Positive parity states inBe11

Abstract

The low-lying positive parity states of Be are calculated in a coupled channels treatment of a valence neutron interacting with a deformed core. The loosely bound nature of the valence neutron is taken into account by using a Woods-Saxon potential. Comparisons are made to shell-model predictions and to data. The model reproduces the measured spectrum quite well for realistic parameters of the neutron-core interaction. PACS number(s): 21.10.k, 27.20.+n I. INTR.ODUCTION The development in radioactive beam experiments has made it possible to study the properties of nuclei far from stability. An example is Be which has already been studied in several fragmentation experiments [1 -3] and more detailed Ineasurements of the two-body breakup Be~Be+n have recently been performed [4]. The analyses commonly employ a simple 8 wave for the valence neutron in its ground state, and we shall in particular investigate the validity of this approximation. The nucleusBe is of particular theoretical interest because of the parity inversion near the ground state, i.e. , the ground state is a 1/2+ state, and not a 1/2 state as one naively would expect from a spherical shell-model.We have recently discussed this parity inversion [5] on the basis of shell-model wave functions and pointed out three competitive efFects, viz. the quadrupole excitation of the core, the Pauli blocking of the pairing correlations, and the narrowing of the shell gap due to the protonneutron monopole interaction. Thus we found that the 1/2+ ground has a large overlap with the 0+ ground state of the Be core coupled to an Sqy2 single-particle state.A loosely bound s~y2 state has a large root mean square (rms) radius and a large dipole strength at low excitation energies, and both of these two features are clearly needed in order to explain the recent fragmentation measurements [1 -4].In the present study we focus on a realistic description of positive parity states in Be. Our approach is to solve coupled equations for a neutron interacting with a deformed. core, including the efFect of the quadrupole excitation of the core. This approach, also known as the weak coupling limit of the particle-rotor model, has previously been applied by many authors, for example to A = 13 nuclei. An early application [6] made use of harmonic oscillator wave functions. It was found that this approach provides a reasonable picture of A = 13 nuclei, whereas the strong coupling limit, implied by the Nilsson model, contradicts measurements. Later applications made use of a Wood-Saxon plus spin-orbit single-particle Hamiltonian, see, e.g. , Refs. [7,8]. We shall use a similar parametrization since it provides a more realistic description of weakly bound orbitals; this was clearly demonstrated in the study of the strong dipole transitions in [9]. The main purpose of our study is to see how well one can predict the positive parity spectrum of Be from the knowledge of the structure of the Be core. The lowlying negative parity states, on the other hand, have a complicated structure because ...

Study of 14Be with core excitation

“…The behavior of the lowest 5/2 + state is, in general, consistent with that of the 1d 5/2 curve on a Nilsson plot. The behavior of the 1/2 + state is consistent with that observed for the 1/2 + state in 11 Be [27,24].…”

“…The previous inclusion of such core excitation, of 10 Be, made it possible to describe 11 Be as a 2s-intruder state nucleus. The approach was also used by Esbensen et al [27] to describe positive parity states of 11 Be and by Ridikas et al [28] to study a series of carbon isotopes.…”

Asymptotic normalization of mirror states and the effect of couplings

“…To illustrate this model, let us consider the particular mirror pair 17 O/ 17 F. The core of both nuclei is 16 The main difference between both mirror nuclei is the B-x Coulomb interaction. We should stress that in this work our approach is strongly phenomenological.…”

“…17 O/ 17 F: For this 16 O+x mirror pair, our model reproduces both the 5/2 + (ε n 5/2 + = −4.144 MeV or ε p 5/2 + = −0.601 MeV) and 1/2 + (ε n 1/2 + = −3.273 MeV or ε p 1/2 + = −0.106 MeV) bound states as predominantly 1d 5/2 and 2s 1/2 valence nucleons coupled to the 0 + ground state of 16 O. For 16 O we consider the effect of the coupling between the 0 + ground state and either the 2 + excited state at ǫ 2 + = 6.917 MeV or the 3 − excited state at ǫ 3 − = 6.129 MeV.…”

“…It was shown that deviations from the analytic formula increased. A simple framework of including multiconfiguration and excitation in the single nucleon overlap functions is provided by the core + N phenomenological model [15][16][17]. In the nineties, this model was applied to a number of light nuclei, including the one-neutron halos 11 Be [15] and 19 C [18].…”