Photoresponse of60Ni below 10-MeV excitation energy: Evolution of dipole resonances infp-shell nuclei nearN=Z<

Abstract

Background: Within the last decade, below the giant dipole resonance the existence of a concentration of additional electric dipole strength has been established. This accumulation of low-lying E1 strength is commonly referred to as pygmy dipole resonance (PDR). Purpose: The photoresponse of 60 Ni has been investigated experimentally and theoretically to test the evolution of the PDR in a nucleus with only a small neutron excess. Furthermore, the isoscalar and isovector M1 resonances were investigated. Method: Spin-1 states were excited by exploiting the (γ, γ ) nuclear resonance fluorescence technique with unpolarized continuous bremsstrahlung as well as with fully linearly polarized, quasimonochromatic, Comptonbackscattered laser photons in the entrance channel of the reaction. Results: Up to 10 MeV a detailed picture of J = 1 levels was obtained. For the preponderant number of the individual levels spin and parity were firmly assigned. Furthermore, branching ratios, transition widths, and reduced B(E1) or B(M1) excitation probability were calculated from the measured scattering cross sections. A comparison with theoretical results obtained within the quasiparticle phonon model allows an insight into the microscopic structure of the observed states. Conclusions: Below 10 MeV the directly observed E1 strength [ B(E1) ↑= (153.8 ± 9.5) e 2 (fm) 2 ] exhausts 0.5% of the Thomas-Reiche-Kuhn sum rule. This value increases to 0.8% of the sum rule [ B(E1) ↑= (250.9 ± 31.1) e 2 (fm) 2 ] when indirectly observed branches to lower-lying levels are considered. Two accumulations of M1 excited spin-1 states near 8 and 9 MeV excitation energy are identified as isoscalar and isovector M1 resonances dominated by proton and neutron f 7/2 → f 5/2 spin-flip excitations. The B(M1) ↑ strength of these structures accumulates to 3.94(27)μ 2 N .

Statistical γ -decay properties of Ni64 and deduced ( n,γ ) cross section of the Campo1, Garrote2, Eriksen3 et al. 2016Phys. Rev. C

“…Moreover, in the mass region considered in this work, big differences in the γ,0 values are often seen between isotopes of similar mass. Therefore, to normalize the γ -strength function of 64 Ni, photoneutron crosssection measurements were used to extract the γ -strength function of 60 Ni [51] and 66 Zn [52] at the giant dipole resonance (GDR) via the relationship…”

“…This is reflected in the Thomas-Reiche-Kuhn (TRK) sum rule, which relates the energy-weighted E1 summed strength to the atomic and mass numbers of the nucleus [53]. The application of the TRK sum rule gives values of 896, 945, and 988 mb MeV for 60 Ni, 64 Ni, [38] and it is therefore reasonable to believe that the low-energy tail of the GDR for 64 Ni should be, at least to a great extent, very similar to that for 60 Ni and 66 Zn. The experimental data on 60 Ni and 66 Zn were used to obtain an estimate of the GDR parameters and their uncertainties within a generalized Lorentzian (GLO) model [39].…”

“…The physical mechanisms underlying this enhancement are still not clarified. There is currently no consensus on whether its character is mostly of E1 or M1 nature and so far only for 60 Ni are there strong indications of the M1 character of this upbend [21]. In the last years, two main theories have been proposed.…”

Current Status of Nuclear Physics Research

“…Heavier nuclei with large amount of excess neutrons have been produced as secondary beams, and the existence of the Pigmy resonance was clearly established in, e.g., 134 Sn [189,[191][192][193]. These nuclei with a thick neutron skin, were produced at high energies.…”

“…The excess nucleons in the halo nuclei results in two important effects. The excitation of low lying dipole mode, the Pigmy Dipole Resonance (PDR) (also known as the Ikeda resonance) [188][189][190][191][192][193], and the close-to-threshold breakup. These two effects were found to influence the fusion in two distinct ways.…”