Asymmetry Dependence of Proton Correlations

Abstract

A dispersive optical model analysis of p+ 40 Ca and p+ 48 Ca interactions has been carried out. The real and imaginary potentials have been constrained from fits to elastic scattering data, reaction cross sections, and level properties of valence hole states deduced from (e, e ′ p) data. The surface imaginary potential was found to be larger overall and the gap in this potential on either side of the Fermi energy was found to be smaller for the neutron-rich p+ 48 Ca system. These results imply that protons with energies near the Fermi surface experience larger correlations with increasing asymmetry.PACS numbers: 21.10. Pc,24.10.Ht,11.55.Fv In the independent-particle model, nucleons in the nucleus move in a mean-field potential generated by the other nucleons. All nucleon levels up until the Fermi energy (E F ) are fully occupied, while those above are empty. Although this model enables an understanding of various aspects of nuclear structure, a full description of nuclei and nuclear matter requires consideration of the correlations between the nucleons. These include short-range, central and tensor interactions and longer range correlations associated with low-lying collective excitations [2]. As a result, for closed-shell nuclei, singleparticle (sp) levels below E F have an occupancy of only 70-80% and the levels at higher energy have a nonzero occupancy [3]. The strength of the sp levels are spread over energy, with narrow peaks or broad distributions (depending on their separation from E F ). In addition, there is strength at very high momentum [4].Although there are numerous studies of the effect of correlations on the properties of sp levels for nuclei near stability, there are only a few studies for very neutron or proton-rich nuclei. From neutron knock-out reactions, Gade et al. [5] infer the occupancy of the 0d 5/2 neutron hole state in the proton-rich 32 Ar nucleus is considerably reduced relative to those for stable nuclei.An alternate method to study sp strength is through the use of the dispersive optical model (DOM) developed by Mahaux and Sartor [6]. This description employs the Kramers-Kronig dispersion relation that links the imaginary and real parts of the nucleon self-energy [7]. This procedure links optical-model (OM) analyses of reaction data at positive energies to structural information at negative energies. In the present work, the properties of proton levels in Ca nuclei as a function of asymmetry δ= N −Z A are investigated with the DOM. Previously measured elastic-scattering and reaction-cross-section data for protons on 40 Ca and 48 Ca as well as level properties of hole states in these nuclei, inferred from (e, e ′ p) reactions, were simultaneously fit. The dependence on δ is extracted and used to predict level properties of 60 Ca.In the DOM, the complex energy-dependent potential felt by the protons is comprised of a real V, volume W v and surface W s imaginary components, plus spin-orbit V so and Coulomb V c potentials,are used. The real part of the nuclear potential is assumed...

Reduction of spectroscopic strength: Weakly-bound and strongly-bound single-particle states studied using one-nucleon knockout reactions

“…The USDB shell-model calculations (in agreement with the mirror nucleus) predict a 27 P first excited state with spin and parity 3/2 + at 990 keV, just above the proton separation energy of 861 (27) keV [43]. However, in qualitative agreement with the high 3/2 + excitation energy reported in Ref.…”

“…The USDB shell model predicts these states to be almost degenerate. In the γ -ray spectrum, taken in coincidence with 27 S, a weak low-energy transition cannot be excluded but its identification is obscured by background and hampered by low statistics. As outlined below, this is consistent with the level ordering proposed by the shell model.…”

“…It was suggested that this may indeed reflect enhanced correlation effects, absent from the underlying effective-interaction theory, linked to the large asymmetry in the Fermi energies of the neutron and proton states in nuclei near the drip lines [15]. Recent theoretical work, involving a dispersive optical model analysis of nucleon elastic scattering data [27], also indicates that nucleons of the deficient species will become more strongly correlated with increasing asymmetry, whereas the opposite holds for the nucleons of the excess species, in qualitative agreement with the observations reported above [15].…”

Forging the Link between Nuclear Reactions and Nuclear Structure

“…This idea was implemented in the dispersive optical model (DOM) by Mahaux and Sartor [5]. By employing dispersion relations, the method provides a critical link between the physics above and below the Fermi energy with both sides being influenced by the absorptive potentials on the other side.The DOM provides an ideal strategy to predict properties for exotic nuclei by utilizing extrapolations of these potentials towards the respective drip lines [6,7]. The main stumbling block so far has been the need to utilize the approximate expressions for the properties of nucleons below the Fermi energy that were developed by Mahaux and Sartor [5] to correct for the normalizationdistorting energy dependence of the Hartree-Fock (HF) potential.…”

“…The DOM provides an ideal strategy to predict properties for exotic nuclei by utilizing extrapolations of these potentials towards the respective drip lines [6,7]. The main stumbling block so far has been the need to utilize the approximate expressions for the properties of nucleons below the Fermi energy that were developed by Mahaux and Sartor [5] to correct for the normalizationdistorting energy dependence of the Hartree-Fock (HF) potential.…”