16}O(0 _sp{2}{+}) GDR, proton capture on ^{15}N was measured in the energy range from E_{p } = 10.0 to 22.0 MeV, and ^3 He capture on ^{13}C in the energy range from E_{rm 3_{He}} = 3.5 to 24.0 MeV. The ^{40}Ca(0_sp {2}{+}) GDR was examined using proton capture on ^{39}K in the energy range from E_{p} = 7.2 to 19.2 MeV. All these experiments used large solid angle electron detection systems to identify gamma rays from capture to the first excited states by coincident observation of an electron or positron from the decay of that state. A pronounced concentration of dipole strength is observed in the ^{15}N( p,gamma_1) ^{16}0 reaction at excitation energies between 23 and 27 MeV which is interpreted as the giant isovector E1 responance built on the ^{16}O(0_sp{2 }{+}) state. Significant capture strength is observed in the ^{13} C(^3He,gamma _1)^{16}O reaction only for energies below 27 MeV of excitation. In the ^{39}K(p, gamma_1)^{40 }Ca reaction, there is a concentration of capture cross section between E_{x} = 18 and 23 MeV, which is interpreted as the giant isovector E1 resonance built on the ^{40} Ca(0_sp{2}{+} ) state. A similar coincidence technique has been used to measure the proton spectroscopic factor of the ^{40}Ca(0_sp{2 }{+}) state. The value of S _{p} is found to be 0.12 +/- 0.03. The integrated strengths of both the ^{15}N(p, gamma_1)^{40 }Ca and ^{39}K( p,gamma)^ {40}Ca reactions are larger than would be predicted by proportionality to the respective spectroscopic factors, suggesting that there are contributions to the cross sections from mechanisms other than direct-semidirect capture. The gamma-ray energy for both first excited GDR's is lower than that for the respective ground state GDR's, in contradiction to the Brink hypothesis.