Phenomenological analysis ofB(E2)transition strengths in neutron-rich carbon isotopes

Abstract

Recent experimental results related to the quadrupole collectivity in neutron rich Carbon isotopes are analyzed in a phenomenological approach. B(E2; 2 + 1 → 0 + 1 ) transitions rates derived from lifetime measurements are interpreted in terms of the mixing of basic neutron and proton 2 + excitations. A seniority inspired scheme is used to describe the neutron component. The observed increase in collectivity can be explained with a corresponding increased role of proton excitations. This is likely due to the reduction of the proton p 3/2 − p 1/2 spin orbit splitting caused by the tensor and two-body spin orbit components of the force between the protons and the added neutrons in the (d 5/2 + s 1/2 ) shells.

Probing the Z = 6 spin-orbit shell gap with (p,2p) quasi-free scattering reactions

“…This case is particularly interesting since it is anticipated that a quenching of the splitting will occur due to effects of the tensor and two-body spin-orbit forces acting on the 1p protons when neutrons are added in the d 5/2 and s 1/2 orbits. Indeed, in our earlier work [5] we have attributed the observed increase in the B(E2; 2 + 1 → 0 + 1 ) values from 16 C to 20 C, as a manifestation of increased in-shell proton excitations (p 1 1/2 p −1 3/2 ), due to a weakening of the 1p 1/2 − 1p 3/2 spin-orbit splitting at Z = 6 towards the dripline. It is worth pointing out a similar effect has been discussed in Ref.…”

“…To determine the proton contribution to the 2 + 1 state wave function from our cross section measurements, we consider a simple shell-model picture for the 0 + 1 and 2 + 1 state as discussed in Refs. [5,8]. We assume that the 0 + ground state of the carbon isotopes can be described as the simulation is depicted for the background induced by the protons (blue) and the γ rays de-exciting the first 2 + state (red).…”

“…Following on from Refs. [5,7,8], we report in this Letter the results of an experiment designed to study the proton component of the 2 + 1 state in 16,18,20 C using Quasi-Free Scattering (QFS) (p,2p) reactions on 17,19,21 N. As described in more detail in Section 4, the proton component (p 1 1/2 p −1 3/2 ) in the carbon isotopic chain can be uniquely accessed through (p,2p) reactions; the population of the 2 + state is expected to proceed only through this component since the π1p 1/2 coupled to the 2 + state in the carbon isotopes cannot contribute to the 1/2 − ground state of the nitrogen isotopes. Our results show an increase in the proton component, and signals a quenching of the Z = 6 1p 1/2 − 1p 3/2 gap towards the dripline, casting doubt on the strong statement given by the authors of Ref.…”

How to Study Efimov States in Exotic Nuclei?

“…In fact, the structure of the carbon isotopes is in itself of much interest to nuclear structure [22], in particular to benchmark state-of-the-art shell model codes and ab initio calculations. Spectroscopic factors of low-lying states will provide key structural information and will be obtained in this type of experiments.…”

Structure of 16C from analyses of proton scattering data and the B(E2) problem

“…This has been assumed by Wiedeking et al [9] and in the shell model calculation of Corragio et al [7]. Extensions to that model suggest the inclusion of proton configurations would influence the B(E2) [11], while the inclusion of more complicated neutron sd shell configurations may also explain it [12]. In all cases, an effective neutron charge of ∼ 0.4e has been required in order to fit the measured/adopted value.…”